Ferromagnetic Relaxation. I. Theory of the Relaxation of the Uniform Precession and the Degenerate Spectrum in Insulators at Low Temperatures

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Abstract
A scheme is proposed for the relaxation processes at low temperatures accompanying excitation of uniform precession spin waves in a ferromagnetic resonance experiment, with particular reference to highly pure yttrium iron garnet. The processes are: (1) scattering of uniform precession spin waves into the degenerate spin-wave modes by polishing imperfections on the sample surface; (2) equalization of the populations of the degenerate spin-wave modes, also by surface imperfections; and (3) relaxation of the degenerate modes by Raman scattering of thermal spin waves through the magnetic dipole interaction. Relaxation times for the three processes are calculated and compared with experimental values with reasonable agreement.