Experimental observation of the optical spin-orbit torque

TL;DR

This paper reports the experimental observation of an optical spin-orbit torque in a ferromagnetic semiconductor, induced by non-equilibrium spin-polarized photo-holes, on ultrafast timescales without external polarizers.

Contribution

It presents the first experimental observation of the optical counterpart of spin-orbit torque, expanding understanding of ultrafast spin manipulation in magnetic materials.

Findings

Detection of magnetization excitations independent of laser polarization

Observation of non-equilibrium spin-orbit coupled photo-holes

Magnetization dynamics occurring on picosecond timescales

Abstract

Spin polarized carriers electrically injected into a magnet from an external polarizer can exert a spin transfer torque (STT) on the magnetization. The phe- nomenon belongs to the area of spintronics research focusing on manipulating magnetic moments by electric fields and is the basis of the emerging technologies for scalable magnetoresistive random access memories. In our previous work we have reported experimental observation of the optical counterpart of STT in which a circularly polarized pump laser pulse acts as the external polarizer, allowing to study and utilize the phenomenon on several orders of magnitude shorter timescales than in the electric current induced STT. Recently it has been theoretically proposed and experimentally demonstrated that in the absence of an external polarizer, carriers in a magnet under applied electric field can develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) acting on the magnetization. In this paper we report the observation of the optical counterpart of SOT. At picosecond time-scales, we detect excitations of magnetization of a ferromagnetic semiconductor (Ga,Mn)As which are independent of the polarization of the pump laser pulses and are induced by non-equilibrium spin-orbit coupled photo-holes.