Ferromagnetic Relaxation. II. The Role of Four-Magnon Processes in Relaxing the Magnetization in Ferromagnetic Insulators

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Abstract
The role of four-magnon processes in relaxing the uniform precession, in relaxing magnons of nonzero wave vector in parallel pumping experiments, and in establishing thermal equilibrium is considered. It is shown that the effect of the exchange-induced four-magnon process should be observable in parallel pumping experiments, but it is found that the four-magnon processes arising from dipole, cubic anisotropy, and exchange coupling are not very effective in relaxing the uniform precession. Both three- and four-magnon processes are important in the later stages of the relaxation scheme by which thermal equilibrium is established.