Thermally induced monochromatic microwave generation in magnon polaritons

We propose thermally induced generation of monochromatic microwave radiation in magnon-polaritons. The mechanism of thermal to microwave energy transformation is based on intrinsic energy loss compensation of coupled magnon and microwave cavity oscillators by thermally induced ``negative damping.'' A singularity at an exceptional point is achieved when at the critical value of negative damping, the damping of the system is fully compensated. At the exceptional point, the input energy is equally distributed between the magnon and photon subsystems of the magnon-polariton. The efficiency of transformation of thermal energy into useful microwave radiation is estimated to be as large as $17%$ due to magnon-photon coupling mediated direct conversion of spin current into microwave photons.