The spontaneous magnetization of alloys and compounds II. Ferrites

The paper describes the second stage of a project to determine the temperature variation of spontaneous magnetization for various materials. An unequivocal determination of the spontaneous magnetization requires, in addition to magnetic intensities, the measurement of the magnetocaloric effect. Three extrapolation techniques were used to deduce the spontaneous magnetization from the observations, the results being considered with reference to Neel’s theory of ferrimagnetism. Two typical ferrites, manganese ferrite and magnesium zinc ferrite, molecular compositions MnO. Fe2O3 and 1/2MgO.1/2ZnO. Fe2O3 respectively were selected for the investigation. Owing to the low conductivity of this class of substances particular care is required in the thermal measurements. The different extrapolation techniques lead to substantially the same values of the spontaneous magnetization. In contrast to the behaviour of the ferromagnetic metals, this agreement is maintained in the Curie point region, the spontaneous magnetization-temperature curves showing a smaller ‘tail’ near the Curie temperature. For both materials the departure of the curve of reduced magnetization against reduced temperature from that predicted by the quantum modification of the Weiss theory is verified by the present work. While the saturation magnetization of the ferrites and the reduced magnetization curve for manganese ferrite are in satisfactory agreement with Neel’s theory, the variation with temperature for the specimen of magnesium zinc ferrite does not correspond to any of the solutions predicted.