Light-Induced Torque in Ferromagnetic Metals via Orbital Angular Momentum Generated by Photon Helicity

We investigated photon-helicity-induced magnetization precession in Co_{1-x}Pt_{x} alloy thin films. In addition to field-like torque, attributable to magnetic field generation owing to the "inverse Faraday effect," we observed nontrivial and large damping-like torque that has never been discussed for single ferromagnetic layer. The composition dependence of these two torques is effectively elucidated by a model that considers mutual coupling via spin-orbit interaction between magnetization and the electronic orbital angular momentum generated by photon helicity. This Letter significantly enhances our understanding of the physics relevant to the interplay of photon helicity and magnetization in magnetic metals.