Nanospintronics for Data Storage

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Encyclopedia ofNanoscience andNanotechnology

Nano Spintronics for Data Storage

Yihong Wu

Department of Electrical and Computer Engineering, National University of Singapore,Singapore 117576, and Data Storage Institute, Singapore 117608

CONTENTS

1. Introduction2. Basics of Magnetic Data Storage

and Spintronics3. Magnetoresistance

and the Associated Read Sensors4. Magnetic Aspects of Spin Valves5. Electronic Aspects of Spin Valves6. Magnetic Tunnel Junctions7. Magnetic Random Access Memory8. Semiconductor Spintronics9. Future Trend of Data Storage

and the Role of SpintronicsGlossaryAcknowledgmentsReferences

1. INTRODUCTIONNanotechnology is one of the hottest research fields in thescience and engineering arena. Of the many potential appli-cations of nanotechnology, one of the most promising onesis in data storage, particularly the hard disk drives. Thisis because the physical size of the recording bits of harddisk drives is already in the nanometer regime, and contin-ues shrinking due to the ever-increasing demand for higherrecording densities. If the pace of areal density increaseis maintained at the current level for about ten years, thedimension of the recording bit will reach the sub-10 nmregime. At this level, both the writing and reading processeswill become extremely challenging, if not impossible.

The rapid shrinkage of bit size poses formidable chal-lenges to the read sensors. This is because the sensor mustbe made smaller or at least comparable to the bit size, and atthe same time, its sensitivity must be improved continuouslyso as to compensate the loss in signal-to-noise ratio due tothe decrease in the bit size. The former has to rely heavily on

the advance of nanotechnology, and the latter on an emerg-ing field called spintronics [1–4]. The combination of thesetwo fields has played an important role in advancing theareal density of magnetic recording from a few gigabits/in2

to the current level of more than 100 Gbits/in2 [5–8]. Inaddition to hard disk drives, the technologies developedhave also been applied to magnetic random access memo-ries (MRAMs) [9–11]. Further advance in the two fields isthe key to realizing terabits/in2 hard disk drives and giga-bit nonvolatile memories within this decade. This chapter isintended to provide an updated review of nanometer-scalespintronics for data storage applications, with emphasis onthe application of metal-based spintronics in magnetic sen-sors and memories.

To have a clear picture of the scope of this review,Figure 1 illustrates how spintronics is positioned in the hier-archy of various different types of data storage technolo-gies. The data storage devices can be categorized into threemajor groups, that is, magnetic and optical data storage,and solid-state memory. Each of them can be divided fur-ther into several subgroups, and here, we only focus onmagnetic data storage and solid-state memory. Among themagnetic storage devices, the hard disk drive (HDD) isthe dominant secondary mass storage device for comput-ers, and very likely also for home electronic products inthe near future. The HDD is an integration of many keytechnologies, including head, medium, head-disk interface(HDI), servo, channel coding/decoding, and electromechan-ical and electromagnetic devices. Among them, the readhead is the only component that has experienced the mostchanges, including some revolutionary ones in terms of boththe operating principle and the structural design and fabri-cation processes during the last decade. The ever-increasingdemand for higher areal densities has driven the read headevolving from a thin-film inductive head to an anisotropicmagnetoresistive (AMR) head, and recently, the giant mag-netoresistive spin-valve (GMR SV) head. There are twodifferent forms of GMR SVs, depending on whether thesense current flows in the film plane (CIP) or perpendicu-lar to the film plane (CPP). Currently, CIP SVs are domi-nant, but CPP SVs are more promising for future extremelyhigh-areal-density heads. In addition to CPP spin valves, a

ISBN: 1-58883-001-2/$35.00Copyright © 2003 by American Scientific PublishersAll rights of reproduction in any form reserved.

Encyclopedia of Nanoscience and NanotechnologyEdited by H. S. Nalwa

Volume X: Pages (1–50)

2 Nano Spintronics for Data Storage

DATA

STORAGE

Magnetic

Optical

SolidState

Read only

Phase change

Magnetooptical

Holography

Spectralhole burning

DRAM

SRAM

Flash

FeRAM

MRAM

CRAM

RRAM

Floppy

Tape

HDD

ZIP

Others

Head

Media

HDI

Servo

Channel

Others

Inductive

AMR

GMR

MTJ

SPINTRONICS

Metallic

Semiconductor

HybridInductive

AMR

GMR

MTJ

Nano Spintronicsfor Data Storage

NANOTECHNOLOGY

2 terminal 3 terminal

Johnsontransistor

Monsma transistor

Spin FET

Spin SET

Spin LED/laser

Spin RTD

MTT

Diode

Transistor

DATA

STORAGE

SPINTRONICS

Nano Spintronicsfor Data Storage

Figure 1. Spintronics and its applications in data storage and elec-tronics. The acronyms are: DRAM—dynamic random access mem-ory, SRAM—static random access memory, FeRAM—ferroelectricrandom access memory, MRAM—magnetic random access memory,CRAM—chalcogenide random access memory, RRAM—resistancerandom access memory, ZIP—zip drive, HDI—head–disk inter-face, AMR—anisotropic magnetoresistance, GMR—giant magnetore-sistance, MTJ—magnetic tunnel junction, FET–field-effect transistor,SET—single-electron transistor, LED—light-emitting diode, RTD—resonant tunneling diode, MTT—magnetic tunnel transistor.

magnetic tunnel junction (MTJ) with low junction resistanceis also one of the potential candidates for future read sen-sors. The MTJ is particularly useful for MRAM applications.As is with the case of read sensors, the MRAM started fromthe ferrite core design to AMR, GMR pseudospin valve,and the current MTJ cell design. However, it is only afterthe introduction of MTJ that the MRAM taken off, andis expected to be in mass production in a few years’ time.The GMR and MTJ sensors are generally called magneto-electronic or metal-based spintronic devices, which are thefocuses of this review. The metal-based spintronic devicesare based primarily on the spatial modulation of electronspins through using layered structures of magnetic and non-magnetic materials. The lack of capability in charge mod-ulation in these types of structures may eventually limittheir ultimate performances in terms of both the magne-toresistance and other functionalities. To address this issue,recently, a great deal of effort has been devoted to the devel-opment of magnetic semiconductors which allow the mod-ulation of both the spins and charges. The advance in thisfield may eventually lead to spintronic devices with perfor-mances superior to their metal-based counterparts. In addi-tion to pure metal-based or semiconductor-based spintronicdevices, hybrid devices making use of both technologies alsohave been explored actively in recent years.

The organization of the rest of this chapter is as follows.Section 2 gives a brief introduction to the basics of magneticdata storage and spintronics, and explains why both the spin-tronics and nanotechnology are crucial for magnetic datastorage. Section 3 discusses different types of magnetoresis-tance effects, and their applications in read sensors. In-depthreviews of the magnetic and electronic aspects of spin valvesare given in Sections 4 and 5, respectively. Section 6 dealswith the MTJs, with their applications in MRAMs discussed

in Section 7. Section 8 touches briefly on the semiconduc-tor spintronics. Finally, we discuss the future trend of datastorage and the role of spintronics in future data storage inSection 9.

2. BASICS OF MAGNETIC DATASTORAGE AND SPINTRONICS

2.1. Magnetic Recordingand Magnetic Memory

The hard disk drive at the system level is the integrationof many key technologies and components, which includethe storage medium, read/write transducer, channel cod-ing/decoding, servocontrol, head/disk interface, tribology,and electromechanical and electromagnetic systems [12–14].In this subsection, however, we focus on the physics andmaterials aspects of magnetic recording, that is, topicsrelated to the storage medium and read/write heads [15].

The thin-film medium for a hard disk drive is a stack ofmultiple layer thin films formed on either an NiP-coatedaluminum alloy or a glass substrate [16]. Among the mul-tiple layers of thin films, the early hard disk media onlyemployed a single magnetic layer as the recording layer,which is usually a polycrystalline alloy of Co, Cr, and Pt withadditional elements such as Ta or B to improve the magneticproperties. The latest magnetic media, however, employmore than one magnetic layer as the recording layer, such asthe antiferromagnetically coupled (AFC) media to improvethe thermal stability of the information bits recorded on thehard disk [17–19]. When recording the information onto thedisk, a writer which can generate a sufficiently high magneticfield is used to switch the magnetization of a localized areaof the media to one of two fixed directions, with one of themrepresenting digit “1” and the other representing digit “0.”Each of the localized areas can be considered as one bit.Each bit consists of a number of partially exchange-coupledmagnetic grains, as shown schematically in Figure 2.

When reproducing the information from the disk, theearly disk drive employs the same write head as the read

UNDERLAYER

Al-Mg : NiP or GLASS

SEED

INTERMEDIATE LAYER

MAGNETIC LAYER

INTERLAYER

MAGNETIC LAYER

Magnetic

transition

read

er

writer

shie

ld

shie

ld

writer

shie

ld

shie

ld

Figure 2. Schematic of a magnetic recording head and media. The headtypically consists of an inductive writer and an AMR or GMR reader.

Proof's Only

Nano Spintronics for Data Storage 3

head, that is, the signal is detected by measuring the voltagechange across the coils induced by the flux variation whenthe head passes by the recorded media. Although the per-formance of the inductive head had been improved contin-uously to meet the requirement of rapidly increasing arealdensity, its structural design and working principle did posecertain limitations to the ultimate performance of this typeof head. That is the reason why, in the beginning of the1990s, a new type of head which is based on the anisotropymagnetoresistance of certain magnetic materials was intro-duced by IBM, and later also adopted by other head anddrive companies. This type of head is widely called an AMRhead [20]. The AMR effect stems from the spin–orbit inter-action of electrons, and thus it normally appears in magneticmaterials; its value, however, varies from less than 1% to afew percent, depending on the type of material. The com-mon material that is used for an AMR head is Permalloy,of which the MR ratio is about 2–4%. It decreases almostlinearly with thickness for ultrathin films (<10 nm).

Although the AMR head showed much better perfor-mance in terms of sensitivity and scalability than the thin-film inductive head, its pace of structural miniaturizationand performance improvement was soon outpaced by therapid advance in areal density. The rapid increase of arealdensity requires simultaneously a decrease in both the bitlength and the track width. The former requires a thinnerand more sensitive sensor, while the latter requires a largemagnetoresistance (MR). Unfortunately, it is difficult forthe AMR head to satisfy both requirements simultaneouslybecause the MR ratio of the AMR head decreases rapidlywhen its thickness is reduced to less than 10 nm [21]. Onthe other hand, in 1988, a much larger MR effect, that is,GMR, was discovered in artificially made magnetic and non-magnetic multiple layers [22 23]. The MR ratio of this typeof superlattice structure is more than one order of mag-nitude larger than that of the AMR. The GMR structure,however, could not be applied directly to the read head asit was because of the large magnetic field that is requiredto switch the magnetization of the magnetic layers from onedirection to the other. This has prompted IBM to invent amore practical structure for a read sensor which is called aspin valve [24].

The state-of-the-art spin valve consists of a dozen thinlayers, the heart of which is a trilayer structure consisting oftwo ferromagnetic layers separated by a nonmagnetic spacer,which is usually copper. The signal detection principle isbased on the fact that the resistance of the trilayer is depen-dent on the relative orientation of the magnetization of thetwo ferromagnetic layers. The resistance is high when themagnetizations of the two layers are in opposite directions,and low when they are in the same direction. To have a lin-ear response from the sensor, the angle between the twomagnetizations is normally set at 90 at zero field, with oneof them “pinned” at a direction perpendicular to the mediasurface through exchange coupling with an antiferromagnet,and the other free to rotate in response to the fringe fieldof magnetic transitions recorded on the magnetic media.

There are two different forms of spin-valve sensors,depending on whether the current flows in the plane ofthe stack of layers or perpendicular to them. The former iscalled a current-in-plane, or CIP, spin-valve sensor, and the

latter a current-perpendicular-to-plane, or CPP, sensor [25].So far, CIP is dominant, but CPP is expected to play animportant role in future terabit recording systems. An alter-native to the CPP spin valve is the magnetic tunnel junction,or MTJ, in which the current also flows perpendicular tothe plane [26–29]. The major difference between the CPPspin valve and the MTJ is that the latter is composed oftwo ferromagnetic layers separated by an insulator insteadof a metal. Therefore, the electrical conduction in MTJ isbased on quantum-mechanical tunneling. The MTJ is attrac-tive because its MR ratio is generally more than double thatof spin valves. However, the primary drawback of MTJ isthat its junction resistance is generally larger than that ofthe CIP or CPP sensors, which may affect its performanceas a read sensor due to the increase in thermal noise and thedecrease in bandwidth when the junction size is reduced toa certain value. In addition to this, the quantum-mechanicalnature of electron transport also causes shot noise whichincreases with the intensity of the output signal. Thus, themajor application of MTJ so far is in MRAM. As one ofthe newcomers in the memory hierarchy, the MRAM hasthe potential to replace some of the commercially availablememories in the near future.

Except for the inductive read head, all other read sensorsdiscussed above make use of both the charge and spin prop-erties of electrons. AMR stems from spin–orbit interaction,while the spin valve and MTJ operate based on the spin-dependent scattering and tunneling both inside the magneticlayers and at the magnet/nonmagnet or insulator interfaces.In contrast to Si-based electronics, the GMR-based sensorsare often referred to as a subcategory of an emerging fieldcalled spintronics (see Fig. 1).

2.2. Spintronics

Electrons possess both charges and spins. The motion ofcharges forms the current. The ability to control or mod-ulate the charge transport has made it possible to formfunctional devices such as diodes and transistors. This isso far only possible in semiconductors instead of metalsbecause the latter have too many electrons per unit volume;the variation of charge distribution, if any, is limited to afew atomic layers at the surface that can hardly cause anymeasurable change in the conductance of the metal. There-fore, functional electronic devices have not been realized inpure metals with dimensions larger than the mean-free pathof electrons. Although they belong to the same family ofmetals, metallic magnetic materials have another additionaldegree of freedom which can be used to vary their elec-tronic transport properties—the spin of electrons. As thespin of electrons in magnetic materials can be easily manip-ulated using an external magnetic field without suffering theelectrostatic screening effect as the charges do when theyare subjected to an electric field, it is possible to alter theconductivity of magnetic materials without changing the car-rier distribution inside the material. This forms the basis ofGMR-based electronics or sometimes is also called magne-toelectronics. It is a subfield of spintronics or spinelectronics[1–4].

The spintronics in its broader sense contains all typesof electronics that make use of both charges and spins.


In semiconductor-based electronics, we only control thecharge motion, and ignore the spins. In fact, it is diffi-cult to control the spins in semiconductors because they donot have specific directions under normal conditions, unlessthe electrons are injected into from external sources witha net polarization. On the other hand, in metallic mag-netic materials, it is easy to control the spins due to thestrong ferromagnetic coupling among the spins, but it is dif-ficult to control the charges. Therefore, a question naturallyarises here: Can we have a kind of material in which wecan control both charges and spins? The answer is “yes,”and this type of material is called a magnetic semiconduc-tor [30–32]. The magnetic semiconductor is usually madethrough adding magnetic impurities to host semiconductors.It is not necessary, however, that every semiconductor can bemade magnetic using this approach because some of themstill do not exhibit any magnetic properties even after theyare doped with a substantial amount of magnetic impurities.Some of them, although being magnetic, show a very lowCurie temperature [33 34]. However, the situation changeddrastically in recent years due to the intensive efforts madeby researchers in this field in many research organizations.Several different types of magnetic semiconductors hav-ing a Curie temperature higher than room temperaturehave been found. These include, but are not limited to,GaN:Mn [35 36], CdGeP:Mn [37], GaP:Mn [38], TiO2:Co[39], and ZnO:Co [40]. It should be noted, however, thatall of these are based on preliminary experimental results;further experiments are required to verify the results. Theprogress was made not only in materials themselves, but alsothe application of these materials in creating new functionaldevices such as semiconductor-based magnetic tunnel junc-tions [41 42] and spin-injection devices [43 44]. Althoughthe current technology for a read sensor is based on metallicspintronics, semiconductor-based spintronics has the poten-tial to provide sensors or storage elements with superiorperformances for next-generation data storage devices.

Although the read sensor for magnetic recording and stor-age cells for magnetic memory are based on spintronics, thestorage of information in disk media is still based on clas-sic physics, and it does not involve spintronics. However, asthe bit size continues decreasing, one day the bit size willapproach that of atoms. At this stage, a fundamental changein the information storage principle might be required. Oneof the possible scenarios is to store information in the recip-rocal space or energy domain. In this case, the spin of elec-trons and nuclei instead of the magnetization of magneticgrains will be important. This is closely related to anotheremerging field called quantum information storage [45].

2.3. Nanotechnology and MagneticData Storage

Nanotechnology is indispensable for all kinds of data storagedevices, be it magnetic, optical, or solid-state devices. Thisis because the common driving force for any type of datastorage technology is the areal density, provided that the bit-cost performance also improves with the areal density. As arule of thumb, the bit size shrinks by one order when theareal density increases by two orders. Figure 3 shows thesize of a square bit as a function of the areal density. The bit

1

10

100

1000

0 500 1000 1500 2000

Areal Density (Gb/in2)

Bit

Siz

e(n

m)

2001 2004 2005 2006 2007

Year

80nm 36nm 25nm 8nm

1 500 Gb/in2

1 Tb/in2

10 Tb/in2

100 Gb/in2

Figure 3. Bit size as a function of areal density for square bits. Alsoshown is the year at which the areal density will be reached by assuminga compound annual growth rate of 60%. Inset shows the size of squarebits at areal densities of 100 500 1000 10000 Gbits/in2, respectively.

size falls into the nanometer regime (1–100 nm) when theareal density reaches 70 Gbits/in2, which was already real-ized about two years ago. To write information to and readinformation from these small bits, the write and read headsmust have a size which is smaller than or at least compara-ble to that of the bit. In addition to the read/write head, thespacing between the disk and the head must also be kept assmall as possible, and in a practical case, it is about one fifthof the bit size. Therefore, once the areal density enters thehundreds of gigabits range, the dimensions of almost all ofthe key components in magnetic recording have to be in thenanometer regime.

The continuous shrinkage of bit size poses formidablechallenges to both media and read heads. For low-areal-density recording, each single bit contains tens of thou-sands of small magnetic grains. At this level, one will not“feel” the difference when one or two grains are missing.However, the number of grains per bit decreases monoton-ically with the areal density. At 100 Gbits/in2, the numberof grains is reduced to less than 100. This results in anincrease of media noise due to the particulate nature ofthe media. One of the possible ways to suppress the noiseis to reduce the grain size of the media so that each bitwill contain a greater number of grains. However, this isnot so straightforward because the superparamagnetic phe-nomenon will appear, making the information bit thermallyunstable. Although the recent advance in multiple-layeredmedia has greatly enhanced the thermal stability of themedia with small grains, it is still far from being a perfectsolution. The thermal stability of the media is not only justaffected by the grain size, but also very much by the grainsize distribution. The latter is technically more challengingbecause one must use a mass-production technique whichcan produce not just small grains, but also monodispersedgrains. This is the area where nanotechnology is expectedto play an important role. One of the possible ways to real-ize such a kind of media is to fabricate single-domain par-ticles or dots by using nanofabrication techniques [46–48].Although one of the major applications of nanostructuredmagnetic materials and nanomagnetism is in storage media,we will not discuss this further in this chapter because, so far,

Proof's Only


the media are still in the “passive” form, and do not involvespintronics.

Now, we turn to the read sensor, which is our main focusin this chapter. We are concerned about the sensor whenits size is reduced because, first, the sensor size must bereduced accordingly when the bit size decreases, and sec-ond, the output and impedance of the sensor are dependenton the sensor size. So far, the sensor fabrication has lever-aged the semiconductor manufacturing processes. But thistrend is going to be reversed because the size shrinkage ofread sensors has outpaced the shrinkage of critical dimen-sions of integrated circuits [49]. In a few years’ time, thedimension of the sensor will approach 50 nm, at which pointthere may not even be a solution for the semiconductorindustry at the moment. In addition to the manufacturingissues, one also needs to find a way to improve the perfor-mance of the sensors so that the output of the sensor willnot decrease significantly when its size is reduced.

The above discussion demonstrates clearly the importanceof nanotechnology and spintronics in magnetic recordingand memory devices. In fact, spintronics is closely related tonanotechnology because most of the phenomena related tobulk and surface magnetism have a characteristic length inthe nanometer regime. In the rest of this chapter, we discussin detail the fundamental concepts and latest developmentof metallic spintronics and its applications in magnetic datastorage.

3. MAGNETORESISTANCE AND THEASSOCIATED READ SENSORS

3.1. Anisotropic Magnetoresistance

The resistivity of a nonmagnetic material is usually inde-pendent of the magnetic field. However, this may not bethe case for magnetic materials. The phenomenon that theresistivity of a ferromagnetic material depends on the rela-tive angle between the current and magnetization directionof the material is called the anisotropic magnetoresistanceeffect, or AMR, which was discovered in 1857 by Thomson[50]. A series of theoretical studies carried out a centurylater showed that the AMR effect stems from the spin–orbitinteraction [51]. The importance of the AMR effect wasrecognized in the 1970s when a large AMR was found ina number of alloys based on iron, cobalt, and nickel [52].Materials exhibiting a normal AMR effect show a maximumresistivity when the current is parallel to the magnetizationdirection () and a minimum resistivity when the current isperpendicular to the magnetization direction (⊥). A mea-sure for the size of this effect is the AMR, ratio which isdefined by

MR = − ⊥

(1)

At intermediate angles between the current and magnetiza-tion direction, the resistivity of an AMR material is given by

= ⊥ + − ⊥ cos2 (2)

where is the angle between the current and the magneti-zation direction.

3.2. AMR Sensor

The immediate application of the AMR was in magneticrecording because sensors based on the AMR effect offerhigher output as compared to the thin-film inductive head[53 54]. Although, for Ni (Fe, Co) alloys, the largest AMReffect so far was found for Ni70Co30, 26.7% at 4.2 K and6.6% at 300 K [52], the material of choice for magneticrecording applications is permalloy because of its softness,high permeability, and low magnetorestriction. The typicalAMR ratio for thin permalloy films (30–50 nm) is about 2%,although the AMR of its bulk counterpart can be as high as4% [13 21]. When being used as a sensor, the magnetizationdirection is normally set at 45 with respect to the currentdirection at zero field so as to maximize the sensitivity [55].This is apparent from the fact that the first derivative of is maximum when = 45.

Figure 4 shows a schematic drawing of an AMR sen-sor element. For the sake of simplicity, we assume that theexternal magnetic field points in the y-axis direction. Assum-ing that the entire element is a single-domain particle witha uniaxial anisotropy, the magnetization direction can beobtained from minimization of the energy density [13]:

= −HMs sin +Ku sin2 + 12HdMs sin

2 (3)

Note that here only the magnetostatic energy, anisotropyenergy, and demagnetizing energy are taken into account; allother energy terms are neglected. In Eq. (3), H is the exter-nal magnetic field, Ms is the saturation magnetization, Kuis the uniaxial anisotropy energy constant, Hd is the demag-netizing field, and is the angle between the magnetizationand easy axis direction. Minimizing the energy density gives

sin = H

2Ku/Ms +Hd= H

Hk +Hd(4)

where Hk = 2Ku/Ms is the anisotropy field. SubstitutingEq. (4) into Eq. (2) yields

H = − − ⊥(

H

Hk +Hd

)2

(5)

For soft materials, Hk Hd; therefore,

H = − − ⊥(H

Hd

)2

(6)

It is apparent from the above equation that the simple AMRsensor element exhibits a nonlinear response to the externalfield, which cannot be used as a read sensor for magneticrecording as it is. However, it is not difficult to realize from

x

y

D θI

M

TW

H

easy axis x

y

D θI

M

TW

H

easy axis

Figure 4. Schematic of a single-domain AMR sensor element. H is theexternal field, M is the saturation magnetization, and I is the current.

Proof's Only


Eq. (6) that the sensor can be made linear if an additionalfield which is much larger than that of the external fieldis added to it to make the total effective external field asH ′ = H +HB with HB H . In the case of Hd H , whichis true for magnetic recording, Eq. (6) becomes

H ≈ − − ⊥(HBHd

)2

− − ⊥2HHBH 2d

(7)

It shows that now the sensor responds linearly to the exter-nal field. HB is the so-called bias field or, more precisely,the traverse bias field because it is perpendicular to the easyaxis direction of the sensor element. In actual sensor design,the strength of the bias field is chosen such that the mag-netization direction at zero field is about 45 away from theeasy axis so as to maximize the sensitivity. It is obvious fromEq. (7) that the smaller the demagnetizing field, the largerthe sensitivity.

There are many different ways to form a traverse bias.Among them, the most successful was the soft adjacent layer(SAL) bias scheme, in which a soft ferromagnetic layer islaminated with the sensing layer via a thin insulating spacer,as shown schematically in Figure 5 [55–58]. As the SAL isnormally chosen such that most of the current flows throughthe sensing layer, the magnetic field induced by the sensingcurrent magnetizes and saturates the SAL into one direc-tion (pointing upward in Fig. 5). The fringe field thus gen-erated, in turn, provides a traverse bias to the sensing layeritself. The SAL scheme offers several advantages, such asadjustable bias field, relatively uniform bias field distribu-tion, and reduced demagnetizing field. Although it also hasdrawbacks such as the current shunting effect, it so far hasremained the most successful engineering design. In actualsensors, in addition to the traverse bias, one also needs alongitudinal bias to stabilize the domain structure so as toreduce the Baukhausen noise caused by the domain-wallmotion [55 56].

As we mentioned above, the AMR sensor, intrinsically, isnot a linear sensor. In addition to the nonlinearity issue, italso suffers drawbacks such as thermal asperity [59] and sidereading asymmetry [60 61]. Perhaps the most fatal short-coming of the AMR sensor is that it is difficult to scale it

SAL M

R

_ _ _ _ _ _ _ _ _ _

Insu

lato

r

M

Hbias

θ

D

d

WSAL

MR

_ _ _ _ _ _ _ _ _ _

Insu

lato

r

M

Hbias

θ

D

d

WII

Figure 5. Schematic of an AMR sensor using an SAL bias. The sensorconsists of an MR element as the active layer and a soft adjacent layer(SAL) for traverse bias. The MR element and the SAL are separatedby an insulating spacer.

down in thickness so as to meet the requirement of shrink-age in bit length and reduction in Mrt of the media [21].These intrinsic characteristics of the AMR plus the tremen-dous progress made in GMR in the early 1990s have deter-mined the short lifetime of the AMR sensor in the history ofhard disk drives. It was gradually replaced by the spin-valvesensor, which was first introduced into disk drives by IBMin 1997.

3.3. Giant Magnetoresistance

In the second half of the 1970s, researchers at IBM devel-oped a technique which allows growing ultrathin films withextraordinary accuracy in thickness control. It was basedon the vacuum evaporation technique, but with a severalorders of magnitude lower base pressure as compared to thenormal high-vacuum evaporator. At this base pressure andappropriate partial pressures of the source materials, it ispossible to have a mean-free path of the evaporated materi-als that is larger than the distance between the source mate-rial and the substrate for most of the molecules or atoms.In other words, the evaporated molecules or atoms formbeams, and impinge directly on the substrate surface to ini-tiate the growth. Therefore, the technique is generally calledmolecular beam epitaxy (MBE) [62]. The word “expitaxy”was used because MBE had been mainly employed to fab-ricate semiconductor materials which are normally single-crystalline materials grown epitaxially on the substrates. Themost attractive point of MBE is that it not only allows thegrowth of ultrathin single-layer film, but also makes it pos-sible to grow layered structures of different materials. Thisopened an important field in materials science called arti-ficial structures. In particular, when the layers are a fewatomic layers thick, they form a so-called superlattice struc-ture [63]. It also made it possible to create quantum-wellstructures in which the electrons of one type of materialare confined by the potential steps formed between thismaterial and the other types of materials at both sides.This led to tremendous advances in semiconductor devices.Being inspired by the work carried out by the semiconductorcommunity, researchers in the surface-science communitystarted to use the same technique to study surface mag-netism and magnetic interactions across an ultrathin anti-ferromagnetic or nonmagnetic spacer [64]. These researchactivities led to the discovery of giant magnetoresistance inFe/Cr superlattices [22]. As shown in Figure 6, the resis-tance of such a superlattice structure is high at zero field,decreases when a magnetic field is applied in both direc-tions along the sample surface, and finally saturates at afield Hs of about 2 T. The MR ratio of superlattices with an[Fe(3 nm)/Cr(0.9 nm)] × 60 structure was measured to beabout 45% at 4.2 K, which is much larger than the AMReffect [22]. It was proved soon afterward that the resistanceof the superlattice structure depends on the relative orien-tation of the magnetization of the adjacent magnetic layers.At a Cr thickness of 0.9 nm, the magnetizations of adjacentFe layers are antiferromagnetically coupled at zero field, butare realigned into the same direction when the external fieldis sufficiently high to overcome the exchange-coupling field.

The mechanism of GMR can be understood using thesimple two-current model [65–67, 284, 285]. In this model,

Proof's Only


R/R (H=0)

(Fe 30 Å/Cr 18 Å)30

(Fe 30 Å/Cr 12 Å)35

(Fe 30 Å/Cr 9 Å)60

Magnetic field (kG)

Hs

Hs

Hs0.8

0.7

0.6

0.5

- 40 - 30 - 20 - 10 0 10 20 30 40

1 -

Figure 6. Magnetoresistance of Fe/Cr superlattices. Both the currentand the applied field are along the same [110] axis in the film plane.Reprinted with permission from [22], M. N. Baibich et al., Phys. Rev.Lett. 61, 2472 (1988). © 1988, American Institute of Physics.

the electrical conduction of ferromagnetic materials is con-sidered carried out by two independent channels of spin-upand spin-down electrons. Due to the different density ofstates distribution of spin-up and spin-down electrons nearthe Fermi level, the mean-free path for spin-up electronsis normally larger than that of spin-down electrons due tothe larger scattering probability of s electrons to spin-downd-electron states (when the density of states of spin-downelectrons is higher than that of the density of states of spin-up electrons). When all of the magnetic layers are ferro-magnetically exchange coupled, the spin-up electrons willexperience less scattering when they cross the nonmagneticlayer entering other magnetic layers within the spin-diffusionlength, which is normally larger than the mean-free path.The resistivity of the spin-up electrons in this case can beconsidered as a constant over the film stack, and is denotedby ↑↑. Similarly, the resistivity of the spin-down electronscan also be considered as a constant, and is denoted by ↑↓.As in the simple two-current model, we ignore the spin-flipscattering, that is, no mixing of the two conduction chan-nels; the total resistivity is given by P = ↑↑↑↓/↑↑ +↑↓.However, the situation changes when the magnetic layersare antiferromagnetically coupled. This is because both thespin-up and spin-down electrons will experience frequentscattering when they cross the multiple layers. The two con-duction channels will have the same resistivity, which is givenby ↑↑ +↑↓/2, leading to a total resistivity of AP = ↑↑ +↑↓/4. The MR ratio is thus given by

MR = P − APAP

= −(↑↑ − ↑↓↑↑ + ↑↓

)2

(8)

The negative sign indicates that the resistivity at saturationstate is lower than that at the zero-field state. A quantita-tive treatment of the GMR effect is possible using eitherthe Boltzmann transport equation [68–70] or the quantumKubo formula [71–73] using either the simple parabolicband structure or the more realistic band structures [74 75].These models can predict very well the dependence of GMR

on the material and structural parameters of the multilayers.More details can be found in other review articles [76 77].For a magnetic/nonmagnetic multilayer with the film planeparallel to the xy plane, the linearized Boltzmann equationbecomes [68–70]

g↑↓z vz

+ g↑↓z vvz

↑↓ = eE

mvz

fovvx

(9)

where

g↑↓z v = f ↑↓z v− f0vis the deviation of the electron distribution function fromthe equilibrium Fermi–Dirac distribution f0v. Here, ↑↓

represents the relaxation times for spin-up (spin-down) elec-trons, e is the elementary charge of electrons, m is the elec-tron mass, v is the velocity, and E is the electrical field. Thegeneral solution of Eq. (9) is

g↑↓± z v

= eE↑↓

m

f0vvx

[1+ F ↑↓

± v exp( ∓z↑↓vz

)](10)

where +− is for vz > 0 vz < 0. F ↑↓± v can be obtained

by using the boundary conditions for g↑↓± z v at both theinterfaces and top and bottom surfaces. After g↑↓± z v isobtained, the total current per unit length in the y direction(assuming the field along the x direction) can be obtainedby first integrating −evxg↑↓± z v in v space, followed byintegrating with respect to z. The MR ratio can thus beobtained from the difference in conductivities between theparallel and antiparallel alignment of the magnetic layers.

3.4. Interlayer Coupling

The underlying mechanism of GMR is the long-range mag-netic exchange coupling between transition metals acrossa nonmagnetic spacer, which was first observed in theFe/Cr/Fe(001) system [78]. Although the GMR effect wasoriginally observed in Fe/Cr superlattices with a fixedCr layer thickness, subsequent studies have revealed theoscillatory behavior of interlayer coupling in this systemand many others [79–81]. More importantly, these resultswere obtained not only from high-crystalline-quality sam-ples grown by MBE, but also from polycrystalline sam-ples deposited by sputtering. This has greatly acceleratedthe research in this field because of the wide availabil-ity of sputtering systems. As one of the typical results,Figure 7 shows the oscillatory behavior of magnetoresis-tance of Co/Cu superlattices observed by Parkin et al. on dcmagnetron-sputtered samples [79]. Four well-defined oscil-lations with a period of about 1 nm are seen at both 4.2 and300 K. This system later became the most important systemfor metal-based spintronic devices.

It was soon realized that the oscillation in GMR is ageneral phenomenon in ultrathin magnetic and nonmag-netic multiple layers, and its origin comes from the peri-odical switching from ferromagnetic to antiferromagneticalignment and vice versa of the adjacent ferromagnetic lay-ers when the thickness of the nonmagnetic layer varies every

Proof's Only


40

30

20

10

0

00 10 20 30 40 50 80 120 160

60

40

20∆R/R

(%)

∆R/R

(%)

Si/Fe(45Å)/[Co(10Å)/Cu(tCu)]N

tCu (Å)

300K

4.2K

Figure 7. Dependence of saturation traverse magnetoresistance on Cuspacer layer thickness for a family of related superlattice structures ofthe form Si/Fe(4.5 nm)/[Co(1 nm)/Cu(tCu]N . An additional Cu layerwas deposited on each film structure such that the uppermost Cu layerwas about 5.5 nm thick. The number of bilayers in the superlattice Nis 16 for tCu below 5.5 nm (solid and empty circles) and 8 for tCu above5.5 nm (solid and empty circles). Adapted with permission from [80],S. S. P. Parkin et al., Phys. Rev. Lett. 66, 2152 (1991). © 1991, AmericanInstitute of Physics.

few atomic layers [82]. It was found that both the GMR andthe saturation magnetic field of Fe/Cr multiple layers oscil-late at a period of about 1.5 nm. The latter suggests thatthe relative angle of the magnetization direction of the twoferromagnetic layers oscillates between 0 and 180 becauseit is easier to saturate the two layers when they are in par-allel alignment, and difficult when they are in antiparallelalignment. This was soon confirmed by the neutron [83] andBrillouin scattering experiments [84]. Furthermore, detailedinvestigations have revealed the existence of two couplingperiods [85–90].

Various models have been proposed to account for theoscillatory behavior of exchange coupling of the mag-netic/nonmagnetic multiple layers, which include primarilythe quantum-well model [91–97] and the Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction model [98–101]. Bothmodels have been able to describe the origin of the exchangecoupling, in particular, the oscillations in the sign andstrength of the coupling. In what follows, we explain brieflythe RKKY model by closely following the treatment byBruno and Chappert [100 101] and Fert and Bruno [102].The interaction between two ferromagnetic layers acrossa metallic spacer has its basis in the interaction betweentwo magnetic impurities in a host metallic material. Accord-ing to the RKKY model, the effective exchange interactionbetween two spins Si and Sj is given by

Hij = J Ri − Rj Si · Sj (11)

where the exchange integral is

J Ri − Rj

= −12

(A

V0

)2 V0

2,3

∫d3 q. q exp/i q · Ri − Rj0 (12)

Here, V0 is the atomic volume and

. q = 212B

V0

2,3

∫d3kf k− f k+q+ G

k+q+ G − k(13)

is the susceptibility of the host materials. The integrationsover q and k are to be performed within the first Brillouinzone, and G is a vector such that k + q + G belongs tothe first Brillouin zone. In the case of free electrons at zerotemperature, the Fermi surface is a spherical surface with aradius defined by the Fermi wavevector kF . The probabil-ity distribution function f k drops to zero sharply at theFermi surface, resulting in a logarithmic singularity at q =2kF for . q. The Fourier transforms of . q give oscilla-tions to J with a period of 3 = 4F /2, and 4F is the Fermiwavelength.

For interactions between two ferromagnetic layers (here-after, we refer them to as FM1 and FM2, the interactionintegral may be obtained by summing over all of the pairsof spins belonging to FM1 and FM2. For the sake of simplic-ity, we assume that both layers are one atomic layer thick.Without the loss of generality, we first look at the interac-tion between a single spin located at position O of FM1 andall of the spins belonging to FM2, which is given by

I1O 2 =d

V0S2

∑j∈FM2

J RO − Rj (14)

The above summation can be further simplified by treatingFM2 as a thin layer with a uniform spin distribution, that is,

∑F 2

→ d

V0

∫F2

d2 R (15)

Here, R is the in-plane component of RO− Rj. The inter-action integral is then given by

I1O2z ≈(A

V0

)2

S2m

16,2

d2

z2sin2kF z for z →∝ (16)

which is also an oscillation function with a period of 3 =4F /2. This is too short to explain the long oscillation peri-ods observed experimentally. Subsequent studies pointedout that the contradiction comes from the invalid assump-tion that the thin FM2 layer has a continuous and uniformspin distribution. By taking into account the discreteness ofthe spin distribution in the FM2 plane, the interaction inte-gral is given by

I1O2z = −12

(A

V0

)2

S2d

2,3

∫ +∝

−∝dqz

× expiqzz∫d2 q. q qz

∑R∈FM2

expi q · R (17)

The last term suggests that q cannot take any arbitraryvalue but the in-plane reciprocal lattice vector G; other-wise, its value will be zero. This gives an effective wavevector/2kF 2 −G2

01/2, which is responsible for the multiperiodic-

ity due to differentG in different directions. Figure 8 showsthe cross section of the Fermi surface for an fcc (001) spacer.The first Brillouin zone (FBZ) and the auxiliary prismatic

Proof's Only


(111)

(111) (111)

(111)

(113)

(111) (111) (113)

(113)

(113)

(002)(000)

(002)(000)

(a)

(b)

Cu

z

Figure 8. Cross section of the Fermi surface for an fcc (001) metal andthe corresponding vectors giving rise to oscillatory interlayer couplingin both the (001) and (111) directions (indicated by the horizontal andoblique arrows, respectively). (a) Free-electron model. (b) Cu Fermisurface. Adapted with permission from [100], P. Bruno and C. Chappert,Phys. Rev. Lett. 67, 2592 (1991). © 1991, American Institute of Physics.

zone are represented by the solid and dashed contours. Thewavevectors giving rise to oscillatory interlayer coupling areindicated by the horizontal bold arrows: (a) free-electronapproximation, and (b) Cu Fermi surfaces. Also shown arethe vectors giving rise to oscillations in the (111) direction(oblique arrows).

On the other hand, the long oscillation period can alsobe understood based on the argument that the spacer thick-ness does not vary continuously in Eq. (16); rather, it onlytakes discrete values z= N +1a with N an integer number[99 103 104]. Because of this discrete sampling or aliasingeffect, one obtains an effective period

3 = ,

kF − n,/a(18)

where n is an integer chosen such that 3 > 2a, with abeing the atomic spacing of the spacer in the thickness direc-tion. This gives a period which is much larger than 4F /2(see Fig. 9).

3.5. Spin Valve

The research on read sensors using the GMR effect com-menced soon after the GMR effect was reported. This isbecause it gives a much higher MR ratio as compared toits AMR counterpart. However, the original GMR struc-ture consisting of Fe/Cr, Co/Cu, or other types of magneticmultiple layers could not be applied to read sensors in theoriginal form because of their large saturation fields, andalso the nonlinear response near the zero-field point. Themost straightforward way to reduce the saturation field is

J(L)

L/d

λ—

8

0

9 10 11 12 13 14 15

d1.28=

Figure 9. Full curve: exchange-coupling strength of two ferromagneticlayers across a monovalent fcc (100) metal (arbitrary units) calculatedusing the continuum version of the RKKY model. Broken curve: theactual coupling strength with the experimentally measured periodicityfor L = Nd. Here, L is the thickness of the spacer, d is the atomiclayer spacing in the thickness direction, and N is an integer number.Adapted with permission from [99], R. Coehoorn, Phys. Rev. B 44, 9331(1991). © 1991, American Institute of Physics.

to reduce the exchange coupling between the ferromagneticlayers through increasing the thickness of the spacer. How-ever, a completely decoupled or weakly coupled multilayerstructure does not function properly as a sensor becauseof the possible inconsistent movement of all of the mag-netic layers under an external field. One of the possibleways to overcome the drawback of the decoupled GMRstack is to use two decoupled layers with one of them muchsofter than the other, so that the former will respond toa small field, while the latter will only change its magneticstate when it is subjected to a large field [105 106]. Thiskind of structure, in principle, can form a good sensor, pro-vided that the hard layer is not only truly hard, but alsohas the same crystalline structure as that of the soft layer.Unfortunately, this kind of material is still not available atpresent. Therefore, most of the experiments on decoupledbilayers have been performed on NiFe/Cu/Co or Co/Cu/Cosystems. In the latter case, the two Co layers may have dif-ferent coercivities, and their values are dependent on theindividual layers, thicknesses. Although a relatively largemagnetoresistance with a small saturation field has beenobtained in these kinds of structures, they were still unsuit-able for data storage applications due to the instability ofthe hard layer. However, this type of structure has servedas a good object for studying spin-dependent transport inmagnetic/nonmagnetic multilayers [107 108]. They can alsobe used for MRAMs [109]. The magnetic stiffness of thehard layer may be improved by using an antiferromagnet-ically coupled hard/Ru/soft triple-layer structure to replacethe single-layer structure [110 111].

To address the issues involving noncoupled magnetic mul-tilayers, IBM invented a device called the spin valve in 1991[24 112 113]. A typical spin valve in its simplest form con-sists of two ferromagnetic layers separated by a nonmagneticspacer and an antiferromagnetic (AFM) layer in contactwith one of the ferromagnetic layers (see Fig. 10). The thick-ness of the spacer is chosen such that there is little exchangecoupling between the two FM layers. The magnetization ofone of the FM layers which is in direct contact with theAFM layer is “pinned” by the latter, and thus this FM layeris commonly called a pinned layer. On the other hand, themagnetization of the other FM layer is free to rotate torespond to an external field, and thus it is called a free layer.

Proof's Only


hard softAFM Cu

PinnedLayer

PinningLayer

Spacer FreeLayer

MPL

MFL

hard softAFM Cu

PinnedLayer

PinningLayer

Spacer FreeLayer

MPL

MFL

Figure 10. Schematic of a simple spin-valve structure.

The typical material for the FM layer is NiFe or CoFe, whilethat for the spacer is generally copper. A thin layer of Coor CoFe with high Co composition is often added at the FMand Cu interface so as to increase the MR ratio due to itshigh polarization ratio and less interdiffusion at the inter-face with Cu [114 115]. The choice of AFM is an issue ofhigh complexity due to many practical considerations, whichwill be discussed shortly.

When being used as a read sensor [116], a constant cur-rent is applied to the sensor, and its voltage change causedby the fringe-field variation from the media is detected asthe readout signal. In order to increase the linear density,usually two shielding layers are used to “shut off” the influ-ence of neighboring bits (see Fig. 11). Depending on howthe current is applied, there are two different types of sen-sors, that is, the current-in-plane and current-perpendicular-to-plane sensors, as discussed earlier. As the name suggests,in the former case, the current flows in the plane of the sen-sor stack (see Fig. 11), while in the latter case, the currentflows perpendicular to the plane. The small resistance ofthe metallic multilayers makes it quite challenging to mea-sure the magnetoresistance of the CPP structure. Severalapproaches have been taken to address this issue, includ-ing the use of superconducting electrodes and reductionof the lateral size using lithography [117 118] or selectivegrowth in the pores of nanoporous membranes [119–121].The results obtained showed that the CPP MR ratio ismuch larger than the CIP MR ratio. Alternatively, the GMRcan also be measured with the current-at-an-angle (CAP)through growing the GMR structure on grooved substrates[122]. Since, so far, CIP sensors dominate in practical appli-cations, we discuss the CIP sensors first.

The M–H and MR–H curves of a typical spin valve areshown schematically in Figure 12. Due to the small thick-ness of the spacer, there always exists an interlayer cou-pling between the free and pinned layers. The nature of

Sh

ield

Sh

ield

senso

r

I

I

Figure 11. Spin valve used as a read sensor in CIP mode.

Hin

Hex

Hc1PL

Hc2PL

Hc1FL

Hc2FL

H

M

Hin

Hex

Hc1PL

Hc2PL

Hc1FL

Hc2FL

H

M

Hin

Hex

Hc1PL

Hc2PL

Hc1FL

Hc2FL

H

M

Hin

Hex

Hc1PL

Hc2PL

Hc1FL

Hc2FL

H

Hin

Hex

Hc1PLHc1PL

Hc2PLHc2PL

Hc1FLHc1FL

Hc2FLHc2FL

H

M

MR

H

(a) (b)

MR

H

MR

H

MR

H

Figure 12. Schematics of M–H (a) and MR–H (b) curves of a typi-cal spin valve (Hex : exchange-bias field; Hin: interlayer coupling fieldbetween free and pinned layer; (HFL

c1 −HFLc2 ): coercivity of free layer;

(HPLc1 −HPL

c2 ): coercivity of pinned layer).

the interlayer coupling can either be ferromagnetic or anti-ferromagnetic, depending on whether the orange peel orRKKY coupling dominates. We will come back to this topicshortly. For the particular spin valve whose characteristicsare shown schematically in Figure 12, the free and pinnedlayers are ferromagnetically coupled. Therefore, the magne-tization of the spin valve is at maximum, while the resistanceis at minimum at zero field. When the applied field increasesin the opposite direction of the pinning field (in this case, inthe positive direction), the magnetization of the free layerwas first reversed, resulting in an antiparallel alignment ofthe free and pinned layers. This corresponds to the high-resistance state. When the applied is further increased, itwill eventually reverse the magnetization of the pinned layer.This will realign the magnetization of the pinned layer withthat of the free layer, recovering the low-resistance state.Decreasing of the applied field after the saturation in thepositive direction will cause the magnetization of the pinnedlayer to be reversed first, followed by the reversal of the freelayer, and finally, the two magnetic layers realign with eachother again at negative fields. In general, a large exchangebias field and a small interlayer-coupling field are desirablefor read-head applications.

The performance of a read sensor is normally charac-terized by the size of the MR effect. It can be either theMR ratio or the sheet resistance change :RS . The formeris given by [24, 123–125]

:RSRS

=(:RSRS

)0

(1− cosF − P

2

)(19)

where RS is the sheet resistance and :RS the change in sheetresistance. :RS/RS0 is the maximum MR ratio when themagnetizations of free and pinned layers are switched fromparallel alignment to antiparallel alignment. F and P arethe angles between the parallel direction and the magneti-zation direction of the free and pinned layers, respectively.It is apparent from this equation that a linear responsecan be obtained through setting P to 90. This is becausesin F is proportional to the external field, as we discussedin Section 3.2 for AMR sensors. In addition to its linearresponse, the spin valve also exhibits a much higher MRratio as compared to the AMR sensors. However, eventhough that is the case, there has been and there is still astrong demand for the improvement of the spin-valve sen-sors. Figure 13 shows the estimated track width and required

Proof's Only


0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

35

30

25

20

15

10

5

0200 400 600 800 10000

Rea

dtr

ack

wid

th(µ

m)

Required

sensorsensitivity

(mV

/µm)

Read track width= 0.8 × track pitch

Read track width= 0.6 × track pitch

Areal density (Gbits/in2)

Aspect ratio = 4

Figure 13. Estimated read track width and required sensor sensitivitiesat different areal densities by assuming an aspect ratio of 4:1. The solidand dashed curves are obtained by assuming the read track width tobe 80 and 60% of the track pitch, respectively. The sensitivity require-ment is obtained by assuming that the sensor would generate an outputvoltage of 1 mV.

sensor sensitivity at an aspect ratio of 4:1 for different arealdensities by assuming that the sensor has to generate anoutput voltage of 1 mV. The solid and dashed curves areobtained by assuming a read track width of 80 and 60%of the track pitch, respectively. From this figure, one cansee that a threefold increase of the sensor sensitivity isrequired for increasing the areal density from 100 Gbits/in2

to 1 Tbits/in2.From Eq. (19), one can see that the MR ratio of a spin

valve is affected by two factors of different origin. Theprefactor of the right-hand-side term of Eq. (19), that is,:RS/RS0, is mainly determined by the spin-dependentelectronic transport properties of the spin valve, while therest is more or less determined by its magnetic properties.As summarized in Figure 14, the improvement in spin-valvedesign has proceeded from both the magnetic and electronicaspects. In a certain sense, the original spin valve proposedby IBM can be regarded as the first step in the improve-ment of the pseudospin valve that still remains the simplest,yet the most attractive design should the materials problembe solved in the future. In what follows, we first discuss themagnetic aspect of spin valves, followed by a discussion onthe electronic aspect. We will refer back to Figure 14 againwhen necessary.

4. MAGNETIC ASPECTSOF SPIN VALVES

Like a transistor which needs a proper bias to work properly,the spin valve also needs a proper magnetic bias so as toachieve a linear response with minimized asymmetry, if any[13 15 55 123]. The main tasks of magnetic design of spinvalves are: (1) to set the magnetization of the pinned layer asrigid as possible in the vertical direction (see Fig. 10), (2) tomake the free layer as soft as possible, and to align its initialmagnetization direction parallel to the media surface, (3) to

PL FLAFM Cu PL1 FLAFM CuPL2Ru

SLHL Ru FLCu I-HL PL FLCu

Improvement of magnetic properties

PL FLAFM Cu HCL PL FLAFM Cu HR

PL FLI-AFM Cu HR PL FLAFM CuPLNOL NOL

Improvement of electronic properties

SLHL Cu

PL1 PL2AFM NOLPL2Ru Cu CuFL NOL

(d) (e)

(f) (g)

(h)

(j)

(i)

(k)

(a)

(b)

(c)

Figure 14. Evolution of spin valves. (a) Pseudospin valve. (b) Advancedsingle spin valve. (c) Advanced dual spin valve. (b) and (c) are theresults of continuous improvements in both the magnetic (d)–(g) andelectronic (h)–(k) designs of the pseudospin valves. (d) Original spin-valve invented by IBM. (e) Synthetic spin valve. (f) Synthetic spinvalve using a hard magnet. (g) Exchange spring spin valve using aninsulating hard magnet. (h) Spin-filter spin valve using a back layeror a high-conductance layer. (i) Specular spin valve. (j) Specular spinvalve using an insulation AFM. (k) Specular spin valve using nano-oxides. The acronyms used are: HL—hard layer, SL—soft layer, PL—pinned layer, FL—free layer, PL1 (2)—first (second) portion of thepinned layer, I-HL—insulating hard layer, HCL—high-conductancelayer, HR—high-specularity reflective layer, NOL—nanooxide layer,AFM—antiferromagnetic layer, I-AFM—insulating antiferromagneticlayer.

eliminate Baukhausen noise induced by domain wall move-ment, and (4) to make the sensor thermally stable at ele-vated temperature under normal operation conditions andcorrosion resistive. Of course, in practical cases, one alsohas to consider the ease of manufacturing and reliability ofthe devices. To achieve all of these goals, one needs to set aproper traverse and longitudinal bias for the spin valve usingappropriate materials. The former is employed to “pin” themagnetization of the pinned layer as rigid as possible, whilethe latter’s role is to maintain the single-domain status ofthe free layer, and at the same time, without sacrificing itssoftness.

4.1. Traverse Bias

4.1.1. Exchange BiasTraverse bias is achieved through the use of exchange bias(EB) formed at FM/AFM interfaces [Fig. 10 and Fig. 14(d)],which was discovered about half a century ago [126]. Theexchange bias at the FM/AFM interface is usually estab-lished through a magnetic annealing and cooling process, inwhich the sample is first heated up to above the Néel tem-perature of the AFM, and then is cooled down to room tem-perature in the presence of a magnetic field with a strengthof one to a few tesla. The exchange-biased FM/AFM bilayeris characterized by a shift of the hysteresis loop in the oppo-site direction of the exchange-bias field direction and anincrease of the coercivity of the FM layer, as shown schemat-ically in Figure 15(a).

The exchange bias in AFM/FM bilayers is a very com-plex issue. Despite intensive studies, the phenomenon itself

Proof's Only


M

H

HE

HC

FM

AFM/FM

(a)

0

50

100

150

200

250

300

350

400

450

0 6 10IrMn thickness (nm)

HE

,HC

(Oe)

Hc

HE

(b)

42 8

Figure 15. (a) Schematic hysteresis curves of a single FM layer (dottedline) and an FM/AFM exchange-coupled bilayer (solid line). (b) Depen-dence of exchange-bias field on the thickness of AFM material for anNiFe (3 nm)/IrMn bilayer system.

is still not fully understood [127–130]. A number of theoret-ical models have been proposed to account for the experi-mental data, which include, but are not limited to: (1) thecoherent rotation model [131], (2) the Néel model [132],(3) the random interface model, (4) the AFM domain wallmodel [133 134], (5) the interface spin canting model [135],(6) the random and compensated interface model, and soon [136–139]. Obviously, the coherent rotation model is thesimplest one which assumes that: (1) both the FM and AFMlayers are single-crystalline materials, (2) the AFM layerhas a rigid spin configuration which is not affected by anystrength of an external field, and its interfacial layer is fullyuncompensated, and (3) the magnetization reversal of theFM layer is accomplished through coherent magnetizationrotation. Under these assumptions, the exchange field isgiven by

HEX = 2J SFM · SAFM

a2MFMtFM(20)

where J is the exchange parameter, SFM SAFM is the spinof the interfacial atom of the FM (AFM) layer, MFMtFMis the magnetic moment (thickness) of the FM layer, anda is the cubic lattice constant. Although the above equa-tion does agree with the experimental observation that HEXdecreases with tFM, it yields a value for HEX that is by ordersof magnitude larger than that of the experimental data. Thesimple model fails to explain the asymmetry observed in thehysteresis either. This has prompted the proposal of variousimproved models. Among them, two models have been quitesuccessful in explaining the observed value of HEX. Mauriet al. proposed a model that is able to explain the observedexchange bias without the need to remove the condition ofstrong interfacial coupling at the FM/AFM interface [134].This is made possible by allowing the formation of paralleldomain walls, either in the AFM layer or in the FM layer,wherever the energy is lower. By assuming that the domainwall forms at the AFM side of the interface, they obtainedthe following expression for the exchange field:

HEX = 2√AAFKAF

MFMtFM(21)

where AAFKAF is the exchange stiffness (magnetocrys-talline anisotropy energy constant) of the AFM layer.

Malozemoff argued that any realistic model should not bebased on the assumption of an atomically perfect uncom-pensated AFM surface, and proposed a random interfacemodel [133]. It was shown that, for a real interface withfinite roughness, it becomes energetically favorable for theantiferromagnet to break up into domains with perpendic-ular domain walls. This model yields an exchange-couplingfield of

HEX = 2z√AAFKAF

,2MFMtFM(22)

where z is the number of order unity. Therefore, theexchange field predicted by both the AFM domain wallmodel and the random interface model is of the same order;this is because both are determined by the characteristicdomain wall energy.

The parallel domain wall model requires the existence ofa spiral spin structure in the AFM layer [140], which hasbeen observed recently in both the conventional FM/AFMbilayers [141] and artificial FM/AFM bilayers [142]. The for-mation of an AFM domain wall is found to be affected bythe magnetization of the FM layer. The exchange couplingbetween the FM and AFM layers occurs on a domain-by-domain basis [143]. Due to the dominant role of the AFMdomain walls in exchange coupling, the exchange bias is gen-erally sensitive to the defects in the AFM layer.

4.1.2. AFM MaterialsAbove is a far too brief introduction of the exchange-coupling mechanism at the FM/AFM interfaces. An in-depthdiscussion of this topic can be found in several excellentreview papers [127–130]. In what follows, we turn to differ-ent material systems that have been developed for practicalapplications in spin valves [144 145]. As discussed earlier,the key to the proper operation of a spin valve is to “pin”the magnetization of the reference layer in a direction thatis perpendicular to the media surface, which forms a 90

angle with respect to the easy axis of the free layer at zerofield. The former is achieved using the exchange bias at theAF/AFM interface. To realize a reliable and linear operationof the spin valve, the following issues have to be consideredcarefully when choosing the AFM materials.

1. Size of the exchange bias field: As a subtle rotation ofthe magnetization of the pinned layer will cause asym-metry and degradation of the output signal, the pin-ning field must be sufficiently high (>300 Oe) so asto make the reference layer “rigid” under a moderateexternal field.

2. Thermal stability: The exchange-bias field usuallydecreases with temperature. In what way and how fastit decreases, however, depends not only on the intrin-sic properties of the materials, but also their structuralproperties, thickness, and techniques that are used toprepare the materials. The temperature at which theexchange-bias field vanishes is called the blocking tem-perature. A high blocking temperature (>250 C) isrequired to ensure that the sensor functions properlynot only at room temperature, but also at elevated

Proof's Only


temperatures. This is important because the temper-ature of the active region of the sensor may rise to60–120 C during normal operation.

3. Large resistivity of the AFM layer: As the AFM layeritself does not contribute to the MR effect, a high-resistivity material is desirable for suppressing thecurrent-shunting effect. In this sense, the ideal AFMmaterial should be an insulator. Unfortunately, mostof the oxide-based AFM materials like NiO can hardlysatisfy other requirements, and thus are unsuitable asAFM materials for practical device applications.

4. Small critical thickness: The exchange-bias field isfound to be quite insensitive to the thickness of theAFM when it is sufficiently thick. However, it starts todrop rapidly when the thickness decreases to a certainvalue, which is largely determined by the properties ofthe AFM material. As an example, Figure 15(b) showsthe results for an Ni80Fe20(3 nm)/IrMn bilayer system.The onset thickness for this process is called the criticalthickness. Considering the temperature dependence ofthe exchange field, the thickness of the AFM must bechosen well beyond the critical thickness in practicaldevices. However, a large AFM thickness will result ina large total thickness of the sensor, which is undesir-able for sensors for high-areal-density hard disk drives.Therefore, AFM materials with smaller critical thick-ness are desirable for spin-valve applications.

5. Good corrosion resistance: AFM materials withpoor corrosion resistance are unsuitable for practicalapplications.

6. Low-temperature process: The thermal annealing pro-cess may affect the performance of spin valves if

Table 1. Properties of widely studied AFM materials.

Hex (Oe) Required Resistivity CorrosionMaterials (NiFe 4 nma) thickness (nm) TB

C) (1< · cm) resistance Annealing

FeMn 420 7b 150b c , 165e, 150, Poor Nob

175f , 130b

170j

IrMn 200 7d , 190/230d , 200, Gooda k Nok

7.5e, ∼250e f , 325d Moderatee

10l 130l

NiMn >390 >25b , 450b>425d , 175b , Moderate Requiredb

35d , 430g , 210d Goode k

30e 400f j

PtMn 500 30 380p , ∼200 Good Required400f ,310j

PdPtMn 280 25dm 300m 350d f 185d Good RequiredCrMnPt 220 30a o , 380a o , 310–345a, Good No

35d 300d 300–350o

360d

NiO 200 35b , 200b h i 190e Insulator, Good Nob

30e, ≥108b Excellente

50h

A–Fe2O3 40–75 100 200–250, Insulator Good No250q ,

280–300r

References: a[144], b[146], c[147], d[145], e[148], f [149], g [150], h[151], i[152], j [153], k[154], l[155], m[156], o[157], p[158], q [159], r [160].

the annealing temperature is too high. Therefore, alow-temperature process is preferred.

The AFM materials investigated so far include: (1) Mn-based alloys, (2) Cr-based alloys, and (3) Fe, Co, or Ni-based oxides [144 145]. Among them, Mn-based alloys arethe most widely studied, and several of them have alreadybeen applied to practical spin-valve sensors. The Mn-basedalloys can be roughly divided into two groups. One groupincludes FeMn, IrMn, RhMn, RuMn, and the other includesNiMn, PtMn, PdMn, and some of the ternary alloys of theseelements. The crystalline structure of the first group is fcc,while that of the second group is fct (CuAu–I). FeMn/NiFe(CoFe) is the most widely studied system which exhibits anexchange bias of about 420 Oe for a 4 nm thick NiFe withoutthermal annealing. However, it has a low blocking temper-ature of about 150 C and poor corrosion resistance, andthus is unsuitable for read sensor applications. Another sys-tem that does not require thermal annealing is IrMn/CoFe(or NiFe). It has a better corrosion resistance and smallercritical thickness as compared to FeMn. Both are desir-able for sensor applications. However, its moderate block-ing temperature of 250 C limits its applications in a realdrive environment. Although, in general, a thermal anneal-ing process (at 200–300 C) is required to establish the AFMphase, the (Ni, Pt, Pd) Mn family gives a much higher block-ing temperature (350–450 C), better corrosion resistance,and a higher exchange-bias field (500–800 Oe), and thus isbeing used in real devices. The primary drawback of thisgroup of materials is their large critical thickness, which maylimit their applications in ultrahigh-density recording appli-cations. Table 1 lists the properties of some widely studiedAFM materials [144–160].


4.1.3. Synthetic AFMAs discussed earlier, the exchange-bias field is inversely pro-portional to the thickness of the FM layer. For practicalapplications, however, one needs a certain thickness for thepinned layer so as to optimize the electrical properties ofthe sensor. This affects the sensor in two aspects. One isthe reduction in the exchange-bias field, and the other is themagnetostatic coupling with the free layer. Although onecan use the sense current field to counterbalance the mag-netostatic field, it may sacrifice the stability of the pinnedlayer. A novel structure which can solve both problems is thesynthetic ferrimagnet which consists of two antiferromagnet-ically coupled FM layers via a thin Ru layer [see Fig. 16(a)].The corresponding synthetic antiferromagnet in which oneof the FM layers of the synthetic ferrimagnet is exchangecoupled to an AFM layer can be used as the pinning andpinned layers for spin valves [161–171].

The partial cancellation of the magnetic moments fromboth layers gives a smaller net moment, and thus anincreased exchange-bias field and a reduced magnetostatic-coupling field. In addition to these advantages, it alsoprovides a better thermal stability, although the blockingtemperature itself remains the same. Shown in Figure 16(b)is a schematic drawing of a typical spin valve using the syn-thetic AFM as the pinned layer [also see Fig. 14(e)]. Asthe exchange bias is inversely proportional to the magneticmoment thickness product of the pinned layer, comparedto parallel alignment, the exchange-bias field in antiparallelalignment is enhanced by a factor of [21]

fP−AP = M1st1 +M2st2M1st1 −M2st2

(23)

where M1sM2s and t1t2 are the magnetic moment andthickness of the first (second) pinned layer, respectively. Onthe other hand, the demagnetizing field is proportional tothe magnetic moment thickness product of the pinned layeritself. Therefore, the stability of the pinned layer is effec-tively increased by a factor of

/M1st1 +M2st2/M1st1 −M2st202

Although it is not shown in Figure 16, the synthetic ferrimag-net can also be used as the free layer, which can effectivelyreduce the magnetic thickness of the free layer without sacri-ficing its physical thickness [172]. However, the longitudinalbias may affect the stability of the synthetic layer [173].

FM1RuFM2

AFM PL1 Ru PL2 Cu FL

(a) (b)

Figure 16. Schematic of a synthetic ferrimagnet (a) and spin valve usingthe synthetic ferrimagnet as a pinned layer (b).

4.1.4. Hard/Ru/Soft Synthetic FerrimagnetThe use of AFM as the pinning layer has two drawbacks:(1) all of the useful AFMs reported so far for spin valvesare metals which contribute negatively to the MR due tothe current-shunting effect, and (2) most AFMs have amoderate or low blocking temperature, and some of themalso show poor corrosion resistance. Based on this back-ground, there have been several works in which hard/Ru/softsynthetic ferromagnets have been used as the pinning andpinned layers [see Fig. 14(f)] [110 111 174]. Preliminaryexperimental results showed that these types of spin valveshave a good thermal stability [110 111]. However, it wouldbe quite challenging to improve other properties of the spinvalves due to either the mismatch of the crystalline structureof the hard and soft layer or the insufficient hardness of thehard layer.

4.1.5. Exchange SpringFurther simplification of the synthetic ferrimagnet would bethe hard/soft exchange spring structure [Fig. 14(g)]. Thistype of structure was proposed recently by Carey et al.using cobalt ferrite as the pinning layer [175]. A high MRratio, 12.8%, and a high pinning field, 1500 Oe, have beenobtained without compromising the softness of the freelayer. In this particular case, the pinning layer is an insula-tor which may also help to reduce the shunting current, andthus is desirable for obtaining a high MR ratio.

4.2. Longitudinal Bias

The traverse bias is to set a proper direction for the mag-netization of the pinned layer. For a spin valve to functionproperly, one also needs to set a proper longitudinal biasfor the free layer. This is to both obtain a good linearitywith less asymmetry and to suppress the Baukhausen noise.The former is achieved through setting the magnetizationof the free layer at 90 with that of the pinned layer, par-allel to the media surface. This, in turn, can be achievedthrough first inducing an easy axis in an appropriate direc-tion during deposition of the free layer, and then using theshape anisotropy to stabilize it in the same direction. How-ever, the shape anisotropy alone may become insufficient asthe aspect ratio of the sensor decreases. The following addi-tional magnetic fields need to be taken into account whendesigning the sensor: (1) the fringe field from the pinnedlayer, (2) the RKKY and orange-peel coupling field from thepinned layer, or so-called interlayer coupling field, (3) thecurrent-induced field from highly conductive layers such asthe spacer layer, and (4) other fields such as imaging fieldsfrom different sources [123]. Figure 17 shows schematicallythe flux lines of various field components exerted on the freelayer. It is worth noting that the direction and magnitude ofthe current-induced fields are adjustable through controllingthe sensing current, whereas the various field componentsfrom the pinned layer are fixed once the sensor fabrica-tion is completed. Therefore, in practice, it is of importanceto ensure that magnetic fields from sources other than thesense current be able to be compensated later by the cur-rent field under normal operation conditions. Ideally, all ofthe external fields other than the signal field from the media

Proof's Only


++

++

+

Shied 1 Shied 2

Image 1 Image 2

Fringefield of P L

Currentfield

++

++

+

++

++

+

Field from image 1 Field from image 2

Figure 17. Different kinds of magnetic fields exerted on the free layer.For a spin valve to function properly, it is of importance to balance outall of the components so as to make the net field as close to zero aspossible. Here, PL (FL) stands for pinned layer (free layer).

should be reduced to zero so as to obtain a high sensitiv-ity, good linearity, and null asymmetry for the read sensor.However, this also means that the sensor is too susceptibleto external disturbances. This will induce noises or base-line popping and a shift in the readout signal, in particular,the domain formation and movement-induced Baukhausennoise. The latter is an issue of high complexity because itdepends on many factors, such as the material and shapeof the free layer and the process to form it and the effectof other layers. Therefore, as in the case of AMR sen-sors, a longitudinal bias of an appropriate strength is nor-mally used to suppress the multidomain formation in thefree layer of spin-valve sensors [55 116 176 177 178 179].Most of the longitudinal biasing techniques for spin-valvesensors stems from the earlier work on AMR sensors, andmay be divided into two groups. The first group is based onexchange bias between a ferromagnet and an antiferromag-net [see Fig. 18(a)–(d)], and the second group is based onthe magnetostatic interaction or exchange coupling betweena ferromagnetic soft film and a permanent or hard magnet[see Fig. 18(e)–(h)].

4.2.1. Permanent Magnet-Based BiasContiguous Junction So far, the most widely studiedbias scheme which is also employed in real disk drives is thecontiguous (or abutted) junction using a permanent mag-net (typically, a CoCrPt alloy), as shown schematically inFigure 18(e) [180–182]. The key to forming a proper biasin this scheme lies not only in the selection of a propermaterial with an appropriate thickness, but also in the con-trol of the junction shape between the permanent magnetand the active element of the sensor [183–188]. The over-hanging of the permanent magnet on top of the free layershould be minimized; otherwise, it will cause hysteresis orkinks in the transfer curves, resulting in instability in thereadback signal [189–191]. The ratio between theMrt of thepermanent magnet and the Mst of the free layer or the softlayer should be optimized so as to obtain a good tradeoffbetween sensitivity and stability. The typical value is about1.1–1.4 or even higher, depending on the shield-to-shield dis-tance [123, 186, 192–194]. A small shield-to-shield distancerequires a stronger permanent magnet. The microstructure

AFM FM

PM

lead

FMCuFM

AFM

PM PM

lead lead

FMCuFM

AFM

PM PM

lead lead

FMCuFM

AFM

PM

lead

FMCuFM

AFM

lead lead

FMCuFM

AFM

lead lead

(a

(e (

(g

AFM AFM AFM AFM

FMCuFM

AFM

AFM

NM

lead lead

(c (d

(b

FMCuFM

AFM

AFM

insulator insulator

lead

lead

FMCu

AFM

PM

insulator insulator

lead

lead

FM

spacer

(h

PM

lead

FMCuFM

AFM

PM PM

lead lead

FMCuFM

AFM

PM PM

lead lead

FMCuFM

AFM

PM

lead

FMCuFM

AFM

lead lead

FMCuFM

AFM

lead lead

(a)

(e) (f)

(g)

AFM AFM AFM AFM

FMCuFM

AFM

AFM

NM

lead lead

(c) (d)

(b)

FMCuFM

AFM

AFM

insulator insulator

lead

lead

FMCu

AFM

PM

insulator insulator

lead

lead

FM

spacer

(h)

A

Figure 18. Schematics of different types of longitudinal bias schemes.(a) Patterned exchange. (b) Lead-overlaid patterned exchange.(c) Long-distance exchange. (d) Same as (c), but in CPP mode.(e) Contiguous junction with PM. (f) Lead-overlaid contiguous junc-tion with PM. (g) Patterned synthetic ferrimagnet. (h) Parallel PM.The acronyms used are: PM—permanent magnet, FM—ferromagnet,AFM—antiferromagnet, NM—nonmagnetic layer. In (a)–(d), the topand bottom AFMs are two different types of materials.

of the permanent magnet also affects the performance of thesensor. The shape and cleanness of the junction are impor-tant for forming low-contact-resistance junctions, which isthe key to suppressing the temperature rise of the reader[195]. To avoid oxidation and contamination at the junc-tion interface, the permanent magnetic and contact are nor-mally formed using a self-aligned lift-off process using a“mushroom” resist pattern formed on top of the sensor ele-ment [178 179 184]. The CoCrPt layer is typically grownon a Cr seed layer to have a higher coercivity, typically1500–2500 Oe [196]. The uniform coverage of the seed layeron the junction edge is crucial to obtain a high-coercivityfilm along the entire junction. Poor seed-layer coverage mayresult in instability in the MR curve of the read sensor [197].

Lead-Overlaid Patterned Permanent Magnet The con-tiguous junction longitudinal bias is by far the most widelyused bias scheme in real products. The drawback of thisscheme, however, is that the bias field usually is not uni-form across the longitudinal direction of the sensor. It isnormally stronger at the two edges and weaker at the center.If the center portion is properly biased, then it is unavoid-able that the edge regions will be overbiased, leading to theformation of so-called dead regions. These inactive regions

Proof's Only


will, in general, degrade the sensitivity of the senor. Theinfluence of the dead region becomes more prominent whenthe read track narrows, in particular when the sensor widthdecreases to below 0.3 1m [198 199]. An alternative schemewhich can suppress the effect of the inactive region is theso-called lead overlaid structure in which the contact elec-trodes are extended over the abutted junction, and thus forma direct electrical contact with the inactive region of thesensor [Fig. 18(f)] [200]. Zhang et al. carried out a compara-tive study of magnetic noise in read heads with a contiguousjunction and a lead overlaid design [201]. It was found that,at 0.25 1m magnetic read width and 4 mA bias current, themagnetic noise is twice as large as Johnson noise for the leadoverlaid design, while it is comparable to Johnson noise forthe contiguous junction design. The higher magnetic noiseis attributed to a weaker longitudinal bias field with the leadoverlaid design. This may affect the performance of readheads with a lead overlaid design at higher areal densities.

Patterned Synthetic Ferrimagnet Recently, the author’sgroup proposed an alternative bias design in which pat-terned magnets are placed on top of the two edges of thefree layer via an ultrathin Ru film such that the permanentmagnets are antiferromagnetically coupled to the free layer[Fig. 18(g)] [202]. In conventional overlaid permanent mag-net design or contiguous junction design with overlappingareas, the domain structure near the edges of the free layeris not so stable because the magnetic field generated by thepermanent magnet inside the free layer always changes signat the edges. In the new design, however, the antiferromag-netic coupling between the permanent magnet and the freelayer forms a flux closure surrounding the permanent mag-net, and thus improves the domain stability near the edges.Micromagnetic modeling has shown that, in addition to thedomain stability, the sensitivity of the sensor with the newdesign is also higher than that of the contiguous junctiondesign.

Permanent-Magnet-Based Design for CPP and MTJSensors The contiguous and lead overlaid designs are notsuitable for CPP and MTJ sensors unless insulating per-manent magnets are used to form the longitudinal bias oran additional insulating layer is added between the perma-nent magnet and the electrode at two edges. Redon et al.proposed a modified version of contiguous junction or pat-terned permanent design for an MTJ head through remov-ing the portion of the pinned layer that is overlapped withthe permanent magnet [203]. The results of micromagneticmodeling showed that the modified design is able to improvethe performance of the MTJ head through suppressing theinfluence of the inactive region underneath the permanentmagnet. Sin et al. proposed a design in which the longi-tudinal bias is formed through the magnetostatic couplingfrom a permanent magnet placed at the other side of thepinned layer, as shown schematically in Figure 18(h) [204].This design is suitable for CPP and MTJ sensors becausethere are no conducting materials at the two sides of thesensor element.

4.2.2. Antiferromagnet-Based BiasPatterned Exchange As with the case of permanentmagnet-based longitudinal bias design for spin valves, theexchange-biased designs also stemmed from the workson MR sensors [178 179 205 206]. Among the exchange-domain stabilization designs, the most widely investigated isthe patterned exchange design in which a patterned layer ofantiferromagnetic material is fabricated on top of the freelayer before the electrical lead is deposited [see Fig. 18(a)][199 207 208 209]. The advantage of this structure over theoverlaid abutted junction structure is that the dead regionis almost removed from the edge. In addition to this, thereduction in parasitic resistance associated with the junctionimproves the signal-to-noise ratio. However, in this case, onemay need to use additional process steps to set the pinningdirection of the patterned AFM layer because the exchange-bias direction of longitudinal bias for the free layer is 90

away from that of the traverse bias for the pinned layer (seeFig. 19). Another potential disadvantage of this design isthe side reading due to the portions of the sensors under-neath the patterned AFM layers. This might be suppressedby increasing the exchange-bias strength and the conductiv-ity of the electrical contacts. It might also be reduced usingthe lead overlaid structure shown in Figure 18(b).

Long-Distance Exchange The most straightforward wayto stabilize the domains of the sensing layer of MR andGMR heads is to add an AFM layer on top of the sensinglayer. In fact, this has been employed in the early design ofan MR head [177]. This design, however, lacks the flexibil-ity in precise control of the bias field. It was often foundthat the exchange bias is too strong, which stiffens themagnetization of the sensing layer from rotation into thetraverse direction, resulting in a low sensitivity. However,recently, there has been a revived interest in this designowing to the recent development of our understanding ofthe exchange bias and the ability to control the bias fieldthrough the insertion of an ultrathin nonmagnetic spacer atthe AFM/FM interface [210–213]. Using Cu as the spacer,Nakashio et al. and Mao et al. successfully applied thisdesign to a CIP spin-valve head and an MTJ head, respec-tively [Fig. 18(c) and (d)] [210 211]. This type of design isparticularly suitable for a CPP head.

AFM

electrode

AFM

electrode

Ru CuAFM PL2PL1 FL AFM

Sensor element Longitudinalbias AFM

(a)

(b)

Figure 19. Cross-sectional view (a) and three-dimensional schematicstructure (b) of spin valves with patterned-exchange longitudinal bias.

Proof's Only


5. ELECTRONIC ASPECTSOF SPIN VALVES

Analogous to a transistor, the traverse and longitudinalbiases discussed above are to set a proper working point forthe spin valves. As is the case with transistors, once a properworking point is chosen, the performance of a spin valveis largely determined by its intrinsic properties, that is, themagnitude of the MR caused by the spin-dependent scat-tering at both the interfaces and the ferromagnetic layers.Although the spin valve exhibits a much higher MR as com-pared to the AMR sensor, the ever-increasing areal densityrequires continuous improvement of the performance of thespin valve. To this end, a variety of spin valves have beenproposed and studied both theoretically and experimentally.These include, but are not limited to: (1) a multiple spinvalve (e.g., dual spin valve), (2) a spin-filter spin valve,(3) a specular reflection spin valve, (4) a nanooxide-addedspin valve, and (5) a nanooxide-added dual spin valve. Inwhat follows, we explain briefly the motivation and under-lying mechanism for each of these different types of spinvalves illustrated schematically in Figure 14.

5.1. Spin Filter Spin Valve

The magnetoresistance of a CIP spin valve can be thoughtof as originating from the difference in the mean-free path(MFP) of the majority and minority electrons, at least phe-nomenologically. By making use of this phenomenon, Gur-ney et al. derived a novel method to measure the MFP ofelectrons in different materials such as Cu, AuCu, Co, NiFe,and Fe [214]. In this technique, a back layer is added toa spin valve with a thin free layer, and its effect on :G(=GP −GAP ) is studied systematically at different back-layerthicknesses. Here, GP and GAP are the conductances of thespin valve in parallel and antiparallel configurations, respec-tively. It was found that, to a good approximation, :G canbe written as

:G = :Gf + :GB/1− exp−tB/4+0 (24)

where :Gf is the conductance change due to the free layer,tB is the thickness of the back layer excluding the inactivelayer, if any, at the interface with the free layer, 4+ is theMFP of majority electrons in the back layer, and :GB is theconductance change due to the back layer when tB 4+.The MFPs measured by this technique for Cu, Co, NiFe, andFe are 192 55 46, and 1.5 nm, respectively. This resultimplies that the addition of a back layer with an appropri-ate thickness can enhance the MR of a spin valve with anultrathin free layer (so-called back-layer effect). The ideabehind this is that the back layer helps to retain or evenenhance the MFP of majority electrons, while it has littleeffect on the MFP of minority electrons. Kamiguchi et al.extended this concept to synthetic spin valves, and calledthem spin-filter spin valves [see Fig. 14(h)] [215]. The backlayer is retermed a high-conductance layer (HCL). Note thatthe synthetic pinned layer is not shown in the schematicdrawing in Figure 14(h) because we want to focus on thepure electronic effect. At a free layer thickness of 2–4 nm,the MR ratio can be enhanced by about 10% using the

spin-filter design [216]. It is worth noting that, in additionto the enhancement of the MFP of majority electrons, theback layer or HCL also helps to better balance the current-induced fields at the free layer due to the redistribution ofa certain portion of current to the HCL, leading to a lowerasymmetry of the readout signal. For example, at a freelayer thickness of 4 nm, Ueno et al. showed that the read-out asymmetry can be reduced from 10% for a conventionalspin valve to 3% for a spin-filter spin valve [216]. However,the improvement of the MR in a spin-filter design is onlyup to a certain extent because the HCL also increases theshunting current, which in turn will reduce the MR. There-fore, in most practical cases, the back layer is employed incombination with other techniques such as the nanooxide toimprove the performance of advanced spin valves.

5.2. Dual Spin Valve

The most straightforward way to improve the MR ratioof a spin valve is to increase the number of free/pinnedlayer pairs. This has prompted the proposal and develop-ment of dual spin valves, in which the free layer is sharedby two pinned layers at both sides (see Figs. 14(c) and20) [217–223]. The MR of a typical dual spin valve can be30–60% higher than that of a single spin valve, depend-ing on the materials, device structure, and fabrication pro-cesses [21 217 218 224]. The fabrication of a dual spinvalve requires the use of AFM materials which are able toform exchange bias both at the top and bottom of the FMlayer, and with a small critical thickness. Special cautionsare needed in choosing the thickness of the two Cu spac-ers because two identical spacers may make it difficult toadjust the traverse bias field in the free layer using the sensecurrent once the sensor fabrication is completed [223].

5.3. Specular Spin Valve

The dual spin valve consists of one free layer and two pinnedlayers. Can we increase the MR further by simply increasingthe number of free and pinned layers? The answer is nega-tive because different free layers may respond to the exter-nal field in a slightly different way, which may result in extranoises in the readout signal or degradation of the sensitivity.The only way to overcome this is to use antiferromagneti-cally coupled ferromagnetic/nonferromagnetic pairs. But asdiscussed earlier, this type of sensor is only applicable to thedetection of a large magnetic field. One of the possible waysto increase the effective number of free layers without theneed to add more layers physically is to increase the spec-ular reflectivity of electrons at the outer surface of the free

PL FLAFM Cu Cu PL AFM

Figure 20. Schematic of a dual spin valve.

Proof's Only


and pinned layer, respectively. As shown schematically inFigure 21, if the specular reflectivity at both surfaces is unity,the trilayer can be considered effectively as the repetitionof the same unit in an infinite number of cycles. However,the difference between the equivalent structure and the realstructure of the same type is that the former only has onefree layer physically, and thus does not suffer from the draw-backs caused by the existence of a plural number of freelayers.

5.3.1. Specular Spin Valve UsingInsulating Antiferromagnet

The most straightforward way to implement this kindof structure is to use insulating AFM layers suchas NiO and A–Fe2O3 to form an AFM/active layers/capping layer, AFM/active layers/AFM or AFM/active lay-ers/NM/AFM sandwich structure [Fig. 14(j)]. This hasbeen verified experimentally to be very effective inincreasing the MR ratio [219, 225–229]. The specularreflection enhancement effect was first described quan-titatively by Swagten et al. in spin valves with struc-ture NiO(50 nm)/FM1/NM1/FM2/NM2/NiO(10 nm), withFM1 = Co 2 nmNM1 = Cu 2 nmFM2 = Co (t),NM2 = Cu 12 nm) [225]. The second nonmagnetic layer,in this case, a 1.2 nm thick Cu, is used to isolate theexchange coupling between the top NiO and the secondferromagnetic layer (FM2), so that the top NiO func-tions only as a pure insulator. The decoupling effect ofNM2 has been confirmed in the measured M–H curves,in which FM2 was found to switch at almost zero field.Using this structure, Swagten et al. obtained an MR ratioas high as 25% at 10 K and 15% at room tempera-ture [225]. A semiclassical model has been invoked tointerpret the experimental results, which suggests that theenhancement of the MR is attributed to the enhance-ment of specular reflection at the metal/insulator interfaces.Also using the same NiO oxide, but in a dual spin valvewith a structure NiO50/Co2.5/Cu1.8/Co4/Cu1.8/Co2.5/NiO50 (thicknesses are in units of nanometers), Egelhoff et al.obtained an MR ratio of 21.5% at room temperature [219].The large MR ratio was attributed to both the dual spin-valve structure and the enhanced specular scattering ofelectrons at the Co/NiO interfaces. Although NiO has adesirable electric property, its low exchange bias field andlow blocking temperature make it an unfavorable choicefor real read sensors. Hasegawa et al. carried out thefirst study on the possibility of using A–Fe2O3, anotherinsulating AFM, to construct specular spin valves [226].The MR ratio that has been obtained for a spin valvewith structure glass/A–Fe2O3/Ni80Fe206/Cu2.2/Ni80Fe209/Cu4

FL

PL

Cu

Oxide

Equivalent to

Repetition ofthe basic unit

Figure 21. The number of the free layer (FL) and pinned layer (PL)can be increased effectively by increasing the specular reflectivity ofboth the top and bottom surfaces to unity.

(thicknesses are in units of nanometers) was only about3.7%. Sugita et al. [227] and Kawawake et al. [228] grewA–Fe2O350/Co2/Cu2/Co5/Cu0.4 (thicknesses are in unitsof nanometers) spin valves on a polished (110) planeof an A–Al2O3 single-crystalline substrate. The MR ratioobtained was as high as 18%. It was further increased to27.8% for a dual spin valve with a structure A–Al2O3/A–Fe2O350/Co2/Cu2/Co5/Cu2/Co2/A–Fe2O350 (thicknesses arein units of nanometers). In either case, however, the unidi-rectional exchange-bias field is very small. The pinned layeris more or less a hard magnet instead of a soft layer pinnedby an AFM. The coercivity of the bottom Co layer is morethan 1200 Oe. The large MR ratio again was attributed tothe enhanced specular reflection at A–Fe2O3/Co interfaces.

In addition to oxides, metals can also be used as thecapping layer to increase the MR of spin valves. Althoughthe mechanism of MR enhancement is not fully under-stood, it is believed that the potential step at both the freelayer/capping layer interface and the capping layer/air inter-face may have helped to enhance the specular reflection ofelectrons. Compared to oxide capping layers, the metalliccapping layer has certain limitations in thickness because toothick a capping layer will cause an increase in the shunt-ing current, and thus leads to a decrease in the MR ratio[230 231]. Although it has not been investigated in detail,this type of structure might be useful for CPP spin valves.

5.3.2. Specular Spin Valve UsingInsulating Ferromagnet

Recently, Carey et al. [175] and Maat et al. [232] used insu-lating cobalt ferrite as pinning layers to improve the per-formance of spin valves. Spin valves with a high MR ratio(12.8%) and a large pinning field (about 1500 Oe) havebeen successfully fabricated. In this case, pinning is realizedthrough an exchange-spring type of coupling between twoferromagnets [see Fig. 14(g)].

5.3.3. Nanooxide Added Spin ValveAs discussed above, the use of oxide AFM can greatlyenhance the MR of spin valves. These oxides, however,are unsuitable for practical applications due to their lowexchange bias field, low blocking temperature (for NiO), orlarge thickness required. This has led to the proposal ofinserting nanooxides at the middle of the pinned layer ofbottom spin valves. This, in combination with the top oxidecapping layer, was found to greatly enhance the MR of spinvalves. Since the pioneer work of Kamiguchi et al. [215],the nanooxide effect has been studied intensively to enhanceboth the magnetic and electronic properties of advancedspin valves [233–264]. The nanooxide, in general, is insertedat the middle of the pinned layer, and in the case of syn-thetic spin valves, it is normally inserted at the middle of thepinned layer that is nearer to the spacer. It is more effec-tive when being used together with an insulating cappinglayer, leading to the so-called double specular spin valve[see Fig. 14(k)]. The nanooxide can be formed using differ-ent techniques such as natural oxidation, plasma oxidation,remote plasma oxidation or so-called atomic beam oxida-tion, ion beam oxidation, and ion-assisted oxidation. Theplasma- and ion-based techniques were found to be able to

Proof's Only


form denser oxides than natural oxidation. Under optimumconditions, the oxides improve the MR without compromis-ing the magnetic and thermal stability and the ESD robust-ness of spin valves. Read heads with sensitivity in the rangeof 6–10 mV/1m or higher have been demonstrated usingnanooxide-added spin valves [257]. As the results obtainedfrom different works are, to a certain extent, dependent onspecific experimental conditions, the results are not alwaysconsistent with one another. Table 2 summarizes the repre-sentative works on specular spin valves reported so far in thelast few years. Due to the large number of publications, theauthor cannot guarantee that the collection is complete. Inwhat follows, we use our own results as examples to describethe main effect of nanooxide.

The Role of Oxide in Pinned Layers The roles of thebottom oxides are twofold. First, they reduce the surfaceroughness of the subsequently deposited layers, and sec-ond, they enhance the specularity of the pinned layer. Theimprovement of the surface roughness has been confirmedthrough both direct measurement of the surface roughnessusing AFM and measurement of the interlayer couplingfield between the free and pinned layers [249]. In general,there are two major coupling fields between the free andpinned layers of different origin: (1) orange-peel coupling,and (2) RKKY coupling. The former is dominant in sam-ples with rough interfaces, while the latter becomes moreimportant when the interfaces become atomically flat. Theorange-peel coupling decays exponentially with the thicknessof the spacer, and for a sinusoidal roughness profile, it isgiven by [265]

Hint_OP = ,2A2

√2tF 4

MF exp−2,√2ts/4 (25)

where A and 4 are the amplitude and wavelength of theroughness profile, tF and ts are the thicknesses of the freeand spacer layer, respectively, and MF is the magnetizationof the free layer. The original concept was extended to thinfilms with a structure of hard/spacer/soft by Kools et al. in1999 [266]. Although Eq. (25) is modified slightly, the expo-nential dependence of the coupling field on the spacer thick-ness is still the same.

On the other hand, as discussed before, the strength ofRKKY coupling oscillates with spacer thickness:

Hint_RKKY ∝ 1tF

1t2s

sin(2, ts3

+ B)

(26)

where 3 is the oscillation period and B is the phase factor.Except for a few reports [244 257], the oscillation can hardlybe seen in normal spin-valve samples without nanooxides.However, it can be observed easily in samples with nano-oxides. The suppression of orange-peel coupling has alsobeen achieved in Cu/Co superlattices through the introduc-tion of a proper amount of oxygen during the growth of theentire film stack [233 267].

To confirm both effects of nanooxide, spin valves withthe structure Ta3/NiFe2/IrMn6/CoFe3/Cu(t)/CoFe1/NiFe2/CoFe1/Cu1/Ta1 [t denotes the thickness of the spacer layer;thicknesses are in nanometers, hereafter, we call them con-ventional SVs (CSV)] and SVs with the structure Ta3/NiFe2/

IrMn6 /CoFe1.5 /NOL/CoFe1.5 /Cu(t) /CoFe1/NiFe2 /CoFe1/OX/Cu1/Ta1 [hereafter, we call them nanooxide-added SVs(NOL-SV)] were deposited on Si(100) substrates coatedwith a 1 1m thick thermally oxidized SiO2 layer by usingan ultrahigh vacuum sputtering system operating at a basepressure of 5 × 10−10 torr. Here, NOL denotes the naturaloxidation of the CoFe layer. Oxidation was conducted in aseparate chamber, by exposing the fresh CoFe surface topure oxygen atmosphere. The exposure time was 1 min,but the pressure was controlled from 1 × 10−4 to 1.6 Pa.The specimens were magnetically annealed at 275 C for1 h under magnetic fields of 1 T in a commercial magneticvacuum annealing oven under a pressure of 10× 10−6 torr.The MR was measured by using a linear four-point probemethod. The sheet resistance was calibrated by measuringthe MR on a microstructural bridge with 400 1m lengthand 100 1m width.

Figure 22(a) shows the dependence of the MR ratioand interlayer coupling field on the oxygen exposure doseused to oxidize the CoFe layer. It is found that Hintdecreases almost monotonically with the oxygen exposuredose, decreasing from 31 Oe in a CSV to 4 Oe in an NOL–SV. On the other hand, the MR ratio increases from 9 to14% with an increasing O2 exposure dose from 10−8 to0.1 min Pa, which saturates when the exposure dose is over0.1 min Pa. Shown in Figure 22(b) are the sheet resistanceRsq and its change :Rsq as a function of O2 exposure dose.Rsq decreases initially, reaches a minimum at an oxygenexposure dose of about 10−3 min Pa, and increases whenthe oxygen dose increases. The initial decrease of the sheetresistance might be attributed to the enhancement of specu-lar scattering of electrons at the interface between the metaland NOL.

Figure 23 shows the MR (a), interlayer coupling field (b),sheet resistance (c), and change of sheet resistance (d) asa function of Cu thickness. The enhancement of surfacesmoothness can be seen from the oscillation of the inter-layer coupling field with the Cu layer thickness in NOL–SVs, as shown in Figure 23(b) (solid circle). Also shownin the figure are the experimental results for conventionalspin valves (open circle) and the fitting curve according toEq. (25) (dotted line). Although the interlayer coupling isstill ferromagnetic in the entire range of the Cu thicknessthat has been investigated, the clear oscillation in NOL–SVs and the monotonic decrease with Cu thickness in CSVsclearly demonstrate the effectiveness of NOL in improv-ing the surface roughness. Figure 23(c) and (d) shows thedependences of the sheet resistance and its changes onthe Cu thickness, respectively. It is interesting to note thatthe nanooxide-added spin valve has a smaller Rsq, but alarger :Rsq than the conventional spin valve in the entirethickness range for Cu, leading to a larger MR ratio in thewhole range, as shown in Figure 23(a). It is also interestingto note that the maximum of MR corresponds to the mini-mum of the interlayer coupling field. This can be understoodreadily because, at this thickness range, the free and pinnedlayers are antiferromagnetically coupled if one subtracts theorange-peel coupling field from the total coupling field. Veryrecently, we have been able to achieve antiferromagneticcoupling routinely in spin valves with oxide capping layers.


Table 2. Summary of the findings reported in nanooxide-related works.

Structure of spin valveResearch group (thicknesses are in nm) GMR (%) Hin (Oe) Hex (Oe) Remarks Ref.

1. NIST,Univ. Minnesota,Univ. California

NiO 50/Co 2.5/Cu 1.8/Co 4/Cu 1.8/Co 2.5/NiO 50

21.5 First experimentalindication ofspecular reflectivityenhancement

[219]

NiO 50/Co 2.5/Cu 1.9/Co 4/Cu 1.9/Co 2.5/NiO 50

24.8 Oscillating between80 and −20 Oe

Systematic study of therole of background gas(CH4, N2, H2O, C, H2,CO, O2);

[233]

NiO 50/Co 2.5/Cu 2/Co 3/CoO 0.4/TaO 0.4

19% Oxygen was found to act asa surfactant, improvingthe properties of SV

2. EindhovenUniv. Tech.,Philips

NiO 50/Co 2/Cu 2/Co (t)/Cu 1.2/NiO 10

25 at 10 K 15at R.T.

First quantitativestudy of specularreflection effect bothexperimentally andtheoretically

[225]

3. Alps glass/A–Fe2O3/Ni80Fe20 6/Cu 2.2/Ni80Fe20 9/Cu 4

3.7% First report on SVusing/A–Fe2O3 as AFM

4. Matsushita A–Fe2O3 50/Co 2/Cu 2/Co 5/Cu 0.4

18% A–Fe2O3 was grownepitaxially on polished(110) plane of anA–Al2O3 singlecrystalline substrate

[227]

A–Fe2O3 50/Co 2/Cu 2/Co 5/Cu 2/Co 2/A–Fe2O3 50

27.8%

Ta 3/PtMn 15/CoFe 2/Ru 0.7/CoFe 1/NOL/CoFe 1.5/Cu 2.2–2.9/CoFe 1/NiFe 1–2/NOL/Ta 3

13 1092 Good thermal stability [245]

Ta/AF (PtMn or A–Fe2O3)/CoFe/NOL/CoFe/Cu/CoFe/Ta or

Ta/AF (PtMn or A–Fe2O3)/CoFe/NOL/CoFe/Cu/CoFe/NOL/CoFe/Ta

12–17.3 414–723 [234]

5. NagoyaUniv.,Matsushita

Ta 3/NOL/Co 2/Cu 2/Co 2/A–Fe2O3 50/Al2O3

21.6 A–Fe2O3 as the AFM SVwith NOL is moresensitive to temperaturebelow 200 K

[235]

6. Toshiba Ta 5/NiFe 2/IrMn 7/PL (withNOL)/Cu 2/CoFe 2/CoFeO 0.4/TaO 0.4

16 400 First report on NOL effectinside the SV stack;

Natural oxidation;Free layer Hc, 14Oe

[215]

underlayer/IrMn 7/CoFe/pin-NOL/CoFe 2/Cu 2.2/CoFe 2/Cu 1/free-NOL

17 7 400 :Rs : 2.5–3 <:Natural oxidation;Testing head for

10 Gbits/in2;Promising for

100 Gbits/in2

[243]

Sub/Ta 3/Ni62Fe16Cr22 3/AF (PtMn or IrMn)/Co90Fe10 1/Co50Fe501 (for FeCo NOL),or Co90Fe10 1 (forCoFe NOL)/

15.7 −2 500 NOL formed by ion-assisted oxidation ismore stable than thoseformed by naturaloxidation;

[259]

continued


Table 2. Continued


oxidation process/Co90Fe10 2/Cu 2.3/Co90Fe10 2/Cu 1/Ta 1

Thick NOL reducesferromagnetic couplingacross NOL

7. Alps+ Toshiba Double specular (Ox–p+Ta–Ox) SAF bottom SV

17–18 1100–1500 Head level characterizationincluding ESD

[258]

8. Fujitsu Underlayer/Pd32Pt17Mn51/CoFeB Ru/CoFeB/Cu/CoFeB/Cu/oxide


>1000 Head for 56.1 Gbits/in2 [260]

Underlayer/Pd32Pt17Mn51/CoFeB Ru/CoFeB/NOL/CoFeB/Cu/CoFeB/Cu/oxide


Head for 106.4 Gbits/in2

Underlayer/PdPtMn/CoFeRu/CoFe/NOL/Cu/CoFe/NiFe/Cu/oxide


>1000 Head for 106.4 Gbits/in2 [241]

Underlayer/PdPtMn/CoFeB/Ru/CoFeB/NOL/CoFeB/Cu/CoFeB/Cu/oxide


Head for 56.1 Gbits/in2 [242]

Underlayer/PdPtMn/CoFeB/Ru/CoFeB/Cu/CoFeB/Cu/oxide

12.3 +2 ∼2000 tCu = 20

Underlayer/PdPtMn/CoFeB Ru/CoFeB/Cu(t)/CoFeB(1.5)/Cu/oxide

8.87 Oscillating(∼−17 Oe whentCu = 2–3 nm)

938 Capping layer effect:Ta capping: MR ratio,

7.55%, interlayercoupling field, 5.1Oe;

Oxide capping: MRratio, 8.87%,interlayer couplingfield, −166%

[236]

9. Philips Ta 3.5/NiFe 2/IrMn 1/CoFe 3/NOL/CoFe (t)/Cu 2.5/CoFe 4/NOL

14 ∼800 Natural oxidation;Al2O3, a good top NOL

[248]

Ta 3.5/NiFe 2/IrMn 8/CoFe 3/Cu 2.5/CoFe 4/NOL

9.5 CoFe is preferred over Cobecause the former allowsfor the formation of ironoxide

[239]

Ta 3.5/NiFe 2/IrMn 8/CoFe 3/NOL/CoFe 3Cu 2.5/CoFe 4/NOL

13

10. INESC Ta 6.7/NiFe 4.2/IrMn 9/CoFe 1.4/NOL/CoFe1.5/Cu 2.2/CoFe 4/NOL/Ta

12.5 3 Temperature-dependent study;Anomalous bumps seen

in MR–T curves ofNOL SV

[251]

Ta 7/NiFe 5/IrMn 9/CoFe 1.4/NOL/CoFe 2.5/Cu 2/CoFe 2.6/NOL/Ta 1–2

13.6 <4 ∼180 Remote plasma oxidation;RBS result indicates a

thickness of 1.5 nmfor NOL

[240]

11. TDK Sub/Ta 5/NiFe 2/NOL/NiFe 1/CoFe 2/Cu 2.5/CoFe 2/NOL/CoFe/PtMn orRuRhMn/Ta 5

11.8 (RuRhMn)

Better thermal stability of SVwith NOLs

[237]

9.4 (PtMn)

12. Sony Glass/Ta 3–5/NiFe 12/PtMn10–20/CoFe 1.1–1.5/Ru 0.8/NOL/CoFe 2–2.2/Cu 1.9–3.2 CoFe 1.5–2.5/Cu 1/Ta (Ta–O) 0.2–5

14.9 Ru/Ox Good thermal stability [261]

continued


Table 2. Continued


13. IBM Al2O3 3/NiCrFe 3/NiFe 1/PtMn 20/CoFe 2/Ru 0.8/CoFe 2.2/Cu 2.2/CoFe 0.9/NiFe 2.7/Cu 8/Al2O3 1/Ta 6


∼3000 >20 Gbits/in2:Al2O3 +NiCrFe as the

seed layer;Strong C111D texture;Good thermal stability;Head level test (good

linearity & smallasymmetry −15%,6.64 mV/1m,20 Gbits/in2)

[244]

14. IBM,San JoseState Univ.

Ta 3–4/Cu 1/IrMn 8.5/CoFe(d)/NOL/CoFe (3.3-d)/Cu 2.4/CoFe 1.6/NiFe 2.2/Ta 3.5

10.4 5.1 460 Natural oxidation;MR ratio highest when

d ∼ 12 nm

[262]

Ta 3–4/Cu 1/IrMn 7/CoFe(d)/NOL/CoFe (3-d)/Cu 2.4/CoFe 3/Cu 2.7/CoFe 3/IrMn 7/Ta 3.5

20.5

15. ReadRite Ta 1.5–3/NiFeCr or NiFe 2–5/PtMn 10–20/CoFe 1.5–2/Ru 0.8/CoFe 1/NOL/CoFe 1.5–2/Cu 1.5–2.4/(CoFe/NiFe or CoFe) 2–3/Ta/TaO 1–5

16.1 (plasmaNOL),

15 (naturalNOL),

12.5 (non-NOL)

Weak oscillation(>0)

5.8 at tCu = 22 nm

1000 >50 Gbits/in2:Plasma oxidation forms

denser and moreuniform NOL;

Optimum Ta cappinglayer thickness∼1 nm;

No degradation inthermal stability;

Head level test(6.5–10 mV/1m,asymmetry, <2% forpositive sensecurrent, good for50 Gbits/in2)

[257]

16. LLNLReadRite

FM1/Cu/FM2FM1 = NiFeCo, NiFeCo/

CoFe, CoFe,FM2 = NiCoFe, CoFe/

NiFe, CoFe/NiFe/CoFe

9.3 (non-NOL),12.9(NOLat oneside), 13.9(NOL atboth side)

Nonexchange-biasedsandwich

[238]

17. VeecoLLNL

Underlayer/NOL/NiFe 2.5/CoFe 1/Cu (t)/CoFe 3/IrMn 7/Ta 2

∼12.5 Oscillation (>0) ∼330–430 Systematic study ofdifferent oxides;

MR increases from 26.5to 36.7% in the orderof AlOx , TaOx , CuOx ,NbOx , CrOx ,NiFeCrOx , NiFeOx ,CoFeOx

[256]

18. Veeco,ReadRite,LLNL, Toshiba

SiO2/Ta 5/NiFe 2/Ir20Mn80 7/CoFe 2/NOL/CoFe 2/Cu 2/CoFe 2/Cu 1/Ta 1

Alternative structures(replacing Ir20Mn80

by Pt50Mn50 15, orPt50Mn50 15/Co10Co90

2/Ru 0.8, or Ir20Mn80

7/Co10Co90 2/Ru 0.8

15.5 ∼480 PVD vs. IBD;PtMn and IrMn;Simple and synthetic;Atomic beam oxidation

gives the highestFM coupling acrossthe NOL

[246]

continued


Table 2. Continued


19. Veeco+LLNL+Toshiba

Top SV: Ta 2/NOL/NiFe 1/CoFe 2/Cu 2.4/CoFe 2/IrMn 6/Ta 2

Bottom SV: Ta 5/NiFe 2/IrMn 7/CoFe (2-t)/NOL/CoFe 2/Cu 2/CoFe 2/Cu 1/Ta 1

Dual SV: Ta 2/NiFe 2/IrMn 6/CoFe (1.5-t)/NOL/CoFe 2/Cu 2/CoFe 2.5/Cu 2/CoFe(2-t)/NOL/CoFe 1.5/IrMn 6/Ta 2

9.8

12.2 (ABO),

11.9 (ABO)

18.5 (ABO)

Positiveoscillation(5–30)

574

Increasefrom300 to350

∼300

Systematic study of atomicbeam oxidation (ABO)or remote plasmaoxidation and ion beamoxidation (IBO);

ABO gives largest MRratio and exchange-biasfield for bottom spinvalves

[245]

20. Univ.Utah+LLNL

Ta 2/NOL/NiFe 2.5/CoFe 1/Cu (t)/CoFe 3/IrMn 7/Ta 1.5

12.25 29 ∼400 Natural oxidation;Cu wedge SV;MR decreases

monotonically withCu thickness

[263]

21. Seoul Nat.Univ.+ KIST

Sub/Ta 5/Ni81Fe19 2/Fe50Mn50 8/Co90Fe10 1.5/NOL/Co90Fe10 1.5/Cu 2.6/Co90Fe10 2.5/Cu 2.6/Co90Fe10 1.5/NOL/Co90Fe10 1.5/Fe50Mn50 8/Ta 5

15.9 ∼250 [252]

Sub/Ta 5/Ni81Fe19 2/ Fe50Mn50

8/Co90Fe10 2/NOL/Co90Fe102/Cu 2.6/Co90Fe101.5/Ni81Fe19 4.5/Ta 5

10.1 NOL may also act as adiffusion barrier for Mn

[253]

22. Nal. TsingHua Univ.

Top SV: Ta 8/Co 5/Cu 2.1/Co 4/FeMn 10/Ta 5

Bottom SV: Ta 8/NiFe 5/FeMn 10/Co 5/Cu 2.1/NiFe 10/Ta 5

7.8 Bottom SV: NOL atFeMn/Co and NiFe/Tainterfaces increasesand NOL at Co/Cuor Cu/NiFe interfacesdecreases the MR ratio;

Top SV: NOL at Ta/Cointerfaces increasesMR ratio

[264]

23. Inst. of Phys.,CAS UST,Beijing Univ.Plymouth,Nordiko

Ta 3.5/NiFe 2/IrMn/6/CoFe 1.5/NOL/CoFe 2/Cu 2.2/CoFe (t)/NOL/Ta

15 ∼380 Bottom NOL is a mixtureof metal and oxide

[255]

Ta/NiFe/IrMn/CoFe NOL/CoFe/Cu/CoFe/Cu/NOL/Ta

15 ∼5 ∼400 Spin filter SV with NOL(already employed byother groups)

[254]

24. DSI/NUS Top SV: Ta 3/NOL/NiFe 2/CoFe 1.5/Cu 2.2/CoFe 3/IrMn 8/Ta 2

10.8 5–15 225–450 MR increases in the orderof CoFeOx , TaOx ,CuOx , AlOx

[250]

Bottom SV: Ta 3/NiFe 2/IrMn 6/CoFe 2.5/Cu 2.2/CoFe 1.5/NiFe 3/Cu 2/NOL/Ta 1

8.8 5–24 520–555 MR increases in the orderof TaOx , AlOx . NiFeOx ,CoFeOx , CuOx

continued


Table 2. Continued


Dual SV: Ta 3/NiFe 2/IrMn 6/CoFe 1.5/NOL/CoFe 1.5/Cu 2/CoFe 1/NiFe 1/CoFe 1/Cu 2/CoFe 2/NOL/CoFe 1/IrMn 6/Ta 2

21.8 ∼280 Best bottom oxide:CoFeOx

Best capping oxide: AlOx

Ta 3/NiFe 2/IrMn 6/CoFe 1.5/NOL/CoFe 1.5/Cu 2.2/CoFe 1/NiFe 2/CoFe/NOL/Cu 1/Ta 1

15.3 Oscillating between0 and 50

∼350 Both specular reflectionenhancement andinterface smootheningeffects have beenconfirmed, but interfacelayer coupling is stillferromagnetic

[249]

Effects of Different Types of Oxides There are manydifferent types of oxides that can be used to improve theperformance of the spin valves. The selection of oxides thathave been studied so far is more or less based on the avail-ability of the elements that are used to form the oxidesin individual deposition systems. In general, one has morechoices for the capping layer than that for the NOLs insertedin the middle of the spin valves because the latter will affectthe magnetic properties of the spin valves. For example, ifit is inserted in the middle of the pinned layer, it cannotbe too thick; otherwise, it will separate the pinned layerinto two sublayers, and result in a substantial decrease ofthe exchange bias. Although it can be made thicker if it isinserted at the AFM/seed layer interface, it cannot be toothick; otherwise, it will affect the crystalline texture of thelayers that are subsequently deposited on top of the oxide.This eventually will lead to a decrease of exchange bias.Bearing all of these in mind, the author’s group has carriedout a systematic study on: (1) how different types of oxidesaffect the performance of the spin valve, and (2) which

∆Rsq

(Ω)

Figure 22. (a) MR ratio (solid circle) and Hint (open circles) as a func-tion of O2 exposure dose. (b) Sheet resistance (open squares) andchange of sheet resistance :Rsq (solid squares) as a function of O2

exposure dose for a series of SVs with the structure Ta3/NiFe2/IrMn6/CoFe1.5/NOL/CoFe1.5/Cu2/CoFe1/NiFe1/CoFe1.2/NOL/Cu1/Ta1 witha deferent O2 exposure dose. The thicknesses are in nanometers.

position is more desirable in terms of performance and man-ufacturability [250].

To investigate the effect of different types of oxides, wefabricated simple top and bottom spin valves with oxidesbeing added at different locations, as shown schematicallyin Figure 24. The oxides that we have investigated includeCoFeOx, TaOx, AlOx, NiFeOx, and CuOx. The effects ofthese oxides on the interlayer-coupling field Hin, exchangebias Hex, and MR ratio are shown in Figure 25. The mainresults can be summarized as follows. First, the exchangebias of the top spin valve is much smaller than that of thebottom spin valve. This is caused by both the property ofthe AFM (in this case, IrMn) itself and the oxide-inducedchanges in the texture of the underlying layers. The latteris especially severe in the case of TaOx and AlOx. CoFeOxand NiFeOx are better because they were formed from theoxidation of CoFe and NiFe, which already had a well-established texture before the oxidation took place. So doesCu because it is usually used as the texture control layer for

1.5 2.0 2.5 3.0 3.5

0

10

20

30

40

50

60

70

tCu(nm)

(b)

Hin

t(O

e)

1.5 2.0 2.5 3.0 3.5

6

9

12

15

(a)

MR

(%)

1.5 2.0 2.5 3.0 3.521

24

27

30

33

36 (c)

1.5 2.0 2.5 3.0 3.5

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

(d)

∆R

sq(Ω

)R

sq(Ω

)

tCu(nm)

Figure 23. Spacer (Cu) thickness dependence of MR (a), Hint

(b), sheet resistance Rsq (c), and sheet resistance change (d) fortwo batches of SVs with the structure Ta3/NiFe2/IrMn6/CoFe3/Cu(t)/CoFe1/NiFe2/CoFe1.2/Cu1/Ta1 (open symbols) and Ta3/NiFe2/IrMn 6/CoFe1.5/NOL /CoFe1.5/Cu(t) /CoFe1/NiFe2/CoFe1.2/NOL/Cu1/Ta1 (solid symbols). The dotted line in (b) was fitted by using Néel’sorange-peel coupling equation. The thicknesses are in nanometers.

Proof's Only


Free layer

Seed layer

Texturecontrol layer

CuCL AFM pinned layer

Nano-oxide

layer

(a) (

Seed layer


Cu pinned layer AFM

Free layer

Nano-oxide

layer

Free layer

Seed layer



Nano-oxide

layer

Free layer

Seed layer



Nano-oxide

layer

(a) (b)

Seed layer


Cu pinned layer AFM

Free layer

Nano-oxide

layer

Seed layer


Cu pinned layer AFM

Free layer

Nano-oxide

layer

Figure 24. Schematic of a top spin valve with a nanooxide inserted atthe free and texture control layer interface (a), and a bottom spin valvewith a nanooxide capping layer (b).

NiFe and CoFe. Second, the interlayer-coupling field of thetop spin valve is, in general, smaller than that of the bot-tom spin valve, except for the case using AlOx. This againsuggests that the nanooxide has the surface smootheningeffect, as discussed earlier. The improvement is particularlyremarkable for CoFeOx. Finally, we can see that the MRratio of the top spin valve, in general, is larger than thatof the bottom spin valve. Comparing these results, we cansee that CoFeOx is, so far, the most desirable bottom oxide,while AlOx seems to be a better choice for the top oxide.For other oxides to be better than CoFeOx, one must find amaterial that has a similar crystalline structure and textureas that of the CoFe layer in spin valves.

Effect of the Position of Oxide In addition to the typesof oxides, we have also carried out a systematic study onthe position of oxide, that is, which position is more suit-able for the bottom oxide. Figure 26 shows schematicallythe different positions that have been investigated. Theseare: (1) inside the AFM layer, (2) at the AFM/first pinnedlayer interface, (3) at the Ru/second pinned layer interface,(4) inside the second pinned layer, (5) at the second pinnedlayer/Cu interface, (6) inside the spacer, (7) at the Cu/freelayer interface, and (8) at the top surface. In what follows,we describe the results for cases (4)–(6).

0

5

10

15

20

25

200

250

300

350

400

450

500

550

600

6

7

8

9

10

11

12

CoFeO TaO AlO NiFeO CuO CoFeO TaO AlO NiFeO CuO

CoFeO TaO AlO NiFeO CuO

Hin

(Oe)

Hex

(Oe)

MR

rati

o(%

)

(a) (b)

(c)

Figure 25. Effect of different nanooxides on the interlayer couplingfield (a), exchange bias (b), and MR ratio (c) of bottom (open cir-cles) and top (open squares) spin valves whose structures are shown inFigure 24.

CuPL2 FL Cu TaRuAFM PL1

1 2 3 5 7 84 6

Nano-

oxid

e

Figure 26. Schematic of the nanooxide-added spin valves with the oxideinserted at different locations. Here, PL1 and PL2 refer to the first andsecond pinned layers, respectively, and FL stands for the free layer.

Figure 27 shows the MR–H curves of case (5), in whichthe spin valve has a structure Ta3/NiF2/IrMn8/CoFe2.5/Ru0.8/CoFe2/NOL/Cu2.2/CoFe2/Cu1/Ta1.5 (thicknesses arein nanometers). Here, NOL stands for the nanooxide layer.The NOL was formed through natural oxidation of the sec-ond pinned layer in a separate chamber, with the thicknessbeing controlled by the oxygen dose. After the oxidationwas completed, the sample was brought back again to thegrowth chamber for the deposition of the rest of the lay-ers. All of the deposition and oxidation processes wereaccomplished in a multiple-chamber ultrahigh-vacuum sys-tem without breaking the vacuum. As can be seen from thefigure, the MR ratio initially increases under light-oxidationconditions, beginning to decrease when the oxygen exposuredose increases, and eventually vanishing out as the oxidationprocess continues. Figure 28 shows the detailed dependenceof the MR ratio and the interlayer coupling on the oxy-gen dose. It is worth noting that the interlayer-coupling fielddecreases and the MR ratio increases in the light-oxidationregime by increasing the oxygen dose. However, the formerincreases sharply when the latter starts to decrease as theoxygen exposure dose is further increased. The behavior ofthe MR ratio is easily understood, but it is not as straightfor-ward to explain the sharp increase of the interlayer-couplingfield. As the interlayer-coupling field normally decreases bydecreasing the pinned layer thickness [266], it is plausible topostulate that the sharp increase of Hin has something to dowith the decrease of the spacer thickness because the sur-face roughness should not change so drastically. Figure 29shows the X-ray diffraction spectra of spin valves under dif-ferent oxidation conditions. Curves (a)–(d) correspond to

-6 -4 -2 0 2 4 60

2

4

6

8

10

MR

(%)

H (kOe)

Without oxidation

Heavy

oxidation

Intermediateoxidation

Light oxidation

Figure 27. MR–H curves of spin valves oxidized under different oxygendoses at the surface of the second pinned layer.

Proof's Only


10-8

10-7

10-6

10-5

10 10-3

10-2

10-1

100

10

20

30

40

50

60

70

0

2

4

6

8

10

Hin

t(O

e)

Oxygen Dose (Pa.s)

-4

MR

(%)

1

2

3

4

Figure 28. Dependence of the interlayer coupling field and MR ratioof spin valves on the oxygen dose that was used to oxidize the secondpinned layer.

cases (1)–(4) shown in Figure 28. The peak at an angle ofabout 41.2 is from the IrMn layer, while that at about 43.7

is from the rest of the layers. It is worth noting that thehigher degree peak splits into two new peaks in case (d):one at an angle of about 42.9, and the other at about 44.Although the details are not well understood at present, itseems that Cu has been oxides partially to form copper oxideor a mixture of copper oxide and CoFe oxide. The sharpnessof the interfaces has been investigated using low-angle X-rayreflectometry. As shown in Figure 30, the four curves corre-sponding to the four different cases shown in Figure 29 canhardly be differentiated from one another. This implies thatthe change in surface roughness is unlikely responsible forthe sharp increase of the interlayer-coupling field shown inFigure 28. The results demonstrate that it is difficult to forma controllable oxide at the interface between two materials ifboth can be readily oxidized. Figure 31 shows schematicallythe possible oxidation processes. Under heavy oxidation con-ditions, the loosely absorbed oxygen atoms on the CoFeOxsurface may result in a partial oxidation of the subsequentlydeposited Cu layer. This may explain the sharp increase ofthe interlayer coupling field when the oxygen exposure doseexceeds a critical value.

From the above discussion, it is clear that adding NOLat the second pinned layer/Cu interface might not bea viable approach due to its narrow process window.The next position that has been widely studied since thefirst report of the nanooxide spin valve is in the mid-dle of the pinned layer. Figure 32 shows the depen-dence of the interlayer-coupling field and the MR ratioon the oxygen exposure dose for which the spin valvehas the structure Ta3/NiF2/IrMn8/CoFe2.5/Ru0.8/CoFe1.5/

0

20

40

60

80

100

120

38 40 42 44 46

Inte

nsit

y(a

.u

.)

2θ ( )o

(a)

(b)

(c)

(d)

Figure 29. High-angle X-ray diffraction spectra of spin valves oxidizedunder different conditions. Cases (a)–(d) correspond to cases (1)–(4),respectively, in Figure 28.

0.5 2.5 4.5 6.5

Inte

nsi

ty(a

.u.)

2 (°)

(a)

(b)

(c)

(d)

Figure 30. Low-angle X-ray diffraction spectra of spin valves oxidizedunder different conditions. Cases (a)–(d) correspond to cases (1)–(4),respectively, in Figure 28.

NOL/CoFe1.5/Cu2.2/CoFe2/Cu1/Ta1.5 (thicknesses are inunits of nanometers). In this case, the oxide was formedin the middle of the second pinned layer. As can be seenfrom the figure, the interlayer-coupling field is almost con-stant at the beginning, decreases, and then increases againwhen the oxygen exposure dose increases. The MR ratio,however, increases first, then decreases, and increases again.Although it is difficult to understand the behavior of theMR, one thing that is clear is that the process window ofadding oxide at this position is much wider than that of theprevious case; thus, it is more desirable from the point ofview of manufacturability.

The last position to be discussed here is the middle ofthe spacer. This position has not been discussed before, andthus it is worth investigating how it affects the performanceof the spin valve. One tends to think that this will affectthe performance adversely because it may reduce the mean-free path of electrons. However, as shown in Figure 33, theMR ratio indeed increases with the oxygen exposure dosethat is used to oxidize the Cu layer in a spin valve witha structure Ta3/NiF2/IrMn8/CoFe2.5/Ru0.8/CoFe2/Cu1.2/NOL/Cu1.2/CoFe2/Cu1/Ta1.5 (thicknesses are in units ofnanometers). This might be attributed to the enhancementof scattering probability at the free layer/Cu/pinned layer

Cu

O

Co

Fe

(a) (b)

(c)

Cu

O

Co

Fe

Cu

O

Co

Fe

(a) (b)

(c)

Figure 31. Schematic illustration of the oxidation process of CoFe (b),and the subsequent deposition and partial oxidation of Cu (c). (a) showsthe CoFe surface before oxidation.

Proof's Only


10-8

10-7

10-6

10-5

10-4

10-3

10-2

100

101

0

5

10

15

20

25

30

9.6

9.8

10.0

10.2

10.4

10.6

10.8

11.0H

int(O

e)

MR

(%)

10-1

Oxygen Dose (Pa.s)

Figure 32. Dependence of the interlayer coupling field and the MRratio of the spin valves on the oxygen dose that was used to form theoxide at the middle of the second pinned layer.

interfaces. In this case, the MR will increase provided thatthe scattering inside the spacer is spin conserved. Furtherstudy is required to understand the underlying mechanism.

As we mentioned above, the study of the nanooxide effectis still at an early stage. A theoretical framework has yetto be formulated to guide the selection of oxides and theirpositions in the spin valves. Further experimental investiga-tions are required to study the structural properties of thenanooxide-added spin valves. Before ending this section, weshow in Figure 34 the MR–H curve of a dual spin valve witha structure Ta3/NiFe2/IrMn5/CoFe1/NOL/CoFe2.3/Cu2.15/CoFe2/Cu2.15/CoFe2.3/NOL/CoFe1.0/IrMn5/Ta1. After theoptimization of structure and processes, we have success-fully obtained an MR ratio of 27.2%, which we believe isthe highest value ever reported for all-metal spin valves.Although the exchange-bias field is too low for practicalapplications, this might be solved by replacing IrMn withother AFM materials such as PtPdMn and NiMn.

5.4. CPP Spin Valves

5.4.1. Why CPP?So far, most of the discussion has been concentrated on CIPspin valves. Although many of the fundamental concepts forCIP spin valves are directly applicable to CPP spin valves,

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

8.8

9.2

9.6

10.0

10.4

10.8

MR

(%)

Oxygen Dose (Pa.s)

Figure 33. Dependence of the MR ratio of the spin valves on the oxy-gen dose that was used to form the oxide in the middle of the spacer.The upper curve was obtained from the major loop, while the lowercurve was obtained from the minor loop. Both show a similar depen-dence on the oxygen dose.

-600 -400 -200 0 200 400 600 800 1000 1200

2.8

3.0

3.2

3.4

3.6

Res

ista

nce

(ohm

)

Field (Oe)

MR: 27.2%

Figure 34. MR–H of a double specular dual spin valve with a struc-ture Ta3/NiFe2/IrMn5/CoFe1/NOL/CoFe2.3/Cu2.15/CoFe2/Cu2.15/CoFe2.3/ NOL/CoFe1.0/IrMn5/Ta1. Here, the thicknesses are given innanometers.

there are some differences in how their performance can beimproved. In this section, we first explain why we still needCPP spin valves for data storage, followed by a discussionon possible ways for improving the performance of CPP spinvalves.

The areal density of a hard disk drive is the product ofthe linear density and track density. The former is deter-mined by many component and system parameters, whilethe latter is largely determined by the capability of lithog-raphy techniques. In what follows, we first focus on lineardensity, which is defined as

linear density = 1bit length

= channel densityPW50

(27)

where PW50 is the full-width-at-half-maximum of the read-out pulse from an isolated transition. The channel density isa parameter that reflects the efficiency of the channel cod-ing scheme (∼2.5–3). The PW50 can be estimated using thefollowing equation:

PW50 =[2g + t/22 + 4deff + a2

]1/2 (28)

where

deff =√dd + F and a = k

(deffMrt

Hc

)1/2

(29)

Here, d is magnetic spacing, g is the gap length between thesensor and the two shields, t is the thickness of the sensor, Fis the thickness of the media, and a is the transition width.Mr and Hc are the remnant and coercivity of the media,respectively. k is a parameter with a value ranging from 1to 2. The above equation suggests that there are many possi-ble ways to increase the linear density, although the reality isthat none of them is an easy task. Although one can alwaystry to push all of the parameters to their individual upperor lower limits, it would make more sense if one tries todetermine which parameter is of more relative importancefirst. To this end, we have tried to find the rate of change ofPW50 with respect to different parameters by assuming thatwe have already achieved 100 Gbits/in2. It turned out that

PW50

d= 26

PW50

MrF/Hcand

PW50

d= 21

PW50

g + t/2 (30)

Proof's Only


where the following parameters have been used for thecalculation: d = 12 nm, F = 12 nm, MrF/Hc = 125 nm,k = 1, and g + t/2 = 45 nm. It is clear that the most sensi-tive parameter is the magnetic spacing, followed by the readgap length, and then media parameters. If the areal densitycontinues increasing at an annual compound rate of 60%,in fewer than five years, we will reach 1 Tbit/in2 [268]. Now,the question is: What kind of sensor is required for this levelof areal density?

To answer this question, we first look at what are therequirements on bit length and track width for terabitrecording. Shown in Figure 13 are the estimated read trackwidth and required sensor sensitivities at different areal den-sities by assuming an aspect ratio of 4:1. The solid anddashed curves are obtained by assuming the read track widthto be 80 and 60% of the track pitch, respectively. The sen-sitivity requirement is obtained by assuming that the sen-sor will have to generate an output voltage of 1 mV. Theread head for terabit recording would require a bit lengthas small as 12 nm, assuming that the read track width is80% of the track pitch. This leads to a pulse width of about36 nm, assuming that the channel density can be increasedto 3. Assume that all other parameters are approaching theirpractical limits, such as d = 5 nm, F = 5 nm, and MrF/Hc =625 nm (maybe even smaller); then g+ t/2 must be as smallas 17 nm. This means that almost no read gap is allowedin sensors for terabit recording. This poses formidable chal-lenges for CIP read sensors because one needs electrodesbetween the two shields. Furthermore, the sensitivity of thesensor also has to be increased by a factor of 2–3, whichmeans that the MR ratio must be around 30%. This mightbe possible for advanced spin valves at the sheet film level(as we have already achieved an MR ratio of about 27%),but it would be difficult to achieve the same figure at thesensor level.

Based on this background, the CPP spin valve is beingstudied as one of the possible candidates for high-densityrecording applications [269–279]. One of the advantages ofthe CPP spin valve over the existing current-in-plane spinvalve is that it allows the elimination of the insulating gaplayer in the read head, which will contribute to the increaseof the linear density [see Fig. 35(a)]. More importantly, theoutput voltage of the CPP sensor does not scale linearly withthe width of the sensor as the CIP does. Assume that onehas a read element with width W , height H , and thicknessT , as shown schematically in Figure 35(b); the outputs ofthe sensor in the CIP and CPP modes are given by

:V CIPmax = :CIPW J

:V CPPmax ≈ :CPPT J (31)

Here, :CIP and :CPP are the resistivity change due tothe GMR effect (nominal values including the nonactiveregions), W and T are the average width and thicknessof the active region, and J is the current density. The aboveequations suggest that the CPP sensor is more suitable forultrahigh-density recording because it is much easier to con-trol the thickness than to control the lateral dimension. Thisis because the former is determined by the film thickness,while the latter is determined by lithography. Therefore, theCPP sensor is also advantageous from the manufacturability

W

D

T

CPP

CIP

sh

ield

sh

ield

electrode

(a) (b)

disk

Figure 35. (a) Schematic drawing of CPP read head in which the shieldscan also be used as the electrodes which help to eliminate the read gap.(b) Schematic of a sensor element used in both the CPP and CIP modes.

point of view. Note that the above discussion is only validfor constant current density operations. The dependence ofthe output signal on the sensor width will be relaxed for theCIP sensor when it is operated under a constant power.

5.4.2. Sensitivity Enhancementof CPP Sensors

Both theoretical and experimental studies on the CPP–GMRstarted shortly after the discovery of CIP–GMR in ferro-magnetic/nonmagnetic multilayers [280–285]. It was foundexperimentally that the CPP–GMR is larger than the CIP–GMR in multilayer samples [280, 281, 286–288]. For spinvalves, Vedyayev et al. estimated theoretically that the CPP–GMR should also be larger than the CIP–GMR [289]. Bystarting with the Boltzmann equation, Valet and Fert derivedan analytical equation for the CPP–GMR [284]. This waslater extended to the treatment of an exchange-biased spinvalve with a typical structure AFM/FM/NM/FM [290 291].In the case where the spin diffusion length is much longerthan the layer thicknesses, Bass and Pratt obtained [291]

A:R = 4G∗F tF + HAR∗F /N

2

2∗F tF + N tN + 2AR∗F /N +ARAF /F

(32)

where tF , tN , and tAF are the thicknesses of the ferro-magnetic, nonmagnetic, and antiferromagnetic layers, andARAF /F is the specific resistances at the anitiferromagneticlayer (AF )/ferromagnetic layer (F ) interface. The bulk spinasymmetry coefficient (G), interface spin asymmetry coeffi-cient H ∗F , and AR

∗F /N are defined as

G = ↓F − ↑F↓F + ↑F

H = AR↓F /N −AR↑

F /N

AR↓F /N +AR↑

F /N

∗F = ↓F + ↑F 4

AR∗F /N = AR

↓F /N +AR↑

F /N

4(33)

where ↑F ↓F and AR

↑F /N AR

↓F /N are the bulk resistivity,

and specific resistances at the F /N interfaces, for the major-ity and minority electrons, respectively. The above equa-tion was obtained by assuming that the specific resistancesbetween the ferromagnetic layer and antiferromagnetic layeror the contacts are independent of the direction of electronspin with respect to the local magnetization direction. On

Proof's Only


the other hand, in another extreme case where the spin dif-fusion length of the ferromagnetic layer is shorter than itsthickness, A:R is given by [291]

A:R = 4G∗F lFsf + HAR∗

F /N 2

2∗F lFsf + 2AR∗

F /N + N tN(34)

where lFsf is the spin diffusion length of the F layers, and allother notations are the same as those in Eq. (32).

When being used as a read sensor, the output voltageof a CPP SV sensor is given by :V = AJA:R. Here, Jis the current density whose maximum value is limitedto ∼107 A · cm−2 due to electron migration and powerconsumption considerations, and A is the head efficiencycoefficient multiplied by the dynamic range coefficient of thesensor, which is about 0.2–0.5, depending on the structure ofthe read head. Assuming A = 02 and J = 5× 107 A · cm−2,an A:R of 10 m< · 1m2 is required to obtain an outputvoltage of 1 mV. A typical spin valve, however, can onlyhave an A:R of about 1–2 m< · 1m2 [270 274 279].Therefore, the performance of the CPP SV has to beimproved before it can be applied to read heads. FromEq. (32), which applies to most of the practical cases,one can see that A:R can be increased by increasing theparameters in the numerator or decreasing the parametersin the denominator. We focus on the numerator first. Themost straightforward way to increase A:R is to increase thethickness of the F layers. However, this is not a preferredapproach because the free layer of a read sensor mustbe made as thin as possible; otherwise, it will degradethe sensitivity of the sensor. Although the values of otherparameters, G H, and AR∗

F /N , can also be increased, theircontribution to the enhancement of A:R is moderatebecause all of these parameters are determined by theproperties of the constituent materials [272]. We are nowleft with only one parameter in the numerator, ∗F , which isgiven by F /1 − G2. If one can find a way to increase theresistivity of the F layers without changing G significantly,then A:R will increase accordingly. It would be even moredesirable should G be increased simultaneously. One of thepossible approaches is to add nanooxides inside the F layers[270 274]. The nanooxide effect is twofold. One of theeffects is to increase the effective resistivity of the F layerswithout significantly changing the spin asymmetry coeffi-cient. The second effect is to enhance the specular reflectionof electrons so as to increase the number of F /N layerseffectively. Nagasaka et al. showed that the nanooxide-added CPP spin valves exhibit an A:R 25 times largerthan that of the conventional spin valves [274]. Focusing onthe denominator, Gu et al. showed that the CPP–GMR inpermalloy-based spin valves can be enhanced by increasingthe spin-memory loss in ferromagnetic layers [292]. Theparticular structure that has been investigated by Gu et al.is Nb250/Cu10/FeMn8/Py12/Cu20/Py(t1)/FeMn(t2)/Py(12-t1)/Cu10/Nb250, where Py stands for permalloy, layerthicknesses are in nanometers, t2 = 05 or 1 nm, and t1was adjustable from 1 to 11 nm. The ultrathin FeMn layerinside the free layer was used to increase the spin memoryloss due to its extremely short spin diffusion length of about1 nm. Although significant improvement in A:R has been

obtained using this technique, the maximum value of A:Robtained is still below 2 m< · 1m2.

Among all of these techniques that have been investi-gated, the nanooxide gives the most significant enhancementto A:R without the need to introduce more layers or toincrease the layer thickness physically; thus, it is desirablefor practical applications. However, the oxide must be suf-ficiently thin; otherwise, it will affect the flow of electronsacross the spin-valve stack in the CPP mode or increase thetotal resistance to an unacceptable level. However, if theoxide layer is too thin, it will lose its original functional-ity as an electron-reflection or resistivity-enhancement layer.Therefore, using oxide alone, one may not be able to obtainthe same effect as what has been obtained in the CIP mode.In addition to the number of layers, A:R also increaseswith the bulk and interface spin asymmetry coefficient, as wediscussed above. This might be achieved through using halfmetals as the electrodes, but so far, no suitable materialshave been found. Based on this background, we have pro-posed a possible solution to overcome this difficulty [277].

It is known from the research work on semiconductordevices that electrons can be reflected not only by a singlepotential barrier of large height, but also by a superlattice-type of structure with moderate barrier heights at the inter-faces, provided that the thickness of each layer is chosensuch that it is one quarter of the de Broglie wave of electrons[293 294]. This type of structure is often called an electronBragg reflector [Fig. 36(a)]. It is relatively easy to realize thiskind of structure in semiconductors because the de Brogliewavelength is relatively large. In metallic materials, how-ever, the electron wavelength is close to the Fermi wave-length, which is just about 0.1–0.2 nm. If this is the “true”length scale, it is almost impossible to build a Bragg reflectorfor electrons using metallic materials. Fortunately, due tothe discreteness of the atomic layers, the actual wavelengthof the electron packet, or so-called envelope functions, isgiven by 2,/:K [295]. Here, :K is the magnitude of the

4 4

Spin-down electrons

Spin-up electrons

FM NM

(b)

(a)

mλ nλ

Figure 36. Schematic of a spin-independent Bragg reflector (a), and aspin-dependent Bragg reflector (b).

Proof's Only


wavevector that spans the appropriate stationary points ofthe Fermi surface. This gives a wavelength of about 1.1 nmfor Cu, for example. The Bragg reflection will occur whenthe conditions dFM = m3FM/4 and dNM = n3NM/4 are sat-isfied. Here, dFM and dNM are the thickness of the ferromag-netic and nonmagnetic layers, respectively, and m and n areodd integer numbers. If one chooses m and n in the rangeof 3–4, the thickness of each layer should be well within thecontrollable range of the state-of-the-art sputtering systems.

Now, a question arises here: How can the electron flowin the CPP mode if one has a perfect Bragg reflector in thespin-valve stack? The point is that the quantum-well statesonly form when all of the ferromagnetic layers are ferro-magnetically coupled, and they only exist for the minorityelectrons [295]. In other words, the Bragg reflector in a mag-netic/nonmagnetic multiple layer is a spin-dependent reflec-tor [see Fig. 36(b)]. It reflects only the minority electrons,but lets the majority electrons flow almost freely. This willeffectively increase the bulk and interface spin asymmetrycoefficient, and thus A:R of spin valves. Note that the pic-ture given here is based on the ballistic transport of electronsinstead of diffusive transport; thus, Eqs. (32) and (34) mightnot be directly applicable to this case.

Figure 37 illustrates some of the possible structures ofCPP sensors using the electron Bragg reflectors, which con-sist of a number of pairs of CoFe and Cu. The thicknessof Cu can be chosen such that it facilitates a strong paral-lel coupling between the adjacent ferromagnetic layers, butat the same time, it satisfies the Bragg reflection condition.If the priority is to achieve ferromagnetic coupling, any dif-ference in the thickness of the Cu layer to the value thatis requested by the Bragg condition can be compensatedby the thickness of the corresponding ferromagnetic layer.The m and n are not necessary to be the same; m canbe small so as to reduce the total thickness of the Braggreflectors. The Bragg reflectors can be used either in thefree layer only [Fig. 37(a) and (b)] or in both the free andpinned layers [Fig. 37(c) and (d)]. To enhance the specular

Cu

PL

AFM

BR FL

Cu

AFM

NOL

NOL

Cu

AFM

Cu

AFM

BR FL

NOL

BR PL

NOL

(a) (b)

(c) (d)

Figure 37. Schematic of some possible CPP spin valves using bothnanooxide and Bragg reflectors. Here, BR stands for Bragg reflector,NOL refers to nanooxide, and FL (PL) is the free (pinned) layer.

reflection effect, ultrathin oxides can be added at both thetop of the free layer and the middle of the pinned layer[Fig. 37(b)]. The same approach can be taken for spin valvesusing Bragg reflectors at both the free and pinned layers[Fig. 37(d)]. Further to these steps naturally will be the onethat adopts dual spin-valve structures, as is the case withcurrent-in-plane spin valves. The functions of the nanooxideare twofold. First, it increases the specular reflection. Sec-ond, it reduces the interface roughness so that the electroninterference effect can occur.

A series of experiments have been carried out to verifythe concept. The sheet films were fabricated using ultrahigh-vacuum sputtering. The detailed sputtering conditions canbe found in [249 250]. After the sheet film was fabricated,it was patterned into elements with different sizes usingthe e-beam lithography and ion-milling processes. The CPPspin valves that have been fabricated include: (1) simplesingle spin valves without nanooxides and Bragg reflectors(type A), (2) single spin valves with a Bragg reflector freelayer (type B), (3) single spin valves with both a Bragg reflec-tor free layer and a nanooxide cap layer (type C), (4) dualspin valves with a Bragg reflector free layer (type D), and(5) dual spin valves with both a Bragg reflector free layerand nanooxides at both the top and bottom pinned layers(type E). Figure 38 shows a schematic drawing of the spin-valve structures. The Bragg reflector used has a structure[CoFe(1.4 nm)/Cu(1.1 nm)] × 5. The antiferromagnet andpinned layers are IrMn (8 nm) and CoFe (3 nm), respec-tively. The top oxide was formed through the oxidation ofNiFe, and the oxide in the middle of the pinned layer wasformed through natural oxidation of CoFe. The total thick-ness of the CoFe pinned layer was always kept constant.

To ensure a reliable measurement of the change in theresistance–area product, we first tried to determine thedependence of the junction resistance on the junction area.Figure 39 shows the typical results for types A, B, C, andE spin valves. As can be seen from the figure, the junc-tion resistance is inversely proportional to the junction area,indicating that current distribution is quite uniform over theentire junction. Similar results have been obtained for typeD sensors. Table 3 lists the RJ and :RJ of the five different

PL FLAFM Cu

BR FLPLAFM Cu

BR FL NOLPL\NOL\PLAFM Cu

Al-O insulator

(a)

(b)

(c)

(d)

(e)

(f)AFM PL1 BR FL Cu PL2 AFM

AFM PL\NOL\PL Cu BR FL Cu PL\NOL\PLAFM

Top Lead

Top Lead

Bo

ttom

Le

ad

Bo

ttom

Lead

CPP

Pillar

Cu

Figure 38. (a)–(e) Schematic drawings corresponding to sensor typeA–E. (f) Optical micrograph of typical CPP sensors.

Proof's Only


1 10 100

0.1

1

10

R(Ω

)

S (µm2)

Type E

RJ=40.0Ωµm

2

∆R=36.0m Ωµm2

1 10 100

0.1

1

10R

(Ω)

S (µm2)

Type A

RJ=1.9Ωµm

2

∆R=5.0m Ωµm2

R(Ω

)

1 10

0.1

1

S (µm2)

Type B

RJ=2.0Ωµm

2

∆R=8.0m Ωµm2

R(Ω

)

S (µm2)

1 10 100

0.1

1

Type C

RJ=5.0Ωµm

2

∆R=15.0m Ωµm2

Figure 39. Dependence of the junction resistance on the junction areafor different types of CPP spin valves.

types of spin valves. In general, we can see that both thenanooxide and the Bragg reflector can improve the perfor-mance of the spin valve. The large junction resistance is dueto the contact resistance at the sensor/electrode interface.We are currently developing a new technique to remove thejunction resistance from the measured data. But the junc-tion resistance will not affect the accuracy of the measuredA:R as much because we have already shown in Figure 39that the current distribution problem is negligible in thesesamples.

The above results show that a well-designed and fabri-cated Bragg reflector may play the same role as a nano-oxide, but the former is more suitable for CPP spin valvesbecause it does not increase the resistance substantially, asthe oxides do. The results shown here are preliminary; wecannot exclude other effects introduced by the insertion ofthe laminated free layers. But we believe that this type ofstructure will become more useful in the future when suit-able magnetic semiconductors are available because it ismuch easier to form Bragg reflectors using semiconductors.Although it is not in spin valves, a similar type of electroninterference effect has already been observed in magnetictunnel junctions grown epitaxially on a single-crystal sub-strate [296].

6. MAGNETIC TUNNEL JUNCTIONSAnother type of sensor that operates in the CPP mode isthe magnetic tunnel junction (MTJ) [26–29]. The key dif-ference between the CPP GMR sensor and the MTJ is thatthe electrical conduction in the former case is based on

Table 3. RJ and :RJ of different types of CPP spin valves.

Type A Type B Type C Type D Type E

RJ < · 1m2) 19 20 50 20 40:RJ (m< · 1m2) 50 80 15 26 36

the spin-dependent scattering effect, both inside the ferro-magnetic layers and at the ferromagnetic/nonmagnetic inter-faces, while that in the latter is based on spin-dependentquantum mechanical tunneling across a thin potential bar-rier. To have a sound understanding of MTJ, we first lookat the basic properties of a normal nonmagnetic tunneljunction. A typical quantum-mechanical tunnel junction is asandwich of two conductive electrodes separated by a thininsulating barrier layer. For a tunnel junction with a suffi-ciently high potential barrier, the tunneling current can becalculated using the transfer Hamiltonian approach, whichreads [297 298]

I = A∫

−M 2D1E + EF 1D2 × E + EF 2 + eV

× /f E + EF 1− f E + EF 2 + eV 0dE (35)

where

M = 2

2m

∫∫SJ1KJ

∗2 −J ∗

2 KJ1 · dS

is the matrix element of the transfer Hamiltonian, D1 andD2 are the density of states for the left and right electrodes,respectively, f is the Fermi distribution function, S is thecross-section area of the junction, m is the electron mass, is the Planck constant, V is the bias voltage, E is the elec-tron energy, EF 1 (EF 2) is the Fermi level of the left (right)electrode, and A is a constant. J1 and J2 are the wavefunc-tions of the electrons at the initial and final states. In thecase where the voltage applied across the junction is small,one has

f E + EF1− f E + EF2 + eV = − fE

∣∣∣∣E=EF

· eV (36)

GV = 0 = dI

dV∝ M 2D1EF D2

EF (37)

where G is the conductance. For tunnel junctions with ferro-magnetic electrodes, Eq. (37) is still valid, except thatD1 andD2 are now dependent on the spin polarization of electrons.This is known to result in a different tunneling conductancebetween the parallel and antiparallel alignment of the mag-netizations of the two electrodes, as shown schematically inFigure 40. The total conductance in the parallel configura-tion is thus approximately given by

G↑↑V = 0 ∝ D↑1 EF D

↑2 EF +D↓

1 EF D↓2 EF (38)

and that in the antiparallel configuration is

G↑↓0 ∝ D↑1 EF D

↓2 EF +D↓

1 EF D↑2 EF (39)

Here, D↑1 D

↓1 and D

↑2 D

↓2 are the density of states of the

majority (minority) electrons of the two electrodes, respec-tively. In deriving the above equation, we have assumed thatspin is conserved during tunneling. The junction magnetore-sistance (JMR) is defined as

JMR = 1/G↑↓0− 1/G↑↑01/G↑↓0

(40)

Proof's Only


)()()()()0( 2121 FFFFEDEDEDEDG

↑↓↓↑↑↓ +∝ FM1 Al2O3 FM2

curren

curren

FM1 Al2O3 FM2)()()()()0( 2121 FFFFEDEDEDEDG

↓↓↑↑↑↑ +∝

Figure 40. Schematic illustration of spin-dependent tunneling across aninsulating barrier.

which can be further reduced to

JMR = 2P1P21+ P1P2

(41)

Here,

P1 = /D↑1 EF −D↓

1 EF 0//D↑1 EF +D↓

1 EF 0

P2 = /D↑2 EF −D↓

2 EF 0//D↑2 EF +D↓

2 EF 0 (42)

are the spin polarizations of electrons in the two ferro-magnetic electrodes. This is the result that has been pre-dicted by the Jullière model [26]. Assuming a typical valueof 0.5 for P1 and P2, this simple model predicts a JMRof 40%. Note that an alternative definition for the tun-nel magnetoresistance (TMR) frequently used in the lit-erature is TMR = 2P1P2/1 − P1P2. Obviously, the TMRis always larger than the JMR, although the actual sizeof the magnetoresistance effect is the same. Although theJullière model predicts a quite large MR, it was not until1995 that MR ratios of about 20% were first reported bytwo groups for magnetic tunnel junctions at room temper-ature [28 29]. Miyazaki and Tezuka [28] reported on anFe/Al2O3/Fe tunnel junction, 1× 1 mm2 in area, that exhib-ited a TMR ratio of 18% at room temperature. The barrierlayer was formed through natural oxidation of 5.5 nm Alin air for 24 h. At about the same time, Moodera et al.fabricated magnetic tunnel junctions with CoFe and Co orNiFe as electrodes and oxygen-plasma oxidized Al as thebarrier [29]. An MR ratio of 18% (in this case, JMR) hasbeen obtained at low-bias voltage and room temperature inCoFe/Al2O3/Co junctions with an area of 6× 10−4 cm2. Thelarge MR ratio was attributed to the smoothness and smallthickness of the electrodes. In both works, the magnetic tun-nel effect was observed in large-area FM/Al2O3/FM junc-tions fabricated by shadow masks. Gallagher et al. reportedon the fabrication of microstructured magnetic tunnel junc-tions using a simple self-aligned lithographic process adopt-ing an exchange-biased structure [299]. The combinationof photo and electron-beam lithographies made it possibleto cover a range of junction areas spanning five orders ofmagnitude (10−2–103 1m3. The MR ratio obtained ranges

from 15 to 22% at room temperature and in fields of a fewtens of Oersteds. The microstructured and exchange-biasedmagnetic tunnel junctions were further developed to havea higher MR ratio (42%) and moderate specific junctionresistance (∼60 < · 1m2, and were applied to prototypesof MRAMs [10]. After these pioneer works, a tremendousamount of effort was devoted to the development of mag-netic tunnel junctions and their application in MRAMs andread heads. In what follows, we try to summarize some ofthe key issues that have been addressed in the literature formagnetic tunnel junctions.

6.1. Electrodes

6.1.1. Spin PolarizationThe simple theoretical model suggests that ferromagneticmaterials with large spin polarizations are the key to achiev-ing a high MR. For a transition metal ferromagnet, the spinpolarization is largely determined by whether the electronsparticipating in electrical conduction have an s-like charac-teristic or a d-like characteristic, or a mixture of these two.As the MR is determined by the spin-dependent density ofstates at the Fermi surface, its value is largely determinedby the extent to which the s and d bands appear across theFermi surface. However, the experimental determination ofspin polarization for each material is quite challenging. Themost straightforward way to determine the polarization isto use the spin-polarized photoemission spectroscopy tech-nique, but it lacks the necessary energy resolution. An alter-native technique that has been pioneered by Tedrow andMoservey is based on the spin-polarized tunneling across anFM-superconductor tunnel junction which gives a submilli-electron volt energy resolution [300]. Recently, a new tech-nique based on the point contact between the sample anda superconductor has been developed [301 302]. This tech-nique measures the polarization ratio using Andrew reflec-tion. The point contact setup allows the measurement ofsamples in various forms, and it also does not require the useof a magnetic field. The values of P measured for some ofthe magnetic materials using both the point-contact Andrewreflection technique and tunneling techniques [301–305] areshown in Table 4 (note that spin polarization of Co and Nishould be negative in the processes where 3d-band electronsare dominant). Among them, NiMnSb, La07Sr03MnO3, andCrO2 are so-called half metals, which theoretically shouldhave a polarization of 100%. As shown in Table 4, mostsoft magnetic materials have a polarization of 40–50%, cor-responding to a JMR of about 30–40% (or a TMR of38–67%). Theoretically, the sign of JMR can either be posi-tive or negative, depending on the signs of P1 and P2. How-ever, most of the experiments using NiFe, CoFe, Co, and Feas the electrodes and Al2O3 as the barrier have producedpositive MRs. Sharma et al. observed a negative MR inNiFe/Al2O3/Ta2O5/NiFe composite barrier junctions, whichwas attributed to the different spin polarizations of NiFe atthe two different interfaces, one with Al2O3 and the otherwith Ta2O5 [306]. Negative MR has also been observed byDe Teresa et al. in La07Sr03MnO3 (LSMO)/barrier/Co tun-nel junctions, when SrTiO3 (STO) was used as the insulatingbarrier [307]. This is in good agreement with the fact thatthe spin polarization of LSMO is positive [308], while that

Proof's Only


Table 4. Measured values of polarizations for different materials.

Materials Polarization (%) Ref.

Co 42 ± 2 [301]37± 2 [302]

Ni 465± 1 [301]32 ± 2 [302]

Fe 45± 2 [301]Ni80Fe20 37± 5 [301]Co50Fe50 53± [301]NiMnSb 58± 23 [301]La07Sr03MnO3 78± 4 [301]La2/3Sr1/3MnO3 95 [303]CrO2 90± 36 [301]Fe3O4 −39 [304]Sr2FeMoO6 60± 3 [305]

of the 3d-band density of states of Co at the Fermi levelis negative. However, when Al2O3 was used as a barrier,the MR turned back to positive, and with a very differentbias dependence. The authors concluded that the electronicstructure of the barrier and the barrier–electrode interfacehas a strong influence on the spin polarization of electrons.Similar results have also been observed for magnetic tunneljunctions using Ni40Fe60 and Fe as the electrodes and STO asthe insulating barrier [309], and in Fe3O4/STO/LSMO [310]and CrO2/natural barrier/Co [311] tunnel junctions. Theseresults imply that the barrier material and/or the electronicstructure of electrode/barrier interface play an importantrole in determining the sign of the spin polarization. Thisis consistent with the ab initio calculations of de Boer et al.on the Co(001)–HfO2 interface [312], Oleinik et al. on theCo/Al2O3/Co interface [313], and Tsymbal et al. on the Fe/Osurface [314], which suggest that the polarization of tunnel-ing electrons is governed by the mechanism of electronicbonding at the metal/oxide interface.

6.1.2. Surface Roughness (Bottom Electrode)One of the major problems that resulted in the failedattempts to produce high-MR MTJs before the early 1990sis related to the surface roughness of the first or bottomelectrode on which the insulating barrier and top electrodeare formed. If the surface roughness exceeds a certain crit-ical value, the MTJ will fail either magnetically or elec-trically or in both ways. The former is mainly caused bythe dipole or orange-peel coupling between the bottomand top FM electrodes, while the latter is caused by pin-holes formed in the thin insulating barrier. Schrag et al.reported on measurements of the magnitude of Néelorange-peel coupling due to interface roughness in a seriesof magnetic tunneling junction devices [315]. The samplesstudied were Si(100) substrate/Ta/Al/NiFe/FeMn/Co(PL2)/Ru/Co(PL1)(pinned)/Al2O3(barrier)/NiFe(free)/Al/Ta withthe thickness of the barrier, free, and pinned layers vary-ing from sample to sample. Results from magnetometry andtransport measurements are shown to be in good agreementwith the theoretical model of Néel. In addition, the authorshave also used transmission electron microscopy to directlyprobe the sample interface roughness, and obtain resultsconsistent with the values obtained by magnetometry and

transport methods. A similar type of study was also carriedout by Li et al. using high-resolution transmission electronmicroscopy [316]. With an in situ scanning tunneling micro-scope, Tegen et al. directly measured the roughness of thefilms, and found a close correspondence between the val-ues for the coupling fields determined by the magnetoopti-cal Kerr effect and the ones computed on the basis of themeasured morphology parameters [317]. The authors con-firmed an increase of the dipole coupling between the mag-netic layers with decreasing barrier thickness, as predicted bythe theoretical model [Eq. (25)]. In addition to topographicroughness, Tiusan et al. showed that, in samples involvingpolycrystalline magnetic films, beyond the orange-peel cou-pling, an important class of interaction is related to the dis-persion fields associated with magnetic inhomogeneities, ormagnetic roughness, arising from the local anisotropy fluctu-ations [318]. The roughness problem becomes more seriouswhen fabricating MTJs for read-head applications due to theroughness of the lower shield layer. Sun et al. reported onthe use of a gas cluster ion beam to smooth the shield layer,and successfully obtained an RA as low as 3.6–6.5 1m2 andan MR ratio of 14–18% for MTJs grown on the smoothedshield layers [319].

6.1.3. Effect of NonmagneticInterfacial Layers

A spin-dependent quantum well forms in an ultrathin non-magnetic metal sandwiched either from both sides by mag-netic materials or from one side by a magnetic material andthe other side by an insulator or vacuum [295 320 321]. Inthe former case, quantum-well states exist only when themagnetic layers are in parallel alignment, and in both cases,they are formed for minority electrons only. The observationof quantum-well states, however, requires atomically sharpinterfaces due to the short wavelength of electrons in met-als. For an ideal magnetic tunnel junction with a structureFM/NM/insulator/FM, theories predict an oscillation in theMR effect as a function of the NM layer thickness becausethe spin polarization of the tunneling electrons oscillates as aresult of the resonant tunneling via the quantum-well states[322–324]. Moodera et al. fabricated Co/Au/Al2O3/Ni80Fe20MTJs on liquid-nitrogen-cooled glass substrates with thethickness of Au varying from 0.1 to 1.2 nm [325]. It wasfound that the JMR decreases with the thickness of Au,which might be understood as the consequence of decreaseof spin polarization [326–328]. But what was interesting wasthat a negative JMR was observed in the thickness range of0.6–0.8 nm, which came along with an unusual bias depen-dence. The authors have shown that both results could beexplained by taking into account the quantum-well statesformed in the NM layer based on the Slonczewski model[329]. LeClair et al. also observed an inversion of sign of theMR in Co/Ru/Al2O3/Co tunnel junctions, but they attributedthis to a strong density-of-states modification at the (inter-diffused) Co/Ru interface [330].

Yuasa et al. fabricated MTJs on single-crystal Cusubstrates of the structure Co(001)/Cu(001)/Al–O/Ni80Fe20using molecular beam epitaxy, and they observed for the firsttime clear oscillation of the JMR as a function of the Culayer thickness [296]. High-resolution transmission electron


microscopy revealed that both the bottom electrode and theCu layer were grown epitaxially on the Cu substrate, leadingto atomically flat interfaces between Co and Cu and Cu andAl–O. As expected, the oscillation period at zero bias wasabout 1.1 nm, which is consistent with the value observedin GMR oscillation. As the spin polarization of electrons inCo and Ni80Fe20 was found to be positive in tunnel junc-tions with Al–O as the barrier, the negative peaks of theJMR observed in these junctions were attributed to the neg-ative polarization of electrons formed in the Cu quantumwells.

6.2. Barrier Materials

6.2.1. Different Kinds of BarriersThe key to fabricating a high-quality MTJ is to forma pinhole-free and uniform ultrathin barrier layer ofAlOx [28 29 298 331]. Although other types of insulatingmaterials have also been investigated as the tunnel bar-rier, such as Ta2O5 [332–335], AlNxOy [336–339], ZrAlOx[340 341], GaAs [342], GaOx [343], diamond-like carbon[344], ZnS [345 346], SrTiO3 [347–350, 307, 351], MgF2[352], ZnSe [353], Cu3N [354], MgO [355–359], HfOx[360–362], TaOx/AlOx [306 363], so far, AlOx was found tobe the most suitable barrier material for MTJ.

6.2.2. Formation and Characterization of AlOx

The AlOx layer can be formed using various methods, such asnatural oxidation [28, 364–370], plasma oxidation [29, 371–377], ultraviolet (UV)-light-assisted oxidation [378–381], ionbeam oxidation [382], radical oxidation [383], ozone oxida-tion [384], atomic layer chemical vapor deposition [385], andother equipment-specific techniques. In general, plasma orother energy beam-based techniques are faster and producedenser and more uniform oxides compared to natural oxi-dation, either in air or in pure oxygen. The property of theoxide formed is also dependent on the types of the processgases. Using Kr–O2 plasma, Tsunoda et al. obtained a TMRof 58.8% at room temperature after annealing the junc-tion Ta5/Cu20/Ta20/Ni–Fe5/Cu5/Mn75Ir2510/Co70Fe302.5/Al–O/Co70Fe302.5/Ni–Fe10/Cu20/Ta5 (thickness in nanometers)at 300 C, while the achieved TMR ratio of the MTJ fabri-cated with the usual Ar–O2 plasma remained 48.6% [386].This improvement is remarkable considering the fact thatthe MR obtained so far using other oxidation methods isabout 50% [387].

Under Oxidation of Al High-resolution transmission elec-tron microscopy study by Bae et al. [388] revealed thatnatural oxidation occurred preferentially through the grainboundary of Al grains, leading to isotropically expandedAlOx grains when Al is fully oxidized. On the other hand,in plasma oxidation, a flat AlOx layer formed uniformly onthe Al layer, leading to a sharp interface with the underlyingmetallic Al layer. Ando et al. conducted a systematic studyof the microstructure of the barrier oxide formed using dif-ferent methods [389]. It was found that it is easier to forma uniform oxide within a short time period using plasmaoxidation. This is because, in addition to the full cover-age of the Al surface, the energetic oxygen species are alsoable to penetrate into the grain boundaries. On the other

hand, in the case of natural oxidation, it is difficult for oxy-gen to penetrate into the grain boundaries. Therefore, theas-deposited MTJs with barrier layers formed by plasma oxi-dation normally show a higher MR as compared to thosewith naturally oxidized barriers. Mitsuzuka et al. studied theinterface structure of magnetic tunnel junctions with natu-rally oxidized AlOx barriers using X-ray photoelectron spec-troscopy (XPS) [390]. The MTJs studied had a structure Fe(50 nm)/AlOx/CoFe (30 nm), where the barrier layer wasformed through natural oxidation of Al with a thicknessranging from 0 to 5 nm. It was found that the MR showeda maximum value when Al is about 2–3 nm in thickness.The XPS analysis showed that an Al layer thicker than 1 nmcovers the entire surface of the lower electrode. However,unoxidized Al remains when Al is thicker than 1 nm. For Allayers greater than 3 nm, the MR ratio is strongly affected byunoxidized Al, probably due to the decrease of spin polariza-tion caused by the unoxidized Al layer. Zhang et al. reportedon the effect of natural oxidation conditions on the per-formance of MTJs. It was found that natural oxidation ata lower oxygen pressure produces an MTJ with a lowerRA and a smaller MR ratio as compared to those formedby a higher pressure oxygen oxidation [368]. Rutherfordbackscattering analysis confirmed the inadequate oxidationof Al at low oxygen pressure. Natural oxidation and UV-assisted oxidation are suitable for formation of MTJs of alow area–resistance product using ultrathin Al layers [391].It was reported that a two-step or multiple steps of naturaloxidation helps to improve the uniformity of the oxidizedbarrier layer [369 392].

Pinholes The quality of the barrier layer plays a domi-nant role in determining the characteristics of the tunneljunction [393]. Allen et al. reported on the imaging of pin-holes by electrochemical decoration of the pinholes usingCu [394]. The results obtained were found to be consistentwith the breakdown voltage analysis [395]. Dimopoulos et al.reported that the “defective” sites of the barrier can alsobe detected using a barrier impedance scanning microscopytechnique [396].

Inhomogeneity and Defects of the Barrier Layer Andoet al. studied the local current distribution of MTJ witha structure Ta5/Ni80Fe205/IrMn15/Co5/Al1.3-oxidation usingconducting atomic force microscopy [397]. Here, thicknessis given in nanometers. It was found that the current distri-bution can be explained by assuming a Gaussian distributionfor the barrier thickness with a mean value of 1.2 nm anda standard deviation of 0.1–0.15 nm. A similar study wasalso carried out by Luo et al. [398]. Rippard et al. used bal-listic electron emission microscopy to study thin aluminumoxide tunnel junction barriers formed both by magnetronsputter deposition and thermal evaporation [399, 400]. Itwas found that the barriers made by oxidation of evapo-rated Al become fully formed at a thickness as small as0.6–0.7 nm, while a thicker Al (0.9–1.1 nm) layer is nec-essary to form a continuous barrier by magnetron sputter-ing. Although the decrease of barrier thickness generallycontributes to the reduction of RA, if the barrier is toothin, it will also cause reliability problems when the MTJsare used as read sensors or storage cell elements. Oliveret al. reported on dielectric breakdown studies on magnetic


tunnel junctions having ultrathin barriers [401]. The MTJs,with a structure Ta5/PtMn25/CoFe2.2/Ru0.9/CoFe2.2/Al-oxidation/CoFe1/NiFe2.5/Ta15 (thicknesses are in nano-meters), were fabricated by magnetron sputtering andpatterned by deep ultraviolet photolithography to a junc-tion area of about 0.2 1m2. The tunnel magnetoresistancewas 15–22% and RA 7–22 < · 1m2 for junctions having a4.75–5.5 Å Al layer oxidized naturally. The authors observedtwo types of breakdown: abrupt dielectric breakdown at aneffective field of 10 MV/cm determined by the thicknessof the tunnel barrier, and a gradual breakdown related todefects in the tunnel barrier. After the breakdown, a metallicpinhole is created, the size of which depends on the max-imum current applied to the junction. The current flowingthrough the pinhole creates a strong circular magnetic fieldthat curls the local magnetization in the free layer aroundthe pinhole. The subsequent free-layer reversal is very sen-sitive to the pinhole location. The electric properties afterbreakdown can be well described by an ohmic resistor and atunnel magnetoresistor connected in parallel. Kikuchi et al.also reported similar types of phenomena in the breakdownprocesses [402]. The existence of pinholes in low-junction-resistance samples has been discussed in the context of asize-dependent breakdown voltage that decreases by increas-ing the junction size.

6.3. Annealing Effect and Thermal Stability

Postgrowth annealing at temperatures in the range of 220–400 C was found to be effective in improving the perfor-mance of magnetic tunnel junctions, beyond which annealwill deteriorate the performances of MTJ [403]. The highesttemperature at which the MR starts to decrease monotoni-cally with temperature is highly dependent on the materialsand device structures. This has a significant impact on theapplication of MTJs in MRAMs because standard back-endtechnology for the metallization of CMOS circuits requiresannealing in the forming gas at 350–450 C. Many attemptshave been made to reveal the mechanisms for MR enhance-ment at intermediate temperatures and the loss of the MRat elevated temperatures. We summarize some of the resultsreported in the literature below.

6.3.1. Effect on the Barrier LayerAnneal at moderate temperatures was found to improvethe quality of the barrier layer through decreasing itsthickness, increasing its barrier height, improving its homo-geneity at interfaces with the electrodes, and reducingthe defects inside the barrier. Parkin et al. reported onthe thermal annealing study of an MTJ with a structureCr80V2025/Co75Pt12Cr1315/Al–plasma oxidation/Co15/Al20or Co88Pt1215/Al20 [404]. Here, the layer thickness is inunits of nanometers. It was found that the MTJ is thermallystable at temperatures greater than 300 C. A comparisonof cross-section transmission electron micrographs of anuntreated sample and a similar one annealed at 350 Cindicates that the thickness of the amorphous tunnel barrieris slightly decreased after annealing. The resistance–areaproduct and the MR of the devices initially increase slightlyfor lower temperature annealing treatments up to about

200–250 C. For annealing at higher temperatures, boththe MR and the resistance–area product decrease mono-tonically, although the MR decreases at a more rapid rate.MR values of ∼10% were still obtained after annealing to∼350 C. Due to the thick barriers used in these particularsamples, the resistance–area product is in the range 5–6M< · 1m2.

Sousa et al. reported on the thermal annealing effectof exchange-biased MTJs. Two series of samples werestudied: one series with resistance–area products of10–13 M< · 1m2, and the other with resistance–areaproducts of 25–30 k< · 1m2 [405]. The former has a struc-ture Ta7/Cu4/Ta7/NiFe6/CoFe3/Al2O3/CoFe3/MnRh18 orTbCo12/Ta3, and the latter has a structure Ta7/NiFe10/CoFe2/Al2O3/CoFe4/MnR17/Ta3. The barriers of sampleswith higher resistance–area products were formed deposit-ing 1.8 nm Al, followed by 90 s plasma oxidation with adensity of 6 mW/cm2, and those of the lower resistance–area product samples were fabricated depositing 1.1–1.3nm Al, and then followed, respectively, by 20 and 15 splasma oxidation at a power density of 4 mW/cm2. Thejunction tunneling magnetoresistance was found to increasefrom 22–26% in high-resistance samples and 22–37% inlow-resistance samples, upon anneal up to 200–230 C.Rutherford backscattering analysis suggested that annealimproved the asymmetry in both the oxygen distributioninside the barrier and the junction parameters. The MRincrease in lower resistance samples was attributed to theincrease of the barrier height. The increase of the barrierheight and decrease of the barrier thickness were alsoobserved by Nowak et al. [406].

Ando et al. studied the annealing effect on MTJshaving the structure Ta/(Cu,Pt)/Fe20Ni80/IrMn/Co75Fe25/Al-oxide/Co75Fe25/Fe20Ni80/Ta [407]. When the Al thickness,oxidation time, and annealing temperature were 0.8 nm,15 s (10 s), and 300 C (250 C), the TMR, RA, andTMR enhancement ratio against the as-grown sampleswere 49% (31%), 1.1 k< · 1m2 (230 < · 1m2, and70% (100%), respectively. In order to investigate theannealing temperature dependence of the TMR ratio,the authors measured the local electrical properties fora Ta/Fe20Ni80/Pt/Fe20Ni80/IrMn/Co75Fe25/Al-oxide multilayerusing the conductive atomic force microscopy technique.The current image became very homogeneous after anneal-ing at around 300 C for 1 h. The increase of the TMR ratioof the junction after annealing was thus attributed to boththe increase of barrier height and the decrease of barrierheight fluctuation. The degradation of TMR after annealingin excess of 350 C was due to the decrease in barrier height,leading to the increase of leakage currents.

Schmalhorst et al. systematically investigated the struc-tural, magnetic, magnetotransport, and tunneling propertiesof CoFe(1.5 nm)/Ru(0.9 nm)/CoFe(2.2 nm)/Al2O3/Ni81Fe19(6 nm)/Ta (5 nm) junctions for different Al thicknesses andoxidation times after isochronal annealing up to 500 C. Themean breakdown voltage of the junction increases with tem-perature, saturates at an annealing temperature of about300–350 C, and remains constant up to almost 500 C. Thisimplies that Al oxide is extremely stable, and its stability isimproved by thermal annealing. The latter might be relatedto the defect reduction inside the barrier layer [408].


6.3.2. Effect on ElectrodesDimopoulos et al. reported that, compared to Co/Ru/Co,Co/Ru/Co50Fe50 is more advantageous when being used asthe pinned layer in MTJs due to its excellent thermal stabil-ity [409]. After successive annealing steps up to 400 C, mag-netic tunnel junctions with structures Cr (1.6 nm)/Fe (6 nm)/Cu (30 nm)/Co (1.8 nm)/Ru (0.8 nm)/Co50Fe50 (2.8 nm)/Al (1 nm)–plasma oxidation/CoFe (1 nm)/ Fe (6 nm)/Cu (5 nm)/Cr (3 nm) as pinned layers were found to stillpresent a significant tunnel magnetoresistance of nearly20%, and an almost intact magnetic rigidity of the hardmagnetic system. The improvement in thermal stability wasattributed to the improvement of the Ru/CoFe/AlOx inter-faces in terms of interdiffusion.

Due to the finite and negative enthalpies of formation ofmost transition metal oxides, the electrodes will also be par-tially oxidized during annealing, as revealed by X-ray pho-toelectron spectroscopy studies. However, the mechanismstill remains unclear regarding how the partially oxidizedinterfacial layers are related to the enhancement or degra-dation of the tunnel magnetoresistance [410]. The oxidationof the electrode might be suppressed, if it has adverse influ-ences on the MTJ, by inserting a thin layer of magneticoxide at the barrier/electrode interface. During the anneal-ing process, due to the much larger and negative enthalpy ofAlOx as compared to most magnetic oxides and the largeraffinity of oxygen for Al, oxygen will tend to move to thebarrier layer instead of the electrode. Using this technique,Zhang et al. fabricated tunnel junctions with an interposedFe oxide layer between the Al2O3 barrier (tAl = 8–9 Å)and the top CoFe pinned layer, and obtained a large tun-neling magnetoresistance of 39% after 40 min anneal at380 C [411 412]. The annihilation of the CoFe oxide dur-ing postgrowth annealing was also observed by Dimopouloset al. in Cr (1.8 nm)/Fe (6 nm)/Cu (30 nm)/Cr (1.8 nm)/Fe(6 nm)/Cu (30 nm)/Co50Fe50 (1 nm)/Al–plasma oxida-tion/Fe (6 nm)/Cu (10 nm)/Cr (5 nm) samples, leading tothe improvement of aluminum oxide’s stoichiometry [413].Matsukawa et al. fabricated MTJs with structures of bottomelectrode/PtMn/CoFe/Ru/ CoFe/Fe1–xPtx/Al oxide/Fe1–xPtx/NiFe/top electrode (x = 005–0.75), in areas from 2 × 2 to30 × 30 1m2, using conventional photolithography and theion-milling method. After a postgrowth anneal at 400 and420 C, the TMRs of MTJs with Fe1–xPtx (x = 01–0.2) werestill about 40 and 30%, respectively [414].

6.3.3. Diffusion of MnIn addition to oxidation of electrodes, overoxidation ofAl will also cause oxidation-induced diffusion of Mn fromthe pinning layer to the barrier layer during the post-growth annealing. Samant et al. investigated the thermalstability of MTJs with a structure Si(100)/0.5 1m thickSiO2/Ti5/Pd15/Ir23Mn77 or Fe50Mn5014/Co80Fe202.4/Al1.3–plasma oxidation/Co80Fe203/Pd1.5 (thickness is givenin nanometers) using near-edge X-ray absorption fine-structure spectroscopy [415]. It was found that structureswith IrMn antiferromagnetic exchange-bias layers are muchmore thermally stable than similar structures with FeMnexchange-bias layers. In either case, however, diffusion of

Mn from the antiferromagnetic layer through thin exchange-biased ferromagnetic layers to the tunnel barrier is observedat elevated temperatures (>300 C). Yoon et al. investigatedthe Mn diffusion in an NiFe/MnIr/CoFe/AlOx multilayerafter annealing at 300 C using Auger electron spectroscopyand X-ray photoelectron spectroscopy, wherein it was foundthat the magnitude of Mn diffusion is correlated to theexcess oxygen generated from the plasma oxidation of theAlOx tunnel barrier [416]. The analysis showed that Mndiffusion was driven by the preferential oxidation of Mnat the oxide insulator interface due to its larger (negative)enthalpy of formation compared to iron and cobalt oxides.Saito et al. reported on the thermal stability study ofdual-spin-valve-type double-tunnel junctions with structuresNi–Fe/Ir–Mn/Co50Fe50/AlOx/Co90Fe10/AlOx/Co50Fe50/Ir–Mn/Ni–Fe and Ni–Fe/Ir–Mn/Co50Fe50/AlOx/Co50Fe50/Ni–Fe/Co50Fe50/AlOx/Co50Fe50/Ir–Mn/Ni–Fe, fabricated usingphotolithography and ion-beam milling [417]. In order toclarify the mechanism of the loss of the MR ratio and thatof V1/2 (bias voltage at which the MR decreases to half ofthe zero-bias value) above 320 C, the authors carried outan XPS study in Ni81Fe19/Ir–Mn/Co50Fe50/AlOx multilayersannealed at various TA. The Mn oxide peaks in addition tothe Mn peaks (both 2p1/2 and 2p2/3 peaks were observed)were observed in the temperature range ≥300 C. It wasfound that a small amount of Mn and Mn oxide reached theCoFe/AlOx interface at TA = 300 C due to interdiffusionof Mn. The oxygen redistribution and homogenization occurbetween AlOx and Mn in the range of TA > 320–350 C,leading to an increase of defect states in the barrier. Thisincrease in defect states above 320–350 C was assumedto be responsible for the decrease of both the MR ratioand V1/2 due to the spin-independent two-step tunnelingvia defect states in the barrier. The Mn diffusion might besuppressed by increasing the pinned layer thickness [418].

6.4. Basic Characteristics of MTJ

A practical exchange-biased magnetic tunnel junction has astructure that is very similar to that of a spin valve, exceptthat the Cu spacer is replaced by an ultrathin insulating layer(see inset of Fig. 41). Figure 41 shows a schematic of a typi-cal MTJ structure. When measuring the magnetic response,a constant current is applied through one of the top andbottom electrodes, and the voltage drops across the othertwo terminals are measured when a magnetic field is sweptwithin a certain range from a positive value to a negativevalue. However, caution must be taken when interpretingthe data because the measured MR could be affected sig-nificantly by the current distribution effect [419–424].

Although the MR ratio is much higher than that of atypical spin valve, the MTJ does have a few drawbacks com-pared to spin valves when they are used as read sensors.

I

V

PL FLAFM I

Figure 41. Typical structure of a magnetic tunnel junction device.

Proof's Only


These include: (1) large junction resistance, (2) bias volt-age dependence of the MR, and (3) temperature depen-dence of MR. The junction RA of an MTJ can be as highas on the order of M< · 1m2. Although it can be loweredby reducing the barrier thickness and height, this will gener-ally result in a lower MR ratio. For example, the MR ratiodrops to about 13% when the RA is reduced to 5 < · 1m2

[425]. As an example, Figure 42 shows the magnetoresis-tance curve of a typical MTJ with a junction area of 80 1m×80 1m fabricated in the author’s group. For this specificdevice with a structure Ta20/Ni81Fe1920/Co90Fe102/Al(1 nm)-oxidation/Co90Fe102/Ni81Fe197/Fe50Mn5020/Ta5 (the thick-nesses of other layers are given in nanometers), the TMRratio is 44.6% at a dc bias of 4.3 mV and the RA is about250 k< · 1m2 [426]. Shown in Figure 43 is the bias volt-age dependence of the MR ratio of this particular MTJ.As can be seen from the figure, the MR drops to almosthalf when the bias voltage is increased to about 0.4 V.Although this value does vary with the MTJ structure, itis roughly in the range of 0.3–0.5 V for most of the MTJstructures [427–435]. Although the mechanism is still notfully understood, it is believed that it is caused by both thebias-induced change in the barrier profile and the energy-dependent density of states for both the majority and minor-ity electrons [436 437]. It is argued that magnon excitationin the magnetic layer might also be one of the possible rea-sons [438]. The energy-dependent density of states modelcan be roughly understood as follows. From Eq. (41), onecan find that

JMRV

= 21+ P1P22

P1P2

V(43)

which reduces to JMR/V = 4P/1+ P 22P/V ,when P1 = P2 = P . For electrodes with positive spinpolarizations, the JMR will decrease with voltage whenP/V < 0. Substituting Eq. (42) into P/V < 0, oneobtains D↓D↑/V −D↑D↓/V < 0, which can befurther simplified to lnD↑/D↑/V < 0. This impliesthat the JMR will drop if the ratio between the density ofstates for majority electrons and that of the minority elec-trons decreases with the bias voltage. As we will discussshortly, the decrease of JMR with bias voltage is undesirablefor practical applications. One of the possible approaches

Magnetic Field (kOe)

Res

ista

nce

(Ω)

Res

ista

nce

(Ω)

-0.440

45

50

55

60

65

-0.2 0.0 0.2 0.4

-40 -20 0 20 4040

45

50

55

60

65

Field (Oe)

TMR-44.5% Voc-4.3mV60µmx40µm

Figure 42. MR–H curve measured at room temperature for an MTJwith a structure Ta20/Ni81Fe1920/Co90Fe102/Al(1 nm)-oxide/Co90Fe102/Ni81Fe197/Fe50Mn5020/Ta5. Inset shows the minor curve. Here, the thick-nesses are given in nanometers. Reprinted with permission from [426],K. B. Li et al., J. Magn. Magn. Mater. 241, 89 (2002). © 2002, Elsevier.

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600

Bias Voltage (mV)

No

rm

ali

zed

MR

ra

tio

Figure 43. Bias voltage dependence of the MR ratio of an MTJwith the structure Ta20/Ni81Fe1920/Co90Fe102/Al(1 nm)-oxide/Co90Fe102/Ni81Fe197/Fe50Mn5020/Ta5. Here, the thicknesses are given innanometers.

for reducing the bias voltage dependence is to use a double-tunnel junction [439]. Using the double junctions, it is pos-sible to raise the voltage at which the MR drops to half ofthe low-bias value to 0.9 V, which is almost double that ofa single-tunnel junction [440]. The voltage dependence canalso be reduced using Hf-inserted AlOx barrier layers [441].

In addition to voltage dependence, the MR ratio of MTJwas also found to decrease with temperature [430, 405, 442–446]. According to Shang et al. [442], Hagler et al. [444],and Wingbermühle et al. [445], the temperature-dependenttotal conductance of an MTJ can be written as

G = GDIT /1+ P1T P2T cos0+GINT (44)

where GDIT indicates the direct tunneling conductanceand GINT represents the spin-independent conductancevia localized states, is the angle between the magnetiza-tions of the two electrodes, and P1T and P2T are thetemperature-dependent spin polarizations. The temperaturedependences of GDIT and spin polarizations [P1T andP2T ] are due to the temperature dependences of Fermidistribution and magnetizations, respectively, and are givenby GDIT = CG0T /sinCT with C = 139 × 10−4d/

√B,

and P1T = P011−A1T3/2 P2T = P021−A2T

3/2. Here,A1 A2 is a material-dependent parameter, d is the barrierthickness in angstroms, B is the barrier height in electron-volts, and G0 is the value of GDIT at zero degrees Kelvin.

6.5. Applications of MTJ in Read Head

Magnetic tunnel junctions can be used either as read sensorsin hard disk drives [447–453] or memory cells in MRAMs.Following are some advantages and drawbacks of MTJscompared to spin valves when they are used as read sensors.

6.5.1. Large Output SignalFor an MTJ sensor element with width W , height H , andthickness T , the maximum output voltage is given by

:VMTJ =I:RA

W ·H = J:RA (45)

where J is the current density, and :RA is the change inresistance–area product of the MTJ. The maximum outputvoltage refers to the value that is obtained when the magne-tizations of the two FM layers are switched from a parallelalignment to an antiparallel alignment or vice versa. Theactual output is normally a fraction of this due to the limiteddynamic range of the read sensor and the head efficiency

Proof's Only


factor. In practice, however, the output is limited by thecharacteristic dependence of JMR on the dc bias voltage,that is [454],

:VMTJV = VB ·MR0 · Vmax − VB/Vmax (46)

where VB is the bias voltage, MR0 is the MR ratio at zerobias, and Vmax is the voltage at which the MR becomes zero.In deriving the above equation, we have assumed that theMR ratio decreases linearly with the bias voltage. The max-imum output signal is thus given by

:VMTJVB = Vmax/2 =12Vmax ·MR0 (47)

Assume that MR0 = 5–10% at device level, and Vmax =800 mV; it gives an output voltage of 20–40 mV. On theother hand, the maximum output signal of a CIP spin valvewith thickness T , width W , current density J , and sheetresistance change :RS is given by

:VSV = J · :RS · T ·W (48)

Using typical values of J= 1×107 A/cm2, :RS = 4 <, T =30 nm, and W = 1 1m, the maximum output voltage willbe given by 12 mV. Of course, the actual output signal forboth cases when being used as a read head is much smallerthan these values due to the low head efficiency. We can seethat the maximum output signal of an MTJ is much higherthan that of a typical spin valve. Moreover, the output of thespin valve scales with its width. Therefore, the MTJ headbecomes more advantageous than the CIP spin-valve headin terms of output signal when the track width shrinks intothe submicron regime, which is already the case in the latestproducts.

6.5.2. Current-Perpendicular-to-PlaneOperation

As is with the case of CPP GMR sensors, it is possible toremove the insulating gap in the MTJ heads, and thus theyare more suitable for higher linear density heads.

6.5.3. Large Junction ResistanceThe primary drawback of MTJ is its large junction resis-tance, in particular, when its size shrinks. The large junctionresistance will result in both a small bandwidth and a lowSNR of the read signal [455]. For high-frequency operations,the sensor resistance is required to be less than 100 < [425].Figure 44 shows the dependence of the junction resistanceas a function of the reader width by assuming that the sen-sor has a square shape and the resistance–area product is1 < ·1m2. Also shown in the figure are the estimated sensorwidths for applications at different recording densities. It isclear that the MTJ may not be suitable for read heads atan areal density of more than 200 Gbits/in2. Jury and Wangreported on a possible approach to extend the bandwidth ofthe MTJ head using a simple buffer amplifier, which leadsto an SNR improvement of 5–10 dB over a no-buffer config-uration for frequencies between 100 MHz and 1 GHz, witha slight reduction of SNR at low frequencies [456].

10

100

1000

10000

0 50 100 150 200 250

Reader width (nm)

Junct

ion

resi

stan

ce(Ω

)

100G

b/i

n2

200G

b/i

n2

500G

b/i

n2

1T

b/i

n2

100Ω

RA = 1Ωµm2

Figure 44. Junction resistance of MTJ as a function of reader width(see text for the assumptions that have been used to calculate theresistance).

6.5.4. Noises in MTJsIn addition to thermal noise, the MTJ also suffers from shotnoise which increases with the sensing current [425 455],1/f , and/or telegraph noise [457–461]. This makes thesignal-to-noise ratio of MTJ smaller than that of the spinvalve, even when the output signal is the same. The shot-noise voltage is given by VS = 2eVRB1/2, while that ofthermal noise is given by Vth = 4kBTRB1/2. Here, e is theelementary charge of electrons, V is the bias voltage, R isthe resistance of the sensor element, B is the bandwidth,kB is the Boltzmann constant, and T is the temperature.Assuming V = 400 mV and T = 300 K, one can easily findthat VS/Vth = 28. The dominance of shot noise in the MTJwas found by Shimazawa et al. in their front flux guide-type magnetic tunnel junction heads [448]. George et al.reported that, depending on the quality of the barrier, themeasured value of shot noise in an MTJ can be much lowerthan that predicted by theory [462]. In tunnel junctions withpinholes, the shot-noise voltage is reduced by a factor of/RP/RT +RP01/2, with RT and RP the ideal tunnel resis-tance and the lumped parallel resistance due to the con-ductive paths of pinholes, respectively. Unfortunately, thedecrease of shot noise due to pinholes does not necessar-ily lead to the increase of SNR because the pinholes alsoresult in a decrease of JMR. Although it is not specific toMTJ, all types of MR heads also exhibit magnetization noisewhich may exceed the thermal noise when the sensor volumebecomes very small [463].

Using the latest MTJ heads, Mao et al. [464] demon-strated an area density of 120 Gbits/in2. Further optimiza-tion in materials, structures, and fabrication processes mayimprove the MTJ technology further, and render them suit-able for higher areal density applications.

7. MAGNETIC RANDOMACCESS MEMORY

7.1. Introduction

Although spin valves and MTJs have been invented forhard disk drive applications, they have also found importantapplications in memory devices. Computer memories featur-ing the following features are always highly demanded: (1)nonvolatility, (2) high density, (3) short cycle time, (4) low-power consumption, (5) low cost, (6) high reliability, (7) infi-nite lifetime, and (8) radiation hard (for special-purpose

Proof's Only


applications). A variety of memories have been com-mercialized. These include DRAM, SRAM, ROM, FlashEEPROM, and FeRAM. However, none of them is able tosatisfy all of the requirements. Furthermore, most of thesememories (e.g., DRAM) store information using charges; itis difficult to perform down-size scaling once it reaches acertain dimension due to the degradation of SNR. MRAMhas been studied intensively as one of the promising candi-dates to replace some of the existing memories [9–11, 465–473]. In the case of disk drive applications, the spin valvesand MTJs are employed as sensors to sense the magneticfields from the information bits recorded on the hard disks.But when being used as cell elements in memories, the sen-sors themselves are used to store the information. The basicstructure of the memory cell is still the same as that of thesensor, but in the former case, one makes use of the satura-tion region of SVs and MTJs rather than the linear regionused in read sensors.

7.2. Different Kinds of MRAM Designs

The concept of magnetic random memory itself is not new,but the early magnetic memory did not become populardue to both the drawbacks of its own design and the rapidadvances made in semiconductor memories. However, theconcept of using two crossed wires to write cells selectivelyis still applicable to current MRAMs. The key differencebetween the early magnetic memory and today’s MRAM isthat the former used an inductive signal to detect the stor-age state, while the latter utilizes AMR [9], GMR [474–479],or the spin-dependent tunneling phenomenon [10 11 299]to detect the signals. This has made it possible to removethe coils from the memory arrays, and at the same time,to increase the readout sensitivity, leading to the miniatur-ization of the cell elements. The first AMR-based MRAMwas developed by Honeywell in the mid-1980s. However, itsapplication was mainly limited to certain niche markets dueto its high cost and relatively poor performance attributableto the low MR ratio. With recent advances in GMR andMTJ technologies, renewed interest has arisen in MRAMssince the mid-1990s. Due to the large read/write currentand a relatively low SNR of large cell arrays of GMR-basedMRAMs, most of the recent works on MRAMs are basedon the MTJ design.

Figure 45 shows a schematic of an MRAM design usingMTJs and the corresponding cell structure using one MTJand one transistor per cell. The memory cells are formed atthe cross junctions of the bit and word lines. Each memorycell consists of an MTJ for storing the information and atransistor for reading the cell selectively. The transistor canalso be replaced by either a diode [480 467] or can even beremoved from the cell, as will be discussed shortly. A typi-cal MTJ cell consists of a pinning layer to pin the referencelayer, a storage layer to store the information, and a thininsulator to separate the two magnetic layers. The signalreadout is based on the spin-dependent tunneling resistanceof the MTJ by supplying a small sensing current to a specificcell. As the sensing current is only used to detect the rel-ative orientation of the magnetization of the two magneticlayers instead of being used to switch the magnetization,it can be kept at a relatively low level. This is in contrast

Bit line

Word line

Selected

cell

(a)

CMOS

wordline

MTJ

gate

source drain

(b)

bit line

Figure 45. Schematic of the MRAM architecture using MTJ (a) andmemory cell design using a transistor and an MTJ (b).

to the readout process of MRAMs using pseudospin valves,wherein larger currents are needed to switch the magnetiza-tion of the free layer from one direction to the opposite soas to detect the state of the stored bit (the so-called dynamicreadout process).

7.3. Write Operation of MRAM

The write operation can be understood by looking at how asingle-domain particle switches under two orthogonal fields,as shown schematically in Figure 46. Assuming that the mag-netization switching occurs via coherent rotation, or basedon the so-called Stoner–Wohlfarth (SW) model, the switch-ing field can be obtained through the minimization of theenergy density:

= −HMs cosB− +Ku sin2

where H is the external magnetic field, Ms is saturationmagnetization, Ku is the anisotropy energy constant, and and B are the angle between the easy axis and Ms and H ,respectively. Note that here only the Zeeman energy andthe anisotropy energy are taken into account. Minimizingthe energy density gives

h⊥ = − cos3 h = sin3 or h2/3 − h2/3⊥ = 1 (49)

Here,

h = HMs/2Ku =H

2Ku/Ms

h⊥ = H⊥Ms/2Ku =H⊥

2Ku/Ms

(50)

are the reduced parallel and perpendicular components ofthe external field. Equation (44) gives an asteroid curve, asshown in Figure 47. To achieve magnetization reversal, themagnetic field vector must be such that its endpoint endsoutside the asteroid (point B, for example). For some of the

θφM

H

Hard axis

Easy axisH//

H⊥

Figure 46. Schematic of the SW model for a single-domain particle.

Proof's Only


B

A

h⊥

h||

1

1

-1

-1

mmh

A

θ*

Figure 47. Asteroid curve for a single-domain particle.

points inside the asteroid, although one of the field com-ponents is larger than that of point B, their magnetizationsstill cannot be switched. This simple mechanism makes itpossible to write the individual cells selectively [481 482].

In practice, however, the magnetization reversal processof the memory element is much more complicated than whatis predicated by the simple single-domain model [469]. Themagnetization switching process of rectangular elements isusually unrepeatable due to the formation of edge domainsat the flat ends of the element. Elements with taperedends have been introduced to suppress the formation ofthe edge domains [483]. Although the switching processbecomes relatively repeatable, the tapered ends result in anincrease in the switching field. An excellent review of themicromagnetics of MRAM was given by Zhu and Zheng[484]. Using Lorentz transmission electron microscopy, Kirket al. [485 486] and Yi et al. [487] found that the switch-ing field of small magnetic elements can either increaseor decrease, depending on whether the ends are gentlycurved or exhibit sharp corners. In element arrays, themagnetostatic interactions among the elements were alsofound to affect the switching process of individual elements[488 489]. A unique design that makes the switching pro-cess repeatable, and yet is more suitable for ultrahigh-density memory is the vertical magnetoresistive randomaccess memory (VMRAM) [471 476 484]. The VMRAMcell consists of a ring-shaped CPP spin valve with two FMlayers separated by a nonmagnetic layer. The formation of astable flux-closure configuration of the magnetization makesthe switching process more robust and predictable.

The formation of edge domains is also sensitive toprocess-related defects in the magnetic element. This wasfound to result in fluctuations of the switching field amongdifferent cells [10]. As one of the examples, Figure 48 showsthe M–H curve for a double-tunnel-junction MRAM withfour cells of the same size fabricated in the author’s group.Although they were fabricated using identical processes andwere placed one next to another on the same wafer, theswitching fields differ from one another. In addition to thefluctuation of switching fields among different cells, the shiftof the switching curve along the field axis also requires alarge tolerance for the switching current [490]. The latterstems from various magnetic couplings between the free andpinned layer, and thus must be minimized by process opti-mization. In the case of mass production, how to achievea narrow distribution for the switching field for millions orbillions of cells is a great challenge.

130

140

150

160

170

-100 -50 0 50 100

Magnetic Field (Oe)

R(kΩ

)

Figure 48. M–H curves of four identical cells with the structure Cr/Cu/Cr/Ta /NiFe /IrMn /CoFe/Ru/CoFe /AlOx /CoFe /NiFe/CoFe/AlOx/CoFe/Ru/CoFe/IrMn/Ta.

In addition to switching field fluctuation reduction, effortshave also been made to reduce the absolute value of theswitching field, which is proportional to the write current.Currently, the write current is about a few milliamperes; itwill increase to more than 10 mA when the element sizeshrinks to less than 100 nm [491]. Tehrani et al. reportedthat the effective write current can be reduced by cover-ing the word line (Cu wire) with a high-susceptibility softmagnetic layer from all sides but the side that is facing theMTJ cell [492]. Under optimum conditions, it is possible toreduce the write current by a factor of 2–4 [491]. Furtherreduction of the write current can be achieved by reducingthe distance between the word line and the MTJ cells. Whenthe demagnetizing field is taken into account, the switch-ing field of an MRAM cell using MTJ is proportional tothe thickness of the free layer and inversely proportional tothe size of the cell. Therefore, it is important to keep thethickness/width ratio constant during down-size scaling ofthe cell. Finally, the write current can also be reduced byreducing the shape anisotropy and increasing the anisotropyof the material itself by using, for example, synthetic ferri-magnets as the free layer [493].

7.4. Read Operation of MRAM

As we mentioned in the beginning of this section, one of thekey features of the MRAM is that the information is storedin the relative orientation of the magnetization of two sep-arate magnetic layers rather than in the charged/dischargedstate of capacitors. Therefore, it is more suitable for scalingdown to small structures, at least, in terms of output signal.As discussed in the MTJ section, the maximum output signalof an MTJ is given by

:VMTJV = Vmax/2 =12Vmax ·MR0 (51)

In principle, it is independent of the cell size, although itmay decrease by reducing the size in actual devices. As theMTJs in MRAMs detect the resistance difference betweenparallel and antiparallel states, the peak-to-peak output volt-age is in the range of 120–160 mV, which is much largerthan the output voltage of a read sensor. The use of double-tunnel junctions may further boost this value [494]. Oneof the key concerns in the readout process of MRAMs atthe product level is the fluctuation of MR values amongthe large number of cells. Fortunately, the rapid progress inprocess development has now made it possible to suppress

Proof's Only


the fluctuation of MR at an acceptable level. For example,Tehrani et al. reported that the mean value and standarddeviation of MR in their particular samples were success-fully suppressed to 5 and 1%, respectively, over a 200 mmwafer [492].

7.5. Diode-Free/Multilayer/Multilevel Designs

The existing 1 MTJ/1 T cell design not only makes theMRAM costly, but also makes it difficult to reduce thedimensions of the entire MRAM cell. A switch-free designis much more desirable for fully making use of the advan-tages of MRAMs [495 496]. Figure 42 shows an example ofsuch a MRAM design using MTJs. As there is no switchingdiode or transistor associated with each MTJ, all of the cellsare electrically mutually connected, forming a large resis-tor network. This will reduce the amplitude of the readoutsignal due to the current-shunting effect, as shown schemati-cally in Figure 49. The author’s group has proposed a simpleperipheral circuitry to reduce the shunting current [496]. Asshown schematically in Figure 50, the simple circuit is basi-cally a voltage follower. It sets an equal electrical potentialfor adjacent bit lines so as to prevent current shunting toother paths when a selected bit is being read. Although itis only shown here for a 2 × 2 cell, it can be easily scaledup to large-scale cell arrays without the need to increase thecomplexity of the peripheral circuit. The effectiveness of thismethod has been verified experimentally [496]. The diode-free design is particularly useful for building multiple-layerMRAMs. In addition to multiple-layer design, multilevel ormultistate design also remains as one of the options for real-izing gigabit MRAMs in the future [497–500].

7.6. Electrical Field Driven Memory

The MRAM in its present form is intrinsically a current-driven device, at least during writing. The most desirablememory should be the one such that only reading involvescurrent, while writing is based on an electrical field effect.This kind of memory is difficult to be implemented usingmetallic magnetic materials because their magnetic proper-ties cannot be altered by a static electric field, at least atthe moment and under normal conditions. The necessity ofgenerating a magnetic field from a current to switch themagnetization in current MRAMs increases the complexityof the design and manufacture of such devices. Althoughmagnetization switching based on “spin transfer” has beenstudied extensively, which has the potential to simplify the

Word line

Bit line

MTJ

VI

Cell being

read

Currentshunting

Figure 49. Schematic diagram of a switch-free 2× 2 MRAM cell array.When a sensing current is supplied to a selected cell to read out thedata, a part of the current is shunted to other cells, resulting in adecrease in the readout signal.

Word line

VBit line

MTJ

VI

Cell being

read

Voltage

follower

+

–

Figure 50. Schematic diagram of a switch-free 2× 2 MRAM cell array.A simple peripheral circuitry is used to reduce the shunting current.

MRAM design, the current that is needed to switch the mag-netization is still comparable to the write current of exist-ing MRAMs [501–508]. Very recently, Zhuang et al. [509]demonstrated the first prototype of an electrical-field-drivenRAM using the colossal magnetoresistance effect (CMR).As it is based on the huge change of resistance at two differ-ent states, it is also dubbed a resistance random-access mem-ory or RRAM. For a 0.8 1m × 0.8 1m cell, the write currentwas 0.2 mA, which is much smaller than the write currentof MRAMs. The RRAM is particularly suitable for multi-level memories due to its large resistance change betweentwo different states (by a factor of 10–1000). Another fieldwhich can be explored for creating electrical-field-drivenmemory devices is magnetic semiconductors. Ohno et al.recently demonstrated electrical-controlled ferromagnetismin (In, Mn)As-based magnetic semiconductors, but so far, ithas not been implemented in any form of functional devices[510].

8. SEMICONDUCTOR SPINTRONICSIn this chapter, we have focused on metal-based spin-tronics and its application in data storage. Anotherimportant emerging field is the spintronics that usesnonmagnetic semiconductors, magnetic semiconductors, andmetal/semiconductor hybrid structures. A number of deviceconcepts have been proposed, and some of them have beenverified experimentally. These include, but are not limitedto: (1) the spin field-effect transistor [511], (2) the spin-valvetransistor [512], (3) the magnetic semiconductor tunnel junc-tion [513 514], (4) magnetic semiconductor diodes [515],(5) the hybrid Hall effect device [516], magnetic tunneltransistors [517], and spin-dependent light-emitting devices[518–520]. The greatest advantage of spintronics using semi-conductors is that it can be integrated into the existingmicroelectronics. Furthermore, it also offers the possibilityof controlling the magnetic properties using an electric field,which may lead to novel nanodevices, which is not possibleusing metal-based magnetic materials. Research activities insemiconductor spintronics have expanded almost exponen-tially in recent years. An in-depth coverage of this topic isout of the scope of this chapter because we focus on datastorage, which at the moment is still dominated by metal-based spintronics. Semiconductor-based spintronic devicesmay find applications in data storage, but they need toovercome many technical hurdles before they can be usedin hard disk drives. This is because most of the semicon-ductors can only be grown on high-quality crystalline sub-strates instead of the AlTiC substrates used currently forsliders. Most of the semiconductor epilayers are also toofragile to sustain the various lapping/polishing processes

Proof's Only


employed in current magnetic head fabrication processes.Therefore, semiconductor-based spintronic devices are moresuitable for future memory and logic devices. The latestreviews of semiconductor-based spintronics can be found in[4 31], and the April 2002 issue of Semiconductor Scienceand Technology.

9. FUTURE TREND OF DATA STORAGEAND THE ROLE OF SPINTRONICS

Most of the existing data storage devices can be categorizedinto three major groups: electronic, magnetic, and opticaldevices. The electronic storage devices store the informationusing charges. Therefore, the absolute number of electronsthat are used to store one bit of information matters becauseit will determine the SNR of the readout signal. To thisend, various types of capacitance enhancement techniqueshave been developed, such as bottle-shaped deep trenches,hemispherical grains, and three-dimensional structures overthe bit line. In addition to the innovation in designs, effortsare also being made to develop suitable high-K capacitordielectrics. In the field of flash EEPROMs, multibit and mul-tilevel technologies are also being introduced to increase thestorage capacity.

Optical storage, on the other hand, has the most diversityamong all kinds of data storage devices. This is because, asshown schematically in Figure 51, light as an electromagneticwave carries more information than its static counterpart ofelectrical and magnetic fields. Depending on which param-eter in the electrical field expression that is used to read orstore the information, various types of optical data storagetechnologies have been developed, and some of them havebeen commercialized. These include: (1) read-only mem-ory (ROM), (2) phase-change (PC) disk, (3) magnetoopti-cal (MO) disk, (4) multiple-layer storage, (5) holographicdata storage, (6) hole-burning memory, and (7) time-domainmemory. The wave nature of light makes it possible to real-ize the storage of information in real space [(1)–(4)], recip-rocal space [5 6], and the time domain. If we focus onlyon real space recording, the areal density of optical stor-age is limited physically by the fact that a certain numberof photons are required to reproduce the information withan acceptable SNR. When the bit becomes very small, thenumber of photons that come back from a tiny bit is lim-ited by the intensity of the light, which is in turn determinedby the damage threshold of the storage medium. Of course,in practice, the areal density of real space optical recordingis largely determined by how small one can focus the laser

]tωrk[expEE ϕ+−⋅=0

ROM

PC

MO

MULTIPLE

LAYER

STORAGE

PHASE

SHIFT

HOLOGRAM

WAVELENGTH

MULTIPLEXING

HOLOGRAM

HOLE

BURNING

MEMORY

TIME

DOMAIN

MEMORY

→ → → →

Figure 51. A wide range of optical data storage technologies that hasbeen developed using the different properties of light waves.

spot into and, at far field, it is determined by the diffraction-limited performance of the optical system. Although near-field optics can reduce the spot size to a certain extent, itis still insufficient to compete with the state-of-the-art mag-netic recording devices.

The magnetic storage devices which have been the focusof this chapter store the information using the spin of elec-trons. Although the output of CIP sensors scales with thesize of the information bit, the output of CPP sensors includ-ing MTJ is relatively insensitive to the size of the sensor.However, even for the latter, the SNR scales inversely withthe bit size. Therefore, the areal density of magnetic record-ing will be limited either by the SNR or by the thermal sta-bility of the bit, whichever gives the lower density. Althoughlarge ballistic magnetoresistance has been obtained in mag-netic nanocontacts [521 522], one still has to find a way toconvert it into a useful sensor for read heads, in particular,the linearity and low-field requirements.

If the bit size of all of these different types of storagedevices keeps shrinking, it will eventually approach the sizeof atoms. When we reach that stage, we will probably haveto seriously look at the information storage in reciprocalspace. As mentioned above, the concept of reciprocal spacerecording has already been explored in optical data storage.A similar concept should also be explored in charge- andspin-based memories. This will lead to the general conceptof quantum information storage. Nanometer-scale spintron-ics is expected to play a bridging role to bring the clas-sic data storage into the quantum information storage era(see Fig. 52).

GLOSSARY

ACKNOWLEDGMENTSThe author is grateful to the members of the Nano Spin-electronics Group at the Data Storage Institute, espe-cially Dr. Kebin Li, Dr. Jinjun Qiu, Dr. Yuankai Zheng,Dr. Zaibing Guo, Dr. Guchang Han, and Ms. Luo Ping fortheir assistance with some of the experimental data. He isalso grateful to Dr. Li Wang and Mr. Yatao Shen for theirassistance with some of the literature survey work. He wouldalso like to acknowledge the support of Prof. Chong TowChong, Dr. Thomas Liew, and Dr. Chang Quan Teck.

100100

101001

001001

000101

100001

100101

101000

101010

10000

|1⟩ |0⟩

λ

λ

λ

2π

Nλ

λ

2π

a = Nλ

Real Space Reciprocal Space

FT

K1K2

K3

Kn

...

(e.g., holographic recording)

(quantum storage)Bits meet atoms

Charges interact

with spins

Real Space Recording Reciprocal Space Recording

Classic scaling

Figure 52. Future trend of data storage.

Proof's Only


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