Abstract: The granular γ-Fe₂O₃ powders, the acicular γ-Fe₂O₃ and CoFe γ-Fe₂O₃ powders are prepared. The particle shapes and mean particle sizes of the samples are determined by transmission electron microscopy. The phases are determined by X ray diffraction. By using vibrating sample magnetometer, the magnetic parameters are measured at different temperatures. By the law of approach to saturation, the effective magnetic anisotropy constants of granular γ-Fe₂O₃ powders at different temperatures are obtained. By using magnetotorquemeter, the effective magnetic anisotropy constants of acicular γ-Fe₂O₃ and CoFe γ-Fe₂O₃ powders are measured. It is found for the first time that variation tendency of effective magnetic anisotropy of granular γ-Fe₂O₃ with the temperature is about the same as that of the magnetic anisotropy with the temperature. The fluctuation field H_f and the activation volume of the samples at different temperatures are measured. The theoretical expression of activation volume is derived, For the granular γ-Fe₂O₃ powders, the calculated activation volumes at different temperatures are consistent with the experimental ones. But for acicular γ-Fe₂O₃ powders, the calculated activation volumes at different tempera tures is different from the experimental ones. It is found experimentally, for the first time, that magnetization reversal of granular γ-Fe₂O₃ at different temperatures is close to the uniform rotation.

Key words: mganetic recording powders, magnetic anisotropy, magnetic viscosity