Abstract
A comprehensive study of the magnetic properties of nonstoichiometric (cation-deficient) titanomagnetites is described. Measurement of the saturation magnetization of synthetic samples provides information about the mechanism by which increasingly nonstoichiometric states evolve. It is concluded that in the spinel structure the inherent availability for oxidation of Fe2+ ions on tetrahedral sites is about 20% of that of Fe2+ ions on octahedral sites. Curie temperature increases with degree of oxidation, showing that the increasing vacancy concentration is offset by the greater number of strongly interacting Fe3+ ions. Comparison of calculated and observed Curie temperatures of highly oxidized compositions indicates that such materials have nonideal u-parameters. Thermomagnetic curves of the oxidized samples are of the “anomalous” type, but none was found having compensation temperatures. Coercivity and anisotropy decrease with decreasing Fe2+ ion concentration. Finally, contours of equal cell edge and Curie temperature are constructed for spinels in the FeO-TiO2-Fe2O3 ternary system. These contours may be useful in the identification of naturally occurring samples, provided they do not contain appreciable amounts of other cations. An extrapolated value of 695°C is found for the Curie temperature of γ-Fe2O3.
