Dynamic equations in the theory of ferromagnetic resonance

In the spin-wave approximation a model of a uniaxial ferromagnet interacting with a transverse magnetic, field is considered. The spin waves that are excited are described by classical amplitudes $a_k$ and $a_k^*$; the thermal magnons, by a nonstationary density matrix $p_m(t)$. The linear and nonlinear cases are investigated. In the linear case the results of the calculation of the complex susceptibility agree with the results obtained previously by means of the Matsubara technique. An expression is obtained for the nonlinear susceptibility and the conditions for a bistable regime of the collective excitations are considered.