Determination of the Directions of Magnetization in Polycrystalline Ferrites

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Abstract
To explain the switching properties of “square-loop” ferrites, assumptions have had to be made as to the detailed configuration of the magnetization because of the lack of experimental evidence. This paper presents a method, using the Kerr transverse magneto-optic effect, to determine directly the directions of magnetization in individual grains across the surface of a ferrite toroid subject to an alternating magnetic field. A rectangular area of the ferrite surface of size 200×160 μ is illuminated by a tungsten filament source, and the intensity of the reflected beam measured with a germanium photodiode. When the magnetization in the ferrite is reversed with an alternating field of 1100 cps, a modulation of the same frequency appears in the output of the photodiode. The depth of this modulation is approximately 0.01% of the dc output and changes markedly as the light probe is traversed across the ferrite, because of the different directions of the magnetization in the separate grains. By choosing a sample with grains of the same size as the light probe, it is possible to examine the light reflected from an individual grain. By rotating the grain around the light probe and recording the variation in size of the modulation with angle, the disposition of the magnetization in the grain as it reverses with the applied field can be determined. Results obtained with a typical square-loop ferrite composition are presented and discussed.