Green's-Function Theory of the Parametric Excitation of Nonuniform Magnon Modes in Strongly Magnetic Systems

Publisher Website
Google Scholar
Abstract
In an ordered, strongly magnetic system, the imaginary part of the susceptibility declines at a much lower signal level than is predicted by the usual linear theory of magnetic resonance. This phenomenon is called premature saturation and is due to the parametric excitation of nonuniform magnon modes which are degenerate with the uniform mode. In order to explain this phenomenon, a model is proposed in which two magnon modes are coupled via one-magnon and two-magnon processes. This system is treated by the method of time-dependent Green's functions. Even in the lowest-order approximation several types of two-, three-, and four-time Green's functions must be calculated. Relations between these Green's functions are investigated, and then the amplitude of the nonuniform magnon mode is calculated through the third order in the amplitude of the applied rf field. A criterion for the onset of parametric excitation is found, and comparison is made with previous semiclassical treatments.