Optical spin-orbit torque in heavy metal-ferromagnet heterostructures

Spin current generation through the spin-orbit interaction in non-magnetic materials lies at the heart of spintronics. When the generated spin current is injected to a ferromagnet, it produces spin-orbit torque and manipulates magnetization efficiently. Optically generated spin currents are expected to be superior to their electrical counterparts in terms of the manipulation speed. Here we report optical spin-orbit torques in heavy metal/ferromagnet heterostructures. The strong spin-orbit coupling of heavy metals induces photo-excited carriers to be spin-polarized, and their transport from heavy metals to ferromagnets induces a torque on magnetization. Our results demonstrate that heavy metals can generate spin-orbit torque not only electrically but also optically. It is known that torques can be exerted on spins in a ferromagnet (FM) layer when an in-plane electric current is injected into a heavy metal (HM) layer in contact with the FM layer. Here, the authors demonstrate that torques can be generated without the current injection by shining instead circularly polarized light on the HM.