Abstract
An empirical method is used to calculate the Curie temperatures of spinel materials containing Fe2+ and Fe3+ ions. The approach taken is to select three simple spinel oxides of known cation distribution and Curie temperature containing the paramagnetic ions Fe2+ and Fe3+. In this way the relative strengths of the Fe3+—Fe3+, Fe3+—Fe2+ and Fe2+—Fe2+ AB interactions are calculated. For titanomagnetites the calculated Curie temperatures agree with the observed values to within 20%. A Curie temperature of 770°C is calculated for maghemite. This is higher than the Curie temperature of magnetite. Similarly an increase in the Curie temperatures of titanomagnetites is predicted on oxidation. This is in agreement with experiment. Finally the method is applied to the ratio of the BB/AB interactions in the spinel structure. This predicts that the apparent ferrimagnetic P-type behaviour of titanium rich titanomagnetites is not explained by the Néel theory. However the behaviour of titanium rich oxidized titanomagnetites is shown to be of the Néel P-type.
