Fokker–Planck method in the theory of parametric resonance of spin waves

A system of parametric spin waves interacting with a thermal reservoir is considered. A generalized Fokker–Planck equation for the distribution function $f(a,t)$, of the amplitudes of the parametric spin waves in a ferromagnet is constructed. An equation for the distribution function $F(n,\sigma,t)$ of pairs of parametric spin waves is obtained on the basis of this equation using the idea of a reduced description. Approximate stationary solutions of the equation for the distribution function $F(n,\sigma)$ are found in the pre-threshold and post-threshold states.