Spintronics aims for generation of spin and orbital current in non-magnetic materials. Among various techniques to measure spin and orbital, the optical method employing magneto-optical Kerr effect has advantages of high sensitivity and vector-and-time resolution. In this talk, I will present three topics of my recent research. First, orbital Hall effect of a light metal Ti (Fig. 1a). The orbital Hall effect refers to the generation of the orbital angular momentum flow transverse to an external electric field. Theoretical studies predict that the orbital Hall effect can be strong even without spin-orbit coupling. Here we directly measure the current-driven orbital accumulation at surfaces of Ti, which has a weak spin-orbit coupling. Second, ultrafast spin current driven by phase transition of FeRh (Fig. 1b). FeRh experiences a phase transition from antiferromagnet to ferromagnet. Measuring spin accumulation on the Cu surface during the phase transition of FeRh in the FeRh/Cu heterostructure, we demonstrate that spin current is generated by the angular momentum transfer between magnons and electrons. Third, dynamics of spin waves in ferromagnetic metals (Fig. 1c). A circularly polarized photon generates ultrafast spin-orbit torque in heavy metal/ferromagnet structure and generates coherent spin waves in ferromagnets. Measuring the dynamics of spin waves, we determine the stiffness and damping constants of spin waves.
Figure 1. a, optical detection of the steady-state orbital accumulation on Ti surface driven by the orbital Hall effect. From the Ti thickness dependence, we determine the orbital diffusion length of ~80 nm of Ti. b, optical detection of the transient spin accumulation on Cu surface driven by the phase transition of FeRh. From the time-resolved dynamics, we prove the mechanism for the spin current generation. c, optical detection of the dynamics of spin waves of ferromagnet. From the oscillation frequency and relaxation time, we determine the stiffness and damping constant of spin waves.