Seminarul National de Nanostiinta si Nanotehnologie MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT...

Seminarul National de Nanostiinta si Nanotehnologie

MAGNETIC NANOSTRUCTURES, MULTILAYERS with

GIANT MAGNETO-RESISTANCE (GMR) and TUNNEL MAGNETO-RESISTANCE (TMR)

Jenica NEAMTU, ICPE-CA, Advanced Research Institute for Electrical Engineering Bucharest


MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT MAGNETO-RESISTANCE (GMR)

and TUNNEL MAGNETO-RESISTANCE (TMR)


Giant Magneto-Resistance is the subject of a huge international research effort due to the numerous technological applications. The largest is the spin-electronics. Other applications are the high density data storage, on-chip GMR sensors for diverse as solid-state compasses, electrical sensors, automotive sensors. GMR and TMR effects has been observed in multilayered nanostructures of the form FM (Å)/NM(Å)/FM(Å), which FM is a transition-metal ferromagnetic layer (Fe,Co,Ni or alloys) and NM is a non ferromagnetic metal (Cr, Cu, Ag etc.) or an isolating (tunnel) barrier in case of Tunnel Magneto-Resistance Effect.

The band structure in a ferromagnetic metal is exchange split, so that the density of states is not the same for spin up and down electrons at the Fermi level. Fermi's golden rule states that scattering rates are proportional to the density of states, so the scattering rates are different for electrons of different spin.




The two-current model, it is assumed that there are two independent conduction channels, corresponding to the spin-up and spin-down electrons.These two channels are characterized by different mean free paths and resistivities.Spin-up electrons (with spin parallel to magnetization) are scattering very weakly in comparison with the spin-down electrons. The first magnetic layer allows electrons in only one spin state to pass easily - if the second magnetic layer is aligned then that spin channel can easily pass through the structure, and the resistance is low. If the second magnetic layer is misaligned then neither spin channel can get through the structure easily and the electrical resistance is high.

The GMR effectively measures the difference in angle between the two magnetizations in the magnetic layers. Small angles (parallel alignment) gives a low resistance, large angles (antiparallel alignment) gives a higher resistance. It is easy to produce the state where the two magnetic layers are parallel - simply apply a field large enough to magnetically saturate both layers.




Spin valves are dedicated GMR systems, in which the electrical resistance is high or low, depending on the direction rather than the strength of the magnetic field. The change in resistance is typical in the range of 5% to 10%.

Contrary to a GMR multilayer the two ferromagnetic layers are magnetically decoupled. This is achieved by increasing the thickness of the space layer. As a further difference the magnetization of one of the ferromagnetic layers is spatially fixed (''pinned”) by an antiferromagnetic layer. Thus it is called the "pinned layer", the other is called the "free layer", because it should easily follow the external magnetic field.

There are two different ways to create a antiferromagnetic pinning layer. It is an "artificial antiferromagnet" (AAF) in contrast to "natural antiferromagnets" such as chromium or manganese. In AAFs the alignment can be changed in high fields at any time, which makes the production of such sensors easy and cheap.




The objectives of our projects are the synthesis of GMR, TMR thin films multilayer structures and studies of the magneto-transport properties correlated with bulk or interface electronic scattering process. The GMR of ferromagnetic/nonmagnetic/ferromagnetic multilayer are dependent of the thickness of thin films, the roughness and the nature of thin film interlayer. The magnetic properties and the magnetoresistance in correlation with microstructural properties of permalloy layers and [NiFe(t)/Cu(s)/NiFe(t)]n and [NiFe(t)/Mo(s)/NiFe(t)] multilayers have been investigated. In this presentation are considered the samples: 1) Si/SiO2/(Permalloy Ni 80%Fe20%) monolayer films; 2) Si/SiO2/Py (10 nm)/Cu (4 nm)/Py(10 nm) in which Py is Ni80Fe20; 3) Si/SiO2/Py(10 nm)/Mo(6 nm)/Py(10 nm); 4) Si/SiO2/[Py(10 nm)/Cu (4 nm)]9/Py(10 nm). The thickness (t) of NiFe layers was ranged from 4 to 12 nm, while the copper and molybdenum layers was ranged from 3 to 8 nm.




-3000 -2000 -1000 0 1000 2000 3000

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Mr=0.021 u.e.m.HC=202 Oe

Si/SiO2/[Py(10 nm)/Cu(4 nm)]*9 - Py (10 nm)

Mom

ent (e

.m.u

.)

H (Oe)

The magnetoresistance effect measurements were performed at room temperature in four-point contact geometry with the contacts in line, using a DC current of 10 mA. The magnetoresistance (MR) is defined as the variation ΔR=(R0-RH) of the resistance due to magnetic field normalized by the resistance R0 at zero magnetic field: MR=ΔR/R0.

Fig. 1. Magnetic properties of GMR thin film multilayers, [Jenica Neamtu, M. Volmer Surface Science 482-485 (2001) p 1010-1014]

Fig. 2. Giant Magneto-Resistance of NiFe(10nm)/ Cu(4nm)/NiFe(10nm) multilayer,deposited in magnetic field, [J.Neamtu, M.Volmer, A.Coraci, THIN SOLID

FILMS 343-344 (1999) p.218-221]




-4000 -2000 0 2000 4000

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015 Si/SiO2/Py (10 nm)/Mo (6 nm)/Py (10 nm)

HC=200 Oe

Mr=0.003 u.e.m.

Mom

ent (e

.m.u

.)

H (Oe)

Fig. 3. Magnetic properties of Py/Mo/Py thin film multilayers, [Jenica Neamtu, M. Volmer Surface

Science 482-485 (2001) p 1010-1014]

Fig. 4. MR effect performed on Py/Mo/Py thin film multilayers, [Jenica Neamtu, M. Volmer Surface Science 482-485 (2001) p 1010-1014]

The magnetoresistance effect, MR, (fig.4) is performed on Py(10 nm)/Mo (6 nm)/Py(10 nm) multilayer with the magnetic field in the film plane. The effect is very small because of small coupling between the magnetic layers with molybdenum spacer. However one can be observed the effect of spin-dependent scattering of the electrons.




The above experimental results for [NiFe(10 nm)/Cu(4 nm)]9/NiFe(10 nm) multilayers show that GMR performance is associated with a low roughness and a sufficiently small average grain size, directly influencing the amount of grain boundary scattering.

Fig. 5. 3D AFM image of [NiFe(10nm)/Cu(4 nm)]9/ NiFe(10nm) multilayer. Average roughness is 6.4265 nm [Jenica Neamtu, M. Volmer Surface Science 482-485 (2001) p 1010-1014]

Fig. 6. 3D AFM image of NiFe(10nm)/Mo(6nm)/ NiFe(10nm) trilayer. The average roughness is 8.5384 nm.Jenica Neamtu, M. Volmer Surface Science 482-485 (2001) p 1010-1014]




For the first time we have performed Hall effect characterisation of multilayers for various orientations of thin film regarding the direction of the applied magnetic field.

In this case Hn is responsible of Hall effect and Hp is responsible of MR effect.

(a) (b)

AMRH UUU

Field dependencies of the output voltage, U, for various orientations of the Py (10 nm) regarding the applied magnetic field. In insert we present the same dependence for =0 in low magnetic field; (b) A simulation of magnetic distributions of remnant state starting from the the saturated state; array of 4x4 single domains (100x100x10 nm3 each ). =45º in order to maximize the AMR effect. Jenica Neamtu, M. Volmer, MRS Fall Meeting, Symposium R 5.5, Boston 2002, publ. in “Journal of Materials Research vol.746” [2003]




0 20 40 60 80 100

0

200

400

600

800

1000

1200

Si/SiO2/Py (10 nm)

Si/SiO2/Py (10 nm)/Cu (4 nm)/Py (10 nm)

HC (

Oe)

(degree)-10000 -5000 0 5000 10000

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

=45o

Si/SiO2/Py (10 nm)/Cu (4 nm)/Py (10 nm)

I=10 mA=0o

UH (

mV

)

H (Oe)

The angular dependencies of HC for Py(10 nm) and Py(10 nm)/Cu(4nm)/Py(10 nm)

Jenica Neamtu, M. Volmer, MRS Fall Meeting, Symposium R 5.5, Boston 2002, publ. in “Journal of Materials Research vol.746” [2003]

Field dependence of the Hall voltage obtained when =0

and =0;the AMR effect is minimized.Jenica Neamtu, M. Volmer, MRS Fall Meeting, Symposium R 5.5, Boston 2002, publ. in “Journal of Materials Research vol.746” [2003]

From Hall effect and magnetoresistance measurements on thin films we can make some assumptions regarding the reversal proceses that take place in the film. For =0, a relatively large transition region suggests a reversal process mainly due to domain wall motion. For >10 the

magnetisation reversal will only take place by coherent rotation.

Seminarul National de Nanostiinta si Nanotehnologie MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT...

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