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

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

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

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

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

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, (2001) 3 Spin transmission across GaAs ? 3 Spin detection by GaMnAs ? GaMnAs GaAs AlAs 17Å Valence band profile (holes)

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   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   m/h; 1x2 surface GaMnAs AlAs Single barrier MTJ HRTEM P. Galtier

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

Magnetoresistance of single barrier MTJ 3 Mr/Ms ~ 30% 3 TMR (low field) ~ 4K, V=1mV Magnetic field parallel to the [110] axis RS ~ 0.1 .cm 2 GaMnAs/AlAs (17Å)/GaMnAs

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 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

Evidence of spin splitting in SC Spin accumulation TMR  spin accumulation Possible for N = SC (small n << cm -3 ) Impossible for N=metal (large n ~ cm -3 ) A. Fert, H. Jaffrès, Phys. Rev. B, 64, (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 ~ cm -3

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 :

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

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%

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