Ferrites

Abstract

With d.c. electrical resistivities several orders of magnitude greater than those of the ferromagnetic metals, and magnetic saturation intensities in some cases less than one order of magnitude smaller, the ferrites gave promise of greater `effective permeability' and lower `eddy-current' loss in high-frequency magnetic fields. Attempts to realize these advantages have had limited success, but have led to a deeper understanding of the ferromagnetic and antiferromagnetic behaviour of non-metallic substances. In the present report, a description of the physical and chemical structure of the ferrites, and of their preparation, is followed by an outline of Néel's theory of ferrimagnetism, and of the Kramers-Anderson theory of superexchange spin coupling. Several possible mechanisms which may account for the observed rather steep fall of permeability with increase of frequency, and for the associated energy absorption, are then discussed. The dielectric properties of ferrites are also of interest in high-frequency applications. The observed very strong dispersion and absorption may be interpreted in terms of the inhomogeneous micro-physical structure of the sintered material, which is known to be sensitive to the oxygen content and heat-treatment conditions. The very low-frequency dielectric absorption may be due to adsorbed moisture in the material. The semiconducting properties are briefly discussed. An extensive bibliography is included.