Optical Spin Transfer in Ferromagnetic Semiconductors

1. 001 0.015 ] 3 0.010 E [meV/nm z z z z z z 0.005 100 110 x x x x x x 0.000 001 110 001 100 mJ=1/2 mJ=-1/2 J=1/2 1 1 1/3 mJ=-3/2 mJ=-1/2 mJ=3/2 mJ=1/2 My,Mz Mx Optical Spin Transfer in Ferromagnetic Semiconductors J.Fernandez-Rossier(1,2,*), A. S. Nuñez1, M. Abolfath1 and A. H. MacDonald1 (1) Department of Physics, University of Texas at Austin, Austin TX (2) Departamento de Fisica Aplicada, Universidad de Alicante, Alicante (Spain). (*)jfrossier@ua.es Motivation (II): Optical injection (of spin polarized carriers) in (Ga,Mn)As Motivation (I): spin transfer in ferromagnetic metals. Summary Spin injection by optical orientation (3). • Material:(Ga1-x,Mnx)As: • Ferromagnetic semiconductor • Equilibrium: holes in valence band • Optical Orientation: • Optical injection of electrons in conduction band and additional holes with spin non parallel to equilibrium magnetization • Optical Spin Transfer(1) (OST) • Spin goes from photo-carriers to collective magnetization • Modification of Landau-Lifshitz equations(2) • Optical control of magnetization Selection rules GaAs (Ga,Mn)As Circularly polarized light -> spin polarized photo-carriers along Light propagation axis. Light Induced Magnetization tilt Current Induced Magnetization Switching E.B. Myers et al., Science 285, 867 (‘99) Oiwa et al., PRL 88, 137203 (2001) Magnetic anisotropy Spin Transfer Magnetization Dynamics e-h recombination: 2-20 ps IN Bands depend on magnetization direction Stochastic Process Precession interrupted M. Abolfath, et al. PRB63, 054418 (2001) OUT Effective field Damping Spin transfer CB spin flip due to Spin Wave Emission4 Anisotropy energy CB Spin decoherence (ps) TRANSFER x=0.055; strain=-0.29% Photo-electrons/(time volume) Spin orbit + shape + applied field Quasiparticle spin orientation (Ga,Mn) As O.S.T. vs Damping Strong pumping: Switching Conclusions Weak pumping: ‘FMR’ • Optical spin injection in (Ga,Mn)As permits magnetization control • Optical Spin transfer (OST) mechanism like metals2 • OST: damping control Linearized LL eq. References • P=0 (no OST), regular damping • P0, n parallel to c. Effective Damping: • (can be both >0 or <0 !!!) • P0, n perpendicular to c [1] JFR, AN, MA, AHM cond-mat0304492 [2] J. C. Slonczewski, JMMM 159,L1 (1996) L. Berger, PRB 54 9353 (1996) [3] Optical Orientation, ed. By F. Meier and BP Zakharchenya (North Holland, NY, 1984) [4] JFR and AHM, in preparation Energy per pulse: 0.1 mJ/cm2 tL: 3 ps; tR= 2ps, p= n= 1.2 1018 cm3 Energy per pulse: 4.0 mJ/cm2 tL: 3 ps; tR= 2ps, p= n= 5 1019 cm3 Poster Presented in the International Conference of Magnetism, Rome, July 2003. Acknowledgement: MCYT Ramon y Cajal Program; Office of Naval Research N000140010951