Anisotropic Ferromagnetic Resonance Linewidth in Ferrites

In disordered magnetic materials such as the ferrites, the dominant source of resonance linewidth can be attributed to processes involving only two elementary excitations: the destruction of a magnon with the creation either of another magnon or a phonon. We here consider only the primary magnon-magnon scattering process. We show that the random variation of the spin-orbit coupling parameters of the disordered ions leads to a resonance linewidth comparable to that observed in ferrites. The particular symmetry of the crystalline fields around the octahedrally coordinated sites causes an anisotropy in the linewidth with a minimum in the [100] directions and a maximum in the [111] directions. This anisotropy of linewidth is in general agreement with experimental observations on typical ferrites, as for example, the measurements of Schnitzler, Folen, and Rado on disordered lithium ferrite.