An electron spin injection driven, paramagnetic solid-state MASER device
  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. 
S. M. Watts and B. J. van Wees