Ferromagnetic semiconductor GaMnAs

ISSN:1369 7021 © Elsevier Ltd 2009APRIL 2009 | VOLUME 12 | NUMBER 414

Ferromagnetic semiconductor GaMnAs

Today’s electronic devices involve two basic properties of the

electron: its charge and its spin. Historically, different classes

of materials have been needed to exploit these two properties.

For example, semiconductors allow control over an electrical

current via the electron charge (as in transistors, diodes, etc.),

while magnetic metals are used to exploit the electron spin

(as in magnetic hard drives, sensors, etc.). A key objective of

spin-electronics (or “spintronics”) research is to develop multi-

functional, practical devices that allow precise and simultaneous

control of both the charge and the spin properties of charge

carriers. The development of a wide range of semiconductor

alloys doped with magnetic ions – referred to as diluted magnetic

semiconductors (DMSs) – represents a major step in this direction,

opening up the prospect of utilizing the charge and the spin of the

electron within the same material in order to create new device

functionalities1-4.

A significant advance in our understanding of the interaction between

charges and spins has been made by research on II-VI-based DMSs,

i.e., materials achieved by alloying a II–VI semiconductor host (such as

HgTe, CdTe, and ZnSe) with magnetic ions, e.g., Mn2+, whose valence

is identical to that of the group II cations5, 6. The random distribution

of magnetic ions over the cation sublattices in DMS materials leads

to important novel magnetic effects, e.g., large Zeeman splittings of

electronic levels, giant Faraday rotation, magnetic-field-induced metal-

insulator transition, and formation of bound magnetic polarons. These

enhanced spin-dependent properties occurring in II-VI DMS systems

were widely studied in the literature and are already summarized in

many excellent review articles7-13. Although these materials have paved

the way for our understanding of the interplay between semiconductor

physics and magnetism, so far they have failed to find realistic

applications in actual devices, because most of their potentially useful

spin properties are manifested at very low temperatures.

The newly-developing spintronics technology requires materials that allow control of both the charge and the spin degrees of freedom of the charge carriers. Ferromagnetic semiconductors (SC) are considered suitable due to simultaneous presence of magnetic order and of semiconducting properties. GaMnAs is one of the most intensively studied ferromagnetic SC. In this paper we will review recent research and accomplishments regarding two technologically important properties – magnetic anisotropy and interlayer coupling -- of GaMnAs-based multilayer structures, with an eye on their potential role in practical devices.

Sanghoon Lee1*, J.-H. Chung1, Xinyu Liu2, Jacek K. Furdyna2 and Brian J. Kirby3 1Department of Physics, Korea University, Seoul, 136-701, Korea2Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA3Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA*Email: [email protected]

MT1204p14_21.indd 14 31/03/2009 14:17:10

mailto:[email protected]

Ferromagnetic semiconductor GaMnAs REVIEW

APRIL 2009 | VOLUME 12 | NUMBER 4 15

A giant stride has been achieved in the last decade by introducing

III-V-based DMSs in which Mn2+ ions replace group-III cations.

Unlike the case of II-Mn-VI DMSs, Mn2+ ions in III-V-based DMSs

not only provide magnetic moments, but also act as acceptors – and

are therefore a source of free holes. One of the important effects

of these extra charge carriers is that they mediate interactions

between magnetic moments localized on the Mn ions, thus leading

to ferromagnetic order in these III-V-based DMSs, with relatively high

Curie temperatures14–17. The first successful growth of III-V-based DMS

involved InMnAs films grown on GaAs substrates18, 19 by low-

temperature molecular beam epitaxy (LT-MBE). Since then many

other III-V-based DMSs - GaMnAs20, 21, GaMnSb22, 23, and InMnSb24

– have been fabricated. The successful growth of GaMnAs is of

special interest, because of the prevalence of GaAs-based devices in

industry already in place, from high speed transistors to light emitting

diodes (LEDs) and laser diodes (LDs). Bringing a spin component into

the picture is therefore of obvious interest, since this holds out the

promise of integrating the existing GaAs-based electronic components

with those involving GaMnAs, and in this way introducing the spin

parameter into the device context in new ways. For example, attempts

have already been made to integrate GaMnAs with GaAs-based non-

magnetic semiconductor systems to form spin-injecting structures25

and spin dependent resonance tunneling devices26. Furthermore,

the magnetic properties of III-V-based DMSs can be controlled by

changing carrier concentration via various external means, such as

light illumination27, 28, application of an electrical gate voltage29, 30,

or external carrier doping31, 32. Such demonstrations of diverse

manipulation techniques provide an indication that spintronic device

applications based on the III-V ferromagnetic semiconductors are

indeed a strong possibility.

The most practical spintronic application of GaMnAs is probably a

spin memory device, in which information is stored via the direction

of magnetization. Since the magnetic anisotropy is one of the major

physical quantities that determine the direction of magnetization in

a ferromagnet, detailed understanding of the anisotropy is a critical

first step for exploring ways to manipulate the magnetic properties in

the GaMnAs-based structures33-35. The next step toward a functional

device based on this material is to achieve a GaMnAs multilayer

structure in which the spin configuration in GaMnAs layers is

switchable between parallel and anti-parallel configurations. Here the

interlayer exchange coupling (IEC) between the GaMnAs layers – either

antiferromagnetic (AFM) or ferromagnetic (FM) – is expected to play a

crucial role in controlling the spin configuration within the structures.

This paper reviews the current understanding of both the magnetic

anisotropy and the IEC in GaMnAs-based structures.

Magnetic anisotropy of GaMnAs filmsThe magnetic anisotropy of GaMnAs has been found to depend on

many parameters, including temperature, strain and carrier density36-38.

This understanding has stimulated intensive studies of GaMnAs via

various experimental techniques, such as superconductor quantum

interference device (SQUID) magnetometry39-41, ferromagnetic

resonance (FMR)42, 43, magneto-transport44-46, and magneto-optical

Kerr effect (MOKE)47, 48. These studies revealed a rather complex

picture of magnetic anisotropy in GaMnAs, arising from cubic and

uniaxial contributions. The behavior of magnetic anisotropy of

a thin ferromagnetic film can be conveniently expressed via the

magnetostatic free energy F. For ferromagnetic thin films with the zinc-

blende structure (such as that of GaMnAs) the complete expression of

F can be written as42, 49, 50

F = -MH[cosθ cosθΗ + sinθ sinθΗ cos(ϕ – ϕΗ)] + 2πM2cos2θ

+1–2 M[– H2⊥ cos2θ − H2||sin2θ sin2ϕ − 1–

2 H4⊥ cos4θ − 1–

4 H4||sin4θ cos2 2ϕ]

(1)

where H is the applied magnetic field and M is the magnetization. The

first term in Eq. (1) describes the Zeeman energy; the second term

refers to the demagnetization energy (or shape anisotropy energy); and

the last term gives the energy due to the uniaxial and cubic anisotropy.

Here H2⊥ and H4⊥ are the perpendicular uniaxial and cubic anisotropy

fields, respectively; H2|| and H4|| are the in-plane uniaxial and cubic

anisotropy fields, respectively; the angles θ and θΗ are defined with

respect to [001]; and the angles ϕ and ϕΗ are defined with respect to

[110] in the (001) plane, as depicted in Fig. 1a. In the absence of an

external magnetic field, the magnetization direction of the GaMnAs

film is totally governed by magnetic anisotropy (i.e., the second and

third terms in Eq. (1)), and thus a precise determination of anisotropy

fields is important for correct understanding of the magnetization.

Since the direction of the magnetization follows the position of

the magnetic free energy minimum in the Stoner-Wohlfarth model51,

angular dependent measurements provide an opportunity for the

experimental determination of anisotropy fields in a ferromagnetic

film. The in-plane (H2|| and H4||) and out-of-plane (H2⊥ and H4⊥)

components of anisotropy fields can be obtained with experimental

configuration shown in Figs. 1b and 1c, respectively. Figure 2 shows

a typical temperature dependence of magnetic anisotropy fields

obtained for a Ga1-xMnxAs film with x = 0.025 grown on (001)

GaAs substrate52. Although the cubic anisotropy fields H4|| and H4⊥

have large values at low temperatures, they decrease rapidly as the

temperature increases. The uniaxial anisotropy fields H2|| and H2⊥ are

temperature independent, and thus become increasingly important at

high temperature.

The temperature dependence of the relative strengths of the cubic

and uniaxial anisotropy fields causes the magnetic easy axes to change

as a function of temperature. This change can be seen more clearly

from the 3D free energy density diagrams shown in Fig. 3, which are

constructed using the anisotropy fields given in Fig. 2. In this specific

case the energy minima appear in the (001) plane, indicating that the

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REVIEW Ferromagnetic semiconductor GaMnAs

APRIL 2009 | VOLUME 12 | NUMBER 416

magnetic easy axes lie in the film plane. The two distinct magnetic

easy axes near <100> at 3K gradually change their directions toward

[11-0] as the temperature increases. This trend eventually makes the

two easy axes merge into one along the [11-0] direction when the

uniaxial anisotropy begins to dominate over the cubic anisotropy at a

sufficiently high temperature. This temperature-dependent change of

magnetic easy axes within the sample plane is typical of GaMnAs films

under in-plane strain, characteristic of GaMnAs films grown on (001)

GaAs substrates.

The magnetic anisotropy of a GaMnAs film changes significantly

under different strain conditions. For example, the Ga0.97Mn0.03As

film grown on a Ga0.8In0.2As buffer layer (which has a larger lattice

parameter than Ga0.97Mn0.03As) is under tensile strain53. Unlike

compressively strained GaMnAs film, the free energy of a GaMnAs

layer under tensile strain is dominated by the out-of-plane components

of the anisotropy, as shown in Fig. 4. The deepest energy minimum is

seen to occur along the [001] direction, indicating that in this strain

condition the film has an out-of-plane magnetic easy axis43, 54. The

transition of the magnetic easy axis from in-plane to out-of-plane

can also be achieved by varying the temperature, as was observed by

FMR44.

The magnetic anisotropy of GaMnAs is also expected to be strongly

affected by the carrier density, due to the fact that the ferromagnetism

in this material is carrier-mediated. The effect of carrier density on

magnetic anisotropy has been investigated for a number of GaMnAs

specimens with different carrier concentrations that were controlled

either by the growth process or by post-growth thermal treatment

in various gas environments (e.g., nitrogen or hydrogen)40, 55.

Direct doping during the growth (e.g., co-doping by Be) produces

inconsistent results due to inadvertently introduced fluctuations of

strain and/or chemical composition, all of which can affect magnetic

anisotropy56-59. The effect of carrier density on the magnetic

anisotropy of GaMnAs was later addressed more systematically in

Ga1-yAlyAs/Ga1-xMnxAs/Ga1-yAlyAs heterostructures (x = 0.062,

y = 0.24), in which the top Ga1-yAlyAs barrier was modulation-doped

with Be. The modulation doping technique allows one to control carrier

density in the GaMnAs layer (i.e., to increase the hole density in the

Be-doped structure relative to the un-doped structure), while keeping

all other parameters constant60.

Figure 5 shows 2D free energy diagrams for the (001) plane

constructed from magnetic anisotropy fields obtained for the

undoped (p=(1.32 ± 0.01) × 1019 cm–3) and the Be-doped

(p=(7.72 ± 0.01) × 1019 cm–3) Ga1-yAlyAs/Ga1-xMnxAs/Ga1-yAlyAs

structures, respectively. Both systems show significant deviations

Fig. 2 Summary plot of anisotropy fields obtained from Ga1-xMnxAs film with x = 0.025 grown on a (001) GaAs substrate. While the uniaxial anisotropy fields are relatively insensitive to temperature, the cubic anisotropy fields decrease rapidly with increasing temperature.

Fig. 1 Coordinate system used to describe crystal directions and experimental configurations. The orientation of the applied magnetic field H is described by θH and ϕH. The resulting equilibrium orientation of the magnetization M is given by θ and ϕ. (b) and (c) Two experimental geometries used in angle-dependent measurements for obtaining anisotropy fields in GaMnAs layers.

(b)(a)

(c)

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from the <100> four-fold symmetry dictated by the cubic anisotropy,

implying a strong influence of uniaxal anisotropy. Note, however, that

in the un-doped structures the directions of the energy minima (and

thus the directions of the easy axes) rotate toward the [1-10], and in

Be-doped structures to the [110] directions, indicating that the uniaxial

anisotropy axis has changed from [1-10] to [110] due to Be doping. Such

complex behavior of magnetic anisotropy, which affects the orientation

of magnetization and complicates the domain structure61, 62, makes

the investigation of IEC difficult in GaMnAs multilayers.

Interlayer exchange coupling in GaMnAs-based multilayers We now consider a current passing across an interface between two

magnetic layers. The nature of charge carrier scattering strongly

depends on the type of spin alignment, either parallel or anti-parallel,

between two adjacent ferromagnetic layers. Thus the magnitude of

Fig. 3 3D plot of free energy density for a Ga1-xMnxAs film with x = 0.025 grown on a (001) GaAs substrate. Energy minima occur in the (001) plane near the <100> directions. The directions of the energy minima rotate toward [11-0] with increasing temperature due to the increased importance of uniaxial anisotropy along the [11-0] direction as the cubic terms decrease.

Fig. 4 3D plot of free energy density for a Ga0.97Mn0.03As film grown on a Ga0.8In0.2As buffer layer. The deepest energy minima appear along the [001] direction, indicating that in this case the easy axis of magnetization is normal to the plane of the film.

Fig. 5 2D plot of free energy densities for two Ga1-yAlyAs/Ga1-xMnxAs/Ga1-yAlyAs structures (x = 0.062, y = 0.24), without and with Be-doping in the top Ga1-yAlyAs barrier. The directions of the energy minima are different in the two structures due to a change of the uniaxal anisotropy from the [1-10] direction in the un-doped sample to [110] in the Be-doped structure.

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electric current passing across the multilayers can be controlled by

manipulating the relative orientations of the magnetization vectors

in these layers, reaching a maximum for parallel and a minimum for

antiparallel configurations. Significant changes of electrical resistance

– the so-called giant magnetoresistance (GMR) – have been observed

in metallic magnetic multilayers when such spin configurations were

controlled by external magnetic field63. Key ingredients for observing a

GMR effect are a field-dependent relative alignment of two magnetic

layers separated by non-magnetic layer, a mobility difference

between majority and minority carriers in the ferromagnetic layers,

and sufficiently weak spin-relaxation throughout the structure. In

the case of the conventional (“current-in-plane”) geometry, the spin-

relaxation length needs to be large compared to the transport mean-

free path. In this paper we concentrate on the IEC, which determines

antiferromagnetic (AFM) or ferromagnetic (FM) spin alignment

between magnetic layers. Spontaneous (AFM) IEC has been observed

in various metallic64-67 and semiconductor68-70 multilayer structures.

The discovery of GMR effect in metallic multilayers led to a host of

new spin based electronic devices, including magnetic random access

memory (MRAM) – a practical spin memory device.

For the realization of GMR-like spin memory devices in the

ferromagnetic semiconductor GaMnAs one needs to obtain a

detailed understanding of the nature of IEC between GaMnAs

layers in multilayer structures. Theoretical studies based on the

k • p kinetic exchange and tight binding models predicted oscillatory

behavior between AFM and FM IEC for carrier-mediated DMS-based

superlattices 71-73. The strength and period of the oscillation were

predicted to depend on both the thickness of the nonmagnetic

spacers and on the carrier concentration in the layers. Experimentally,

the IEC properties of various configurations of GaMnAs-based

multilayer structures – including trilayers and superlattices – were

investigated by various characterization techniques74-79. While

several groups have observed FM IEC in GaMnAs structures,

spontaneous AFM IEC has only very recently been realized in GaMnAs,

achieved by introducing additional charge carriers into the GaAs

spacers that separate the GaMnAs layers80. This observation of AFM

IEC in GaMnAs-based multilayers demonstrated the high potential

of III-V-based ferromagnetic semiconductors for spin memory

applications.

Observation of FM IEC in GaMnAs multilayer structuresThe simplest and most informative structure for studying IEC is the

trilayer structure, in which two magnetic layers are separated by a non-

magnetic spacer. The best known ferromagnetic semiconductor trilayer

structures are Ga1-xMnxAs/Ga1-zAlzAs/Ga1-yMnyAs layer combinations

grown on a GaAs substrate. In these structures, the two GaMnAs layers

differ in either Mn concentration, hole concentration, or layer thickness,

chosen to give the two GaMnAs layers different magnetic properties.

Whether the two magnetic layers are coupled or independent can be

inferred from magnetization and/or magneto-transport measurements.

Since the IEC in the ferromagnetic semiconductor trilayer

structure is expected to be sensitive to the parameters of the non-

magnetic spacer (such as potential height and the layer thickness), the

investigation of IEC in trilayer structures has typically been performed

with series of samples, in which one of the spacer parameters is

systematically varied.

The first experimental investigation of IEC between GaMnAs

layers was performed on Ga1-xMnxAs/Ga1-zAlzAs/Ga1-yMnyAs trilayer

structures by magneto-transport and magnetization measurements74.

In that study the otherwise independent characteristics of the Hall

resistance and of the hysteresis of the two GaMnAs layers were

gradually observed to develop into a coupled behavior as the potential

barrier height of the Ga1-zAlzAs layer was lowered by changing its

alloy composition, or by decreasing its thickness. The systematic

dependence of magnetic properties of the assembly on the non-

magnetic spacer parameters implied the presence of IEC between the

two GaMnAs layers. The study of IEC between GaMnAs layers was

extended using diverse multilayer structures as well as measurement

techniques76-79, 81, 82.

Although the above experimental studies have demonstrated

the existence of the IEC between the GaMnAs layers in multi-layer

structures, all previous observations revealed only FM IEC, regardless

of the parameters of the non-magnetic spacer. The absence of AFM

IEC in the GaMnAs-based multilayers was rather surprising from

the theoretical viewpoint, which predicted both FM and AFM IEC,

depending on the spacer properties. However, the experimental

techniques used in the above studies – SQUID magnetometry, FMR,

and magneto-transport – all have limitations in unambiguously

detecting AFM IEC, since they are sensitive to the collective behavior

of all the layers in a multilayer structure as a whole, and are not able

to directly access the spin configuration in individual GaMnAs layers.

Clearly, to fully understand IEC in a multilayer structure, one requires

an experimental technique that can directly probe the spin alignment

in individual magnetic layers.

Direct measurement of spin alignment in magnetic layersIn the early days of GMR, light scattering from spin waves provided

important evidence of AFM IEC in metallic ferromagnetic trilayers63.

Observation of spin waves via light scattering, however, is limited to

simple structures with high magnetization densities. In the case of

GaMnAs the magnetization is usually very low: it is indeed a diluted

ferromagnet, as Mn2+ ions typically constitute less than 10% of the

total chemical composition, and not all Mn ions contribute to the

ferromagnetic exchange. Thus a probe that is directly sensitive to

even a weak magnetization is crucial in this case. Polarized neutron

reflectometry provides such a probe83, 84. This technique is sensitive

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to the depth profiles of the nuclear density and magnetization in thin

films and multilayers66 even when the magnetization is small, as in the

case of GaMnAs69, 79, 80, 85, 86, 87.

Polarized neutron reflectivity has already been successfully used

by Kepa et al.79 to observe FM IEC in GaMnAs-based multilayers.

Using 50-period multilayers of Ga0.94Mn0.06As/GaAs, Kepa and

co-workers observed magnetic contributions to the reciprocal space

Bragg peak corresponding to the multilayer periodicity79. Since it

is very difficult to obtain a field that is ideally zero at the sample

during measurement, simple observation of parallel alignment

does not provide definitive evidence of spontaneous FM IEC. The

authors79 observed that some samples developed single domains

with net magnetization along the direction opposite to the external

magnetic field, and therefore concluded that the FM IEC was intrinsic.

Later, antiparallel alignments between ferromagnetic GaMnAs

layers were also observed using polarized neutron reflectivity. Using

Ga0.95Mn0.05As/GaAs/Ga0.95Mn0.05As trilayers, where modulation

doping was applied on one side of the structure, Kirby and coworkers

showed that magnetization of ferromagnetic layers can be reversed

individually by using the difference in coercivity87. Although this did

not involve spontaneous coupling, the experiment of Kirby et al.87

demonstrated that polarized neutron reflectivity can be used to directly

observe AFM alignments in GaMnAs-based multilayers.

Very recently, definitive evidence of true AFM IEC was

finally reported by Chung et al.80 using 10-period multilayers

of Ga0.97Mn0.03As/GaAs, in which the carrier concentration was

increased directly in the non-magnetic GaAs spacers by Be doping80.

The sample was deposited on a GaAs (001) substrate by molecular

beam epitaxy, and the Be concentration in the spacers is estimated

as 1.2 x 1020 cm-3. Here we make a more detailed description of the

polarized neutron reflectivity method, which is conceptually depicted

in Figure 6. Polarized neutron beams (arrows) were used to probe all of

the layers in the multilayer stack, revealing a 6.95 nm Mn-doped FM

layer thickness dFM, a 3.47 nm Be-doped p-type spacer layer thickness

dS, and the magnetization orientations of each of the layers under a

range of field and temperature conditions. The neutron spins flip when

they are scattered from layers whose magnetization orientations are

perpendicular to them. This scattering process is called spin flip and

is typically very weak for diluted magnets. On the other hand, the

neutron spins are maintained when they are scattered from layers

whose magnetization vectors are parallel or antiparallel to them.

These two scattering processes are called non-spin flip (NSF). While

the NSF intensities also include non-magnetic scattering, magnetic

components can be extracted because phase shifts occur depending

on the relative orientations between the neutron spins and the

magnetization vectors. Therefore, net magnetization of each layer is

revealed as splittings between two NSF reflectivities with opposite

neutron polarizations, (++) or (--), respectively. Figure 7 shows a

summary of the field-dependent NSF polarized neutron reflectivity

data at 30K. A characteristic spin-split AFM Bragg peak was clearly

observed at a wave vector transfer corresponding to twice the

structural periodicity of the multilayer, Q ≈ 2π/2(dFM + dS) ≈ 0.03 Å-1

(see Fig. 7(a)). This AFM Bragg peak was suppressed when the applied

field was increased beyond the coercive field of the GaMnAs layers, and

spin-dependent changes were observed at the FM Bragg peak position

Q ≈ 2π/2(dFM + dS) ≈ 0.06 Å-1, revealing a switch to FM alignment

of the GaMnAs layers (see Fig. 7(b)). The AFM Bragg peak was not

recovered when the field was lowered to below the coercive field at 6K,

due to the strong cubic anisotropy field in the GaMnAs layers. At 30K,

in contrast, the AFM Bragg peak was repeatedly recovered even after

cycling the field several times, demonstrating that the observed AFM

IEC is spontaneous and robust (see Fig. 7). In contrast, only FM IEC was

observed when the spacers were not doped with Be, indicating that the

Be doping of the non-magnetic spacer layers was responsible for the

observed AFM IEC.

While this observation substantially brightened the prospect of

achieving all-semiconductor-based spintronic devices, we note that

so far only multilayers with spacers of approximately 12 monolayers

(3.5 nm thick) have been investigated with polarized neutron

reflectivity. Thus a large parameter space still remains unexplored,

providing considerable hope for optimization.

CONCLUDING REMARKSGaMnAs thin film structures exhibit very intricate and interesting spin

properties associated with their magnetic anisotropy, including two

magnetic easy axes (i.e., four easy orientations of magnetization) that

originate from strong cubic anisotropy. This feature automatically

offers the opportunity for increasing storage capacity in magnetic

recording devices. For example, a four-level magnetic memory concept

Fig. 6 Schematic representation of a polarized neutron reflectometry measurement used to detect the AFM IEC reported in80. Polarized neutron beams (arrows) probe each of the layers in the multilayer stack, allowing for determination of the individual layer thicknesses and magnetizations.

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had already been demonstrated by Lim et al.88 with GaMnAs grown on

a vicinal GaAs surface, where four distinct Hall resistance states were

realized due to the combined effects of the planar and the anomalous

Hall effects. Later, Lee et al.89 further extended this concept by using

stable muti-domain formations arising from the presence of the four

magnetization directions in a GaMnAs layer. The magnetic anisotropy

of GaMnAs responsible for such rich magnetization behavior depends

in a sensitive manner on the material parameters such as strain

and carrier density, which then provide a handle for manipulating

the magnetization. However, the nature of magnetic anisotropy in

GaMnAs layers – e.g., the origin of in-plane uniaxial anisotropy in this

material – is still not fully understood, and remains to be uncovered

for achieving precise control of magnetization orientation in GaMnAs

device structures.

From the point of view of memory device applications, IEC in the

GaMnAs multilayer structures must also be thoroughly understood.

Although the theoretically predicted AFM IEC was indeed recently

observed, the agreement between theory and experiment is only

qualitative at the present time. So far the AFM coupling was observed

in only a single sample, and the presence of IEC oscillation in GaMnAs

multilayer structures is yet to be revealed. Furthermore, the thickness

of GaMnAs layers which produce AFM IEC in a multilayer geometry

is much thicker than what was considered in theoretical calculations.

The dependence of IEC on the structure parameters must therefore

be further investigated quantitatively, both in theory and in the

laboratory.

The possibility of electrical control of magnetic anisotropy

of a GaMnAs layer90 along with controlling IEC in GaMnAs

multilayers by doping, as discussed in this paper, suggests that such

multilayers can potentially provide significant device advantages

over metallic ferromagnetic multilayers. The remaining obstacle

for the implementation of GaMnAs-based devices is the fact that

ferromagnetic properties of GaMnAs films are now only observed

below room temperature. Increasing the ferromagnetic transition

temperature TC in GaMnAs is therefore the primary challenge that

needs to be addressed. Recent work suggests, however, that there is

hope in this regard, since values of TC as high as 170K have already

been demonstrated in bare GaMnAs films with heavy Mn doping91,

and TC of nearly 250K has been achieved in delta- and modulation-

doped GaMnAs-based heterostructures92. There is also some evidence

that the proximity of an Fe layer can induce room temperature

ferromagnetism in a thin GaMnAs layer93. If room temperature

ferromagnetism can be achieved in GaMnAs, there is real potential

to exploit the novel magnetic anisotropy properties, together

with the demonstrated FM and AFM coupling in GaMnAs-based

multilayers described in this paper, to realize entirely new devices

with unprecedented properties in the area of magnetic memory storage

and manipulation.

Fig. 7 AFM (left) and FM (right) splittings in polarized neutron reflectivity (multiplied by Q4) measured in a Ga0.97Mn0.03As/GaAs:Be/Ga0.97Mn0.03As multilayer at 30K. The data were collected serially in the order from (a) to (g). The solid lines show calculated reflectivity corresponding to multilayer models shown in the middle column.

MT1204p14_21.indd 20 31/03/2009 14:18:07



ACKNOWLEDGEMENTSThis work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST)

(No. R01-2008-000-10057-0); by the Seoul R&DB Program; by Korea University Grant; by KOSEF through the Nuclear R&D Programs (M20701050003-08N0105-00311); and by NSF Grant No. DMR06-03752.

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Ferromagnetic semiconductor GaMnAs

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