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Magnetoresistance of tunnel junctions based on the ferromagnetic semiconductor GaMnAs

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

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Magnetoresistance of tunnel junctions based on the ferromagnetic semiconductor GaMnAs

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  1. 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 UMR CNRS-Université Rennes I, France A. Hamzic, M. Basletic, E. Tafra Department of Physics, Faculty of Science, Zagreb, Croatia

  2. Spin electronics Use the spin of the carriers : • Metallic system (GMR, TMR,..) • Extension to semiconductors • “Source” of spin polarized carriers : Ferromagnetic transition metal • Conductivity mismatch • Chemical reactivity Ferromagnetic semiconductors

  3. GaMnN, ZnCoO: Tc > RT but material optimization still required Ferromagnetic semiconductors Tc : Computed values Dietl et al., Science 287, 1019 (2000) GaMnAs, InMnAs Best knowledge of growth conditions, magnetic & transport properties Heterostructures based on GaMnAs Spin electronics properties

  4. 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

  5. Ga1-xMnxAs thin films • x > 7.5% : • Formation of MnAs clusters • x < 4% : • FMbutinsulating • 4% < x <7% : • FMandmetallic can be used as FM electrodes in MTJs

  6. Single barrier Ga1-xMnxAs (x=5.7%) (300Å) GaMnAs GaMnAs GaAs (10Å)  ~ 0.5eV AlAs (17Å) GaAs (10Å) Ga1-xMnxAs (x=4.5%) (3000Å) AlAs 17Å Double barrier GaAs GaMnAs GaMnAs Ga1-xMnxAs (x=5.7%) (300Å) GaAs (10Å) AlAs (17Å) GaAs (50Å) AlAs 17Å AlAs 17Å AlAs (17Å) • Spin transmission across GaAs ? • Spin detection by GaMnAs ? GaAs (10Å) Ga1-xMnxAs (x=4.5%) (3000Å) Valence band profile (holes) • Spin dependent tunneling from GaMnAs • Detection by GaMnAs  observed by Tanaka • Phys. Rev. Lett. 87, 026602 (2001)

  7. GaAs buffer layer at high temperature on semi-insulating GaAs (001) substrates : Ts=580°C; As4/Ga  25 0.3 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  10 • 0.3 m/h; 1x2 surface GaMnAs AlAs GaMnAs Growth conditions Molecular beam epitaxy (MBE) in a RIBER 2300 system (As4 solid source) : Single barrier MTJ HRTEM P. Galtier

  8. Fabrication process of the tunnel junctions • Four steps of optical lithography • Diameter : 10, 20, 50, 100, 200, 300 µm

  9. GaMnAs/AlAs (17Å)/GaMnAs @ 4K, V=1mV Magnetic field parallel to the [110] axis RS ~ 0.1.cm2 Magnetoresistance of single barrier MTJ • Mr/Ms ~ 30% • TMR (low field)~ 38%

  10. GaMnAs/AlAs/GaMnAs Magnetoresistance of single barrier MTJ R(electrode) ~ 1 k; R(junction) ~ 100 k  R(tunnel) >> R(electrode) TMR (high field) : TMR ~ 675%(progressive saturation of the magnetization)  Large “spin polarization”

  11. 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 Magnetoresistance of double barrier MTJ GaMnAs/AlAs/GaAs/AlAs/GaMnAs @ 4K, V=1mV Magnetic field parallel to the [110] axis TMR in F/I/N/I/F is expected in following case :

  12. TMR(single) ~ TMR(double) AlAs AP AlAs m , µ ­ ¯ m ­ GaMnAs m , µ eVbP m ­ ¯ ~ 1016 cm-3 ¯ GaMnAs GaAs Evidence of spin splitting in SC Spin accumulation TMR - F/I/N/I/F structure TMR  spin accumulation number of spin flips much smaller than injected spin current 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)

  13. Magnon excitation • Tc=60K • Electronic band structure • Exchange coupling smaller • (J ~ 0.1eV) • Barrier shape • Barrier height : ~ 0.5eV •  weak characteristic energies V1/2 ~ 15mV Tunnel properties : Bias dependence Faster decrease compared to metallic junctions : three potential origins :

  14. Without 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 barrier • V1/2 (double) ~ V1/2(single) AlAs Antiparallel AlAs 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  GaMnAs ,µ ~eVb µ GaMnAs GaAs V1/2 ~ 15mV Tunnel properties : Bias dependence

  15. Spin dependent tunneling of epitaxial tunnel junctions based on the ferromagnetic semiconductor GaMnAs •  Large effect of tunnel magnetoresistance ~ 675% Conclusion Theoretical works on TMR in systems with spin-orbit coupled states

  16. Conclusion • New effect due to semiconductor characteristic (low density of states) - TMR in double MTJ with aSCnonmagnetic central layer  non-relaxed spin splitting of the chemical potential Spin accumulation • - Same bias dependence in single and double barrier MTJs • Spin accumulation in double barrier MTJ : • Spin relaxation in SC layers • Diffusion mechanisms in SC layers

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