Coercive force and remanence of monodomain grains with mixed crystalline and shape anisotropy

The thermal dependence of coercive force H c and saturation isothermal remanence I rs has been measured between –196°c and the Curie point for two samples containing monodomain grains of CoFe2O4 and γ-Fe1·95Co0·05O2. The H c −-T data suggest that the first-order crystalline anisotropy constant varies as the 9th to 11th power of spontaneous magnetization above 20°c. Both H c and I rs data indicate a transition from mixed crystalline and shape anisotropy below about 300°c to pure shape anisotropy above. The room-temperature intensity of nearly saturated remanences acquired in the shape range, such as thermoremanence (TRM) and high-temperature isothermal remanence (IRM), is 40–50% less than that of room-temperature IRM, but the high-temperature remanences are more resistant to thermal and alternating field demagnetization. Analogous effects should occur in other crystalline-controlled remanences—for example, anhysteretic remanence (ARM) and low-temperature crystallization remanence (CRM)—and in shape-controlled remanences like high-temperature CRM. Since the magneto-crystalline anisotropy of Fe3O4 and γ-Fe2O3 is much smaller than that of CoFe2O4, it may be difficult to observe these effects in natural rocks.