Amplification of electron-mediated spin currents by stimulated spin pumping

Amplification of spin currents is attractive for fundamental research and practical applications. It has been recently discussed in the context of magnonic spin currents. However, amplification of spin currents that are mediated by electron transport is less familiar. Here we propose a stimulated spin pumping mechanism for amplifying ac electronic spin currents in a solid-state magnetic medium. The mechanism closely resembles the optical stimulated emission process during which a coherent photon is created. Two schemes are discussed: The first is synchronous and consists of phase-locked pulses that perturb a precessing magnetic moment. The second is asynchronous, where a precessing magnetic moment is driven by the dc spin Hall effect and inherently locks its phase during the interaction. Depending on the pulses' timing/dc bias level, the pumped spin current amplifies or absorbs the injected ac spin current mimicking the operation of the optical gain medium as seen from the gain saturation characteristics.