Complex susceptibility of a uniaxial ferromagnet

A method of approximate solution is constructed on the basis of a successive deconpling of the chain of equations for the spin Green's functions. Expressions are obtained for the Green's functions of an anisotropic Heisenberg model; these characterize both the frequency and the damping of homogeneous oscillations of the magnetic moment of the system. The complex (high-frequency) susceptibility is expressed in terms of these functions in accordance with [1]. The Green's function has a complex pole which determines the resonance frequency and also the profile and width of the resonance line. The line width depends on the anisotropy and the demagnetizing factors and also on the correlation functions, which determine its temperature dependence. The latter is found for a wide range of temperatures including the neighborhood of the Curie point.