Simplified theory of nonlinear phenomena in ferrimagnetic materials

The onset of nonlinear phenomena in ferrites is due to the exponential growth of spin waves from their thermal excitation by the coupling of the spin waves to the uniform mode through exchange and demagnetising fields. The purpose of the paper is to present a unified development of nonlinear theory under perpendicular and parallel microwave pumping in terms of the components of magnetisation along the axes of the spin-wave ellipse. The eccentricity of the spin precession appears explicitly, and the final transformation to circular variables, the equivalent of the more usual Hollstein-Primak off transformation, is considerably simplified.