  In response to an external, microwave-frequency magnetic field, a
paramagnetic medium will absorb energy from the field that drives the
magnetization dynamics. Here we describe a new process by which an external
spin injection source, when combined with the microwave field spin-pumping, can
drive the paramagnetic medium from one that absorbs microwave energy to one
that emits microwave energy. We derive a simple condition for the crossover
from absorptive to emissive behavior. Based on this process, we propose a spin
injection-driven paramagnetic MASER device.
