Spin current generation by spin pumping

Magnetization dynamics is coupled with spin currents by exchanging the spinangular momentum. This coupling allows to control magnetization by spin currents; spin injection into a ferromagnet induces magnetization precession. The inverse of this process, namely, spin current emission from precessing magnetization, is spin pumping, which offers a route for generating spin currents in a wide range of materials. This chapter describes experiments on the generation and detection of spin currents using the spin pumping and inverse spin-Hall effect. The inverse spin-Hall effect, conversion of spin currents into an electric voltage through spin-orbit interaction, induced by the spin pumping was first discovered in a metallic film. The spin pumping in this film is quantitatively consistent with a model calculation based on the Landau-Lifshitz-Gilbert equation. This dynamical spin injection, the spin pumping, offers an easy and versatile way for injecting spin currents into not only metals but also highresistivity materials. In a metal/semiconductor junction, the spin pumping is demonstrated to be controlled electrically through the tuning of dynamical spinexchange coupling at the interface. This spin-injection method works without applying a charge current, which makes it possible to generate spin currents from magnetic insulators; the spin pumping appears even in a metal/insulator junction due to finite spin-exchange interaction at the interface. The spin pumping from an insulator enables nonlinear generation of spin currents: nonlinear spin pumping. The combination of the spin pumping and inverse spin-Hall effect provides an essential route for exploring spin physics in condensed matter.