Theory of Bose-Einstein condensation in a microwave-driven interacting magnon gas.

Room temperature Bose-Einstein condensation (BEC) of magnons in YIG films under microwave driving has been recently reported. We present a theory for the interacting magnon gas driven out of equilibrium that provides rigorous support for the formation of the BEC. The theory relies on the cooperative mechanisms created by the nonlinear magnetic interactions and explains the spontaneous generation of quantum coherence and magnetic dynamic order when the microwave driving power exceeds a critical value. The results fit very well the experimental data for the intensity and the decay rate of Brillouin light scattering and for the microwave emission from the BEC as a function of driving power.