Optical spin transfer and spin-orbit torques in thin-film ferromagnets
Junwen Li, Paul M. Haney
We study the optically induced torques on thin film ferromagnetic layers under excitation by circularly polarized light. We include Rashba spin-orbit coupling and assume an out-of-plane magnetization, and consider incident light with an in-plane component of angular momentum (e.g. light with an oblique angle of incidence). Solving the semiconductor Bloch nist-equations, we find simple expressions for the torque per optical absorption rate in the limits of weak and strong spin-orbit coupling. For weak spin-orbit coupling, the torque is bounded by the optical angular momentum injection rate. For strong spin-orbit coupling, the torque can exceed the angular momentum injection rate by a factor $\Δ\tau/\hbar$, where $\Δ$ is the magnetic exchange splitting and $\tau$ is the carrier scattering time.