Relaxation Regimes of Spin Maser

Spin relaxation in a microscopic model of spin maser is studied theoretically. Seven qualitatively different regimes are found: free induction, collective induction, free relaxation, collective relaxation, weak superradiance, pure superradiance, and triggered superradiance. The initiation of relaxation can be originated either by an imposed initial coherence or by local spin fluctuations due to nonsecular dipole interactions. The Nyquist noise of resonator does not influence processes in macroscopic samples. The relaxation regimes not initiated by an imposed coherence cannot be described by the standard Bloch equations.