BEHAVIOR OF FERRITES IN THE MICROWAVE FREQUENCY RANGE

Abstract

Present and foreseen applications of ferrites in the microwave frequency range are reviewed. The material parameters that are required or desirable for these applications are discussed. Special attention is given to the losses observed in polycrystals at biasing fields much smaller or much larger than the field for resonance. The loss may be characterized by an effective linewidth ƊHeff, which is a function of frequency and dc magnetic fieldstrength. In low loss polycrystals ƊHeff is typically much smaller in the off-resonance regions than near resonance, but even in the best polycrystals it is considerably larger than in good single crystals. The near-resonance behavior of ƊHeff can be explained quite well in terms of inhomogeneity induced scattering of energy into spin waves, having wavenumbers comparable to the inverse grain size. The off-resonance behavior is less well understood. Inhomogeneity induced scattering into phonons is discussed as a possible loss mechanism. For fine-grain ferrites the observed dependence of ƊHeff upon grain size and that of the spinwave linewidth (inferred from the instability threshold) upon grain size and frequency is in rough agreement with .the behavior expected theoretically for this mechanism. The microwave properties of partially magnetized ferrites are discussed. A simple theoretical formula, applicable in the demagnetized state, is derived. It is shown to agree reasonably well with the observed behavior both at high frequencies (where the losses are low) and at intermediate frequencies (0.1-3 GHz) (where the losses are high).