Electronic, Magnetic, and Magnetoresistance Properties of ...
Magnetoresistance of tunnel junctions based on the ferromagnetic semiconductor GaMnAs
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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
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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
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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
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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
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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
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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)
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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
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Fabrication process of the tunnel junctions
• Four steps of optical lithography
• Diameter : 10, 20, 50, 100, 200, 300 µm
AlAsGaMnAsGaAss
GaMnAsSi3N4
Ti/Au
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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
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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)
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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
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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
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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 :
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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
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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%
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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