Magnetic Spectra of Ferrites

A brief discussion is given of the meaning of initial permeability, the principles of its measurement, and the nature of "dimensional resonance." In considering magnetic spectra containing only a single resonance, typical data on sintered ferrites and on finely divided ferrite powders are reviewed, and some of the theoretical interpretations previously advanced are criticized. The investigations involving the two natural resonances found at this laboratory in a commercial Mg-ferrite are then summarized, including resonance in single-domain particles as a method for identifying dispersion mechanisms, experimental evidence for the inertia of domain walls, the nature of internal fields, and the temperature dependence of the natural resonances and of the spectroscopic splitting factor. Using the information obtained from these special studies, the following general hypothesis is advanced: In principle, two natural resonances are present in the magnetic spectrum of any sintered ferrite, and the single resonance sometimes observed represents the coalescence of two resonances. Reasons are given for expecting this coalescence to occur in some materials only, as in certain sintered mixed ferrites containing Zn-ferrite. The new hypothesis is consistent with all presently available data, including new spectra, some containing two resonances, that were recently obtained on pure ferrites at this laboratory. Additional confirmatory experiments are described which provide methods for separating or coalescing two resonances, and permit distinguishing between internal fields due to anisotropy and magnetic interactions. The limitations and possibilities of ferrites for obtaining useful microwave permeabilities are discussed from the new viewpoint, the criteria involved in this problem being shown to differ from those used at radiofrequencies.