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UNITE MIXTE DE PHYSIQUE associée à l’UNIVERSITE PARIS SUD. Magnetoresistance of tunnel junctions based on the ferromagnetic semiconductor GaMnAs. R. Mattana, J.-M. George, H. Jaffrès, F. NGuyen Van Dau, A. Fert UMP CNRS-THALES, Orsay, France B. Lépine, A. Guivarc’h, G. Jézéquel - PowerPoint PPT Presentation
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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