Optical spin-transfer and spin-orbit torques

Spin polarized carriers electrically injected into a magnet from an external polarizer can exert a spin transfer torque (STT) whose physical origin is a non-relativistic angular momentum transfer. The phenomenon belongs to the area of spintronics research focusing on the manipulation of magnetic moments by electric fields. A current induced spin-orbit torque (SOT) is a distinct relativistic phenomenon in which magnetization dynamics is induced in a uniform spin-orbit coupled ferromagnet in the absence of the external polarizer. In the ferromagnetic semiconductor (Ga,Mn)As, we observed an optical counterpart of the non-relativistic STT (OSTT) in which a circularly polarized pump laser pulse acts as the external polarizer [1]. We also report the observation of the optical counterpart of the relativistic SOT (OSOT) in (Ga,Mn)As [2]. The absence of an external polarizer in the OSOT corresponds to photo-carrier excitations by helicity independent pump laser pulses which do not impart angular momentum. The OSOT relies on spin-orbit coupling of non-equilibrium carriers, in a direct analogy to the current induced SOT. We will discuss the extensive materials development, pump-and-probe magneto-optical experiments, and theory background [3] that led to the observations of these new effects as part of the continuous research in our Joint Laboratory of Opto-spintronics with the Faculty of Mathematics and Physics of the Charles University.

[1] P. Němec, et al. Nature Phys. 411, 8 (2012)
[2] N. Tesařová et al., arXiv:1207.0307
[3] P. Němec et al., Nature Commun. in press