Magnetic Resonance Phenomena in Ferrites

The resonant frequencies of the rotations of domain magnetization in ferrimagnetic materials are calculated by the use of the molecular field approximation, taking into account the effects of anisotropy, external field, and differences in magnetization and $g$ values between the two sublattices. Resonance is found to occur in two frequency bands: one in a microwave band, which various workers have investigated experimentally, and one in the infrared, predicted earlier by Kaplan and Kittel. It is necessary to discuss critically the physical nature of the anisotropy field entering the equations; when this is done, an excellent agreement is found with the measurements of Yager et al. and Healy on single crystals of nickel ferrite. Finally, the relation between the theory presented here and experiments on sintered samples of ferrite is discussed.