Magnetoresistance of tunnel junctions based on the ferromagnetic semiconductor

GaMnAs

UNITE MIXTE DE PHYSIQUEassociée à l’UNIVERSITE PARIS SUD

R. Mattana, J.-M. George, H. Jaffrès, F. NGuyen Van Dau, A. FertUMP CNRS-THALES, Orsay, France

B. Lépine, A. Guivarc’h, G. JézéquelUMR CNRS-Université Rennes I, France

A. Hamzic, M. Basletic, E. TafraDepartment of Physics, Faculty of Science, Zagreb, Croatia

Spin electronics

Use the spin of the carriers :Metallic system (GMR, TMR,..)Extension to semiconductors

“Source” of spin polarized carriers : Ferromagnetic transition metalConductivity mismatchChemical reactivity

Ferromagnetic semiconductors

Ferromagnetic semiconductors

Dietl et al., Science 287, 1019 (2000)

Tc : Computed values

GaMnN, ZnCoO : Tc > RTbut material optimization still required

GaMnAs, InMnAs

Best knowledge of growth conditions, magnetic & transport properties

Heterostructures based on GaMnAs

Spin electronics properties

Outline

Magnetic and transport properties of thin films

Structures and growth conditions of MTJs TMR in single and double barrier MTJs

Spin accumulation

Bias dependence of TMR Conclusion

x > 7.5% :Formation of MnAs clusters

x < 4% :FM but insulating

4% < x <7% :FM and metallic

can be used as FM electrodes in MTJs

Ga1-xMnxAs thin films

Ga1-xMnxAs (x=5.7%) (300Å)

GaAs (10Å)

AlAs (17Å)

GaAs (10Å)

Ga1-xMnxAs (x=4.5%) (3000Å)

Single barrier

Double barrierGa1-xMnxAs (x=5.7%) (300Å)

GaAs (10Å)

AlAs (17Å)

GaAs (10Å)

Ga1-xMnxAs (x=4.5%) (3000Å)

AlAs (17Å)

GaAs (50Å)

AlAs 17Å

GaMnAsGaMnAs

~ 0.5eVSpin dependent tunneling from GaMnAs Detection by GaMnAs

observed by TanakaPhys. Rev. Lett. 87, 026602 (2001)

Spin transmission across GaAs ? Spin detection by GaMnAs ?

GaMnAsGaMnAs GaAs

AlAs 17Å AlAs 17Å

Valence band profile (holes)

Growth conditionsMolecular beam epitaxy (MBE) in a RIBER 2300 system (As4 solid source) :

GaAs buffer layer at high temperature on semi-insulating

GaAs (001) substrates :

Ts=580°C; As4/Ga 250.3 m/h; As 2x4 surface

Growth of Ga1-xMnxAs & AlAs at low temperature on As rich GaAs

surface (As C4x4) :

Ts=230°C; As4/Ga 100.3 m/h; 1x2 surface

GaMnAs

GaMnAs

AlAs

Single barrier MTJHRTEM

P. Galtier

Fabrication process of the tunnel junctions

• Four steps of optical lithography

• Diameter : 10, 20, 50, 100, 200, 300 µm

AlAsGaMnAsGaAss

GaMnAsSi3N4

Ti/Au

Magnetoresistance of single barrier MTJ

Mr/Ms ~ 30%

TMR (low field) ~ 38%

R(0G)

R(0G)R(H)100TMR

@ 4K, V=1mVMagnetic field parallel to the [110] axis

RS ~ 0.1.cm2

GaMnAs/AlAs (17Å)/GaMnAs

Rmin

RminR(H)100TMR

GaMnAs/AlAs/GaMnAs

Magnetoresistance of single barrier MTJ

TMR (high field) : TMR ~ 675% (progressive saturation of the magnetization)

Large “spin polarization”

R(electrode) ~ 1 k; R(junction) ~ 100 k

R(tunnel) >> R(electrode)

Magnetoresistance of double barrier MTJGaMnAs/AlAs/GaAs/AlAs/GaMnAs

TMR in F/I/N/I/F is expected in following case :

@ 4K, V=1mV

Magnetic field parallel to the [110] axis

Ballistic transmission through the entire I/N/I barrier

is expected to increase considerably the junction resistance.In our case : R(single) ~ R(double)

Sequential tunneling with energy relaxation TMR is due to spin accumulation

Evidence of spin splitting in SC Spin accumulation

TMR spin accumulation

Possible for N = SC (small n << 1016 cm-3)Impossible for N=metal (large n ~ 1023 cm-3)

A. Fert, H. Jaffrès, Phys. Rev. B, 64, 184420 (2001)

TMR(single) ~ TMR(double)

GaMnAs

GaAsGaMnAs

AlAs ,µ

eVbP

AlAs

,µ

AP

number of spin flips much smaller than injected spin current

TMR - F/I/N/I/F structure

b

Nsf

N

Nsf

re

l

t

ln

~ 1016 cm-3

Tunnel properties : Bias dependence

Magnon excitationTc=60K

Electronic band structureExchange coupling smaller(J ~ 0.1eV)

Barrier shapeBarrier height : ~ 0.5eV

weak characteristic energies

V1/2 ~ 15mV

Faster decrease compared to metallic junctions :

three potential origins :

V1/2 ~ 15mV

Tunnel properties : Bias dependenceWithout spin accumulation (metallic case) : V1/2 (double) ~ 2* V1/2 (single)

Montaigne et al. APL 73, 2829 (1998)

With a spin accumulation :Same bias dependence for single and double barrierV1/2 (double) ~ V1/2 (single)

TMR comes from the spin splitting of the potential (µ and µ) in GaAs

The total voltage drop Vb can be concentrated on one of the junction.

,µ

,µ

Antiparallel

µ

GaMnAs

GaMnAs

AlAs

AlAs

GaAs

~eVb

Conclusion

Theoretical works on TMR in systems with spin-orbit coupled states

Spin dependent tunneling of epitaxial tunnel junctions based on the ferromagnetic semiconductor GaMnAs

Large effect of tunnel magnetoresistance ~ 675%

Conclusion

Spin accumulation in double barrier MTJ :

• Spin relaxation in SC layers• Diffusion mechanisms in SC layers

New effect due to semiconductor characteristic (low density of states)

- TMR in double MTJ with a SC nonmagnetic central layer

non-relaxed spin splitting of the chemical potential

Spin accumulation

- Same bias dependence in single and double barrier MTJs