Light-helicity induced orbital angular momentum detected by laser-driven magnetization dynamics in ferromagnetic metals

Interaction between photon's angular momentum and magnetization in matter is one of central subjects in opto-magnetism. One of the relevant phenomenon is so-called the inverse Faraday effect where light can be used to generate magnetic field or angular momentum in matter. Also, angular momentum can be induced by considering absorption of photon's angular momentum due to the conservation law. It has been predicted theoretically that orbital angular momentum (OAM) in metals can be generated by irradiation of light. OAM in matter have attacted attention recently for energy-efficient magnetization manipulation by current injection. However, light-induced OAM has not been clarified experimentally yet. In this work, we show that OAM can be generated by light irradiation which is detected by measuring laser-induced magnetization dynamics in ferromagnetic metals. Circularly-polarized laser-induced magnetization dynamics in Co_1-xPt_x alloy metallic ferromagnets was measured with systematically changing strength of spin-orbital coupling by Pt additions. Magnetization dynamics induced by large non-trivial light-induced damping-like torque was observed for samples with large Pt concentration whereas field-like torque due to light-induced effective magnetic field was observed for Co. The Pt concentration dependence of light-induced torques is elucidated by a model considering coupling between magnetization and light-helicity induced OAM.