1. Magnetoresistance of tunnel junctions based on the ferromagnetic semiconductor GaMnAs UNITE MIXTE DE PHYSIQUE associée à l’UNIVERSITE PARIS SUD 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 : 3Metallic system (GMR, TMR,..) 3Extension to semiconductors “Source” of spin polarized carriers : Ferromagnetic transition metal 3Conductivity mismatch 3Chemical reactivity Ferromagnetic semiconductors

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

4. Outline 3Magnetic and transport properties of thin films 3Structures and growth conditions of MTJs 3TMR in single and double barrier MTJs  Spin accumulation 3Bias dependence of TMR 3Conclusion

5. 3 x > 7.5% : Formation of MnAs clusters 3 x < 4% : FM but insulating 34% < x <7% : FM and metallic can be used as FM electrodes in MTJs Ga 1-x Mn x As thin films

6. Ga 1-x Mn x As (x=5.7%) (300Å) GaAs (10Å) AlAs (17Å) GaAs (10Å) Ga 1-x Mn x As (x=4.5%) (3000Å) Single barrier Double barrier Ga 1-x Mn x As (x=5.7%) (300Å) GaAs (10Å) AlAs (17Å) GaAs (10Å) Ga 1-x Mn x As (x=4.5%) (3000Å) AlAs (17Å) GaAs (50Å) AlAs 17Å GaMnAs  ~ 0.5eV 3Spin dependent tunneling from GaMnAs 3Detection by GaMnAs  observed by Tanaka Phys. Rev. Lett. 87, 026602 (2001) 3 Spin transmission across GaAs ? 3 Spin detection by GaMnAs ? GaMnAs GaAs AlAs 17Å Valence band profile (holes)

7. Growth conditions Molecular beam epitaxy (MBE) in a RIBER 2300 system (As 4 solid source) : 3GaAs buffer layer at high temperature on semi-insulating GaAs (001) substrates : Ts=580°C; As 4 /Ga  25 0.3  m/h; As 2x4 surface 3Growth of Ga 1-x Mn x As & AlAs at low temperature on As rich GaAs surface (As C4x4) : Ts=230°C; As 4 /Ga  10 0.3  m/h; 1x2 surface GaMnAs AlAs 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. Magnetoresistance of single barrier MTJ 3 Mr/Ms ~ 30% 3 TMR (low field) ~ 38% @ 4K, V=1mV Magnetic field parallel to the [110] axis RS ~ 0.1 .cm 2 GaMnAs/AlAs (17Å)/GaMnAs

10. 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)

11. Magnetoresistance of double barrier MTJ GaMnAs/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 3Ballistic transmission through the entire I/N/I barrier  is expected to increase considerably the junction resistance. In our case : R(single) ~ R(double) 3Sequential tunneling with energy relaxation  TMR is due to spin accumulation

12. Evidence of spin splitting in SC Spin accumulation TMR  spin accumulation Possible for N = SC (small n << 10 16 cm -3 ) Impossible for N=metal (large n ~ 10 23 cm -3 ) A. Fert, H. Jaffrès, Phys. Rev. B, 64, 184420 (2001) TMR(single) ~ TMR(double) GaMnAs GaAs GaMnAs AlAs  ,µ  eV b P     AlAs  ,µ  AP number of spin flips much smaller than injected spin current TMR - F/I/N/I/F structure ~ 10 16 cm -3

13. Tunnel properties : Bias dependence 3Magnon excitation Tc=60K 3Electronic band structure Exchange coupling smaller (J ~ 0.1eV) 3Barrier shape Barrier height : ~ 0.5eV  weak characteristic energies V 1/2 ~ 15mV Faster decrease compared to metallic junctions : three potential origins :

14. V 1/2 ~ 15mV Tunnel properties : Bias dependence 3Without spin accumulation (metallic case) : V 1/2 (double) ~ 2* V 1/2 (single) Montaigne et al. APL 73, 2829 (1998) 3 With a spin accumulation : Same bias dependence for single and double barrier V 1/2 (double) ~ V 1/2 (single) TMR comes from the spin splitting of the potential (µ  and µ  ) in GaAs  The total voltage drop V b can be concentrated on one of the junction.  ,µ  Antiparallel  µµ GaMnAs AlAs GaAs ~eV b

15. Conclusion Theoretical works on TMR in systems with spin-orbit coupled states 3Spin dependent tunneling of epitaxial tunnel junctions based on the ferromagnetic semiconductor GaMnAs  Large effect of tunnel magnetoresistance ~ 675%

16. Conclusion Spin accumulation in double barrier MTJ : Spin relaxation in SC layers Diffusion mechanisms in SC layers 3New 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