Surface-sensitive magneto-optical detection of nonequilibrium spins and orbitals

Abstract The magneto-optical effect provides a direct probing tool for angular momentum in materials and has been used to detect ferromagnetic orders. Notably, owing to its surface sensitivity, the magneto-optical Kerr effect has been extended to detect nonequilibrium spin and orbital accumulations in initially nonmagnetic materials. The spatial distribution and temporal dynamics of spins and orbitals provide key aspects of the generation, transport, and relaxation mechanisms. With an accurate determination of the magneto-optic constant for spin and orbital, a distinctive evaluation of the magnitude of the spin and orbital accumulation is possible. The magneto-optical Kerr effect also allows for the investigation of spin–orbit torque (SOT) in bilayers of nonmagnet and ferromagnet. The SOT enables a quantitative analysis of the generation and transport of spins and orbitals, and the orbital-to-spin conversion process. In this article, we review recent studies on magneto-optical detection of nonequilibrium spin and orbital accumulations in single layers of semiconductors and metals and SOT in bilayers including heavy metals, where spin generation is dominant, and light metals, where orbital generation is dominant.