Laser-induced torques in metallic ferromagnets

TL;DR

This paper investigates laser-induced torques in metallic ferromagnets using first-principles calculations, identifying contributions from the inverse Faraday effect and optical spin-transfer torque, and comparing results with experiments.

Contribution

It provides a detailed first-principles analysis of laser-induced torques in different ferromagnets, highlighting the roles of IFE and OSTT contributions and their dependence on material and broadening.

Findings

Torques have two main contributions: IFE and OSTT.

The contributions vary in magnitude depending on the material and broadening.

Good agreement with experimental data for hcp Co.

Abstract

We study laser-induced torques in bcc Fe, hcp Co and $L1_0$ FePt based on first-principles electronic structure calculations and the Keldysh nonequilibrium formalism. We find that the torques have two contributions, one from the inverse Faraday effect (IFE) and one from the optical spin-transfer torque (OSTT). Depending on the ferromagnet at hand and on the quasiparticle broadening the two contributions may be of similar magnitude or one contribution may dominate over the other. Additionally, we determine the nonequilibrium spin polarization in order to investigate its relation to the torque. We find the torques and the perpendicular component of the nonequilibrium spin polarization to be odd in the helicity of the laser light, while the spin polarization that is induced parallel to the magnetization is helicity-independent. The parallel component of the nonequilibrium spin polarization is orders of magnitude larger than the perpendicular component. In the case of hcp Co we find good agreement between the calculated laser-induced torque and a recent experiment.