A spin current, a flow of electron spins in a solid, interacts with magnetization by exchanging the spin angular momentum. This interaction is responsible for spintronic functionalities; spin injection into a ferromagnet transfers the spin angular momentum, enabling switching the direction of the magnetization or driving magnetization precession. The inverse of this process, i.e., the angular momentum transfer from magnetization to carrier spins, is spin pumping; a spin current is emitted from a ferromagnet by the angular momentum transfer from precessing magnetization to carrier spins through dynamical spin-exchange coupling at a ferromagnetic/nonmagnetic interface. This dynamical spin injection offers a versatile route for generating spin currents in a wide range of materials, providing a pathway to explore spin physics in condensed matter.
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