  Stimulated emission is a process during which an atomic system gives away
energy to create a coherent photon. It is fundamental to the operation of the
optical amplifier. Here we propose two mechanisms for amplifying AC spin
currents in a solid-state magnetic medium by a stimulated spin pumping process.
The first is synchronous and consists of phase-locked pulses that perturb a
precessing magnetic moment. The second is asynchronous and is driven by DC spin
currents. The amplification relies on a non-adiabatic interaction taking place
in a ferromagnetic medium in which the magnetic moment emits spin angular
momentum in the form of spin current before equilibrating with the environment.
The pumped spin current amplifies or absorbs the injected AC spin current
mimicking the operation of the optical gain medium as readily seen from the
gain saturation profiles. The mechanisms we propose are a first step towards a
realistic spin current amplifier.
