# Optical spin transfer and spin-orbit torques in thin film ferromagnets

## TL;DR

This paper investigates optically induced torques in thin ferromagnetic films, analyzing conditions for optical spin transfer versus inverse Faraday effect, and explores the influence of Rashba spin-orbit coupling on these phenomena.

## Contribution

It provides analytical expressions for optical torques in ferromagnets and clarifies the roles of spin transfer and Edelstein effects under different conditions.

## Key findings

- Optical spin transfer torque dominates when excitation energy is far from band edges.
- Rashba spin-orbit coupling leads to torques perpendicular to photon angular momentum.
- Analytic expressions for optical torques are derived in the absence of Rashba coupling.

## Abstract

We study the optically induced torques in thin film ferromagnetic layers under excitation by circularly polarized light. We study cases both with and without Rashba spin-orbit coupling using a 4-band model. In the absence of Rashba spin-orbit coupling, we derive an analytic expression for the optical torques, revealing the conditions under which the torque is mostly derived from optical spin transfer torque (i.e. when the torque is along the direction of optical angular momentum), versus when the torque is derived from the inverse Faraday effect (i.e. when the torque is perpendicular to the optical angular momentum). We find the optical spin transfer torque dominates provided that the excitation energy is far away from band edge transitions, and the magnetic exchange splitting is much greater than the lifetime broadening. For the case with large Rashba spin-orbit coupling and out-of-plane magnetization, we find the torque is generally perpendicular to the photon angular momentum and is ascribed to an optical Edelstein effect.