  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.
