The granular γ-Fe2O3 powders are prepared. The particle shapes and mean particle sizes of the samples are determined by Transmission Electron Microscopy (TEM). Using Extracting Sample Magnetometer (ESM) and Vibrating Sample Magnetometer (VSM), the magnetic parameters are measured at different temperatures. By the Law of Approach to Saturation (LATS), the effective magnetic anisotropy constants KE of granular γ-Fe2O3 powders at different temperatures are obtained. By using the mixed-coercivity model, a good explanation is given to the temperature dependence of the coercivity for granular γ-Fe2O3. It is found experimentally, for the first time, that the variation tendency of KE with temperature for granular γ-Fe2O3 is about the same as that of the shape magnetic anisotropy Ksh, and that the magnetization reversal process of granular γ-Fe2O3 is close to uniform rotation at different temperatures.

Granular γ-Fe2O3 powders, acicular γ-Fe2O3, and CoFe–γ-Fe2O3 powders are prepared by different methods. Particle shapes and mean particle sizes of samples are determined by transmission electron microscopy (TEM). Magnetic parameters are measured by a vibrating sample magnetometer (VSM) at different temperatures. Effective magnetic anisotropy constants K E of granular γ-Fe2O3 powders at different temperatures are obtained by using the law of approach to saturation (LATS). KE values of acicular γ-Fe2O3 and CoFe–γ-Fe2O3 powders are measured by a magnetotorquemeter. It is found for the first time that the variation tendency of KE with temperature for granular γ-Fe2O3 is about the same as that of shape magnetic anisotropy Ksh. Fluctuation field Hf and activation volume Vf of samples are measured. A theoretical expression of Vf is derived. For granular γ-Fe2O3 powders, calculated activation volumes are consistent with experimental ones at different temperatures. But as for acicular γ-Fe2O3 powders, calculated activation volumes are larger than experimental ones. Experimental results show that magnetization reversal of granular γ-Fe2O3 at different temperatures is close to homogeneous rotation.

Ultrafine iron particles are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Using the law of approach to saturation (LATS), effective magnetic anisotropy is obtained at different temperatures. KE increases with the decrease of temperature, and decreases with the thickening of the surface oxide layer. The temperature dependence of coercivity has been investigated.

The Fe0.7 Co0.3 - SiO2 granular alloy solid has been prepared using a sol-gel method, and investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The effective magnetic anisotropy KE has been measured by the law of approach to saturation (LATS). The evolution of the magnetic properties for the granular alloy solid during reducing is described and explained by means of particle size distribution and superparamagnetic behaviour. When the measuring temperature changes from 100 to 800 K, the variations of saturation magnetization, effective magnetic anisotropy and coercivity of the Fe0.7 Co0.3-SiO2 granular solid are much smaller than those of existing magnetic recording media. The Fe0.7 Co0.3 - SiO2 granular alloy solid has a potential for application in high density magnetic recording media, which can be used in a wide temperature range.

The irreversible response of a ferromagnetic pervoskite SrRuO3 with a Curie temperature TC = 160 K has been characterized experimentally by measuring the temperature dependence of the field cooled and zero field cooled moments over the temperature interval 5 K≤T≤180 K; in a sequence of applied fields up to the coercive field, and also the major hysteresis loop over the field range ∣ha∣≤40 kOe at a sequence of temperatures which span the ordered phase. The data have been analysed within the framework of a generalized version of the Preisach model of hysteresis which combines thermal fluctuations with the growth of the free energy barriers below TC. The numerical simulations replicate all of the principal structural systematics of the data, and yield details of the spectrum of Barkhausen instabilities in this material.

Zero-field-cooled (ZFC) magnetization, field-cooled (FC) magnetization and coercive fields of Sr1-x Cax RuO3, with x = 0.0, 0.2, 0.4 and 0.6, have been measured over the magnetic ordering temperature, in a sequence of applied fields. An empirical model has been proposed to analyze ZFC behaviors. The calculated ZFC magnetization is in agreement with the measured one. Different ZFC behaviors have been explained by means of the empirical model.

Itinerant ferromagnetic SrRuO3 with a Curie temperature TC 160.7 K was prepared by a conventional solid reaction method. Its zero-field-cooled (ZFC) magnetization and field-cooled (FC) magnetization were measured over a temperature interval from 5–180 K, in a sequence of applied fields up to coercivity. The major hysteresis loops were measured over a range of temperatures, which span the ordered phase. An empirical model was proposed to analyze the ZFC behaviors by means of coercivities at different temperatures. The calculated ZFC magnetization is consistent with the measured ZFC magnetization. Different ZFC magnetization behaviors were explained by using this model. The results indicate that the various ZFC behaviors originate from the comparison between the temperature dependence of FC magnetization and that of coercivity.

Zero-field-cooled (ZFC) magnetization, field-cooled (FC) magnetization, ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160 K. An empirical model is proposed to calculate the measured ZFC magnetization. The result indicates that the calculated ZFC magnetization compares well with the measured one. Based on the generalized Preisach model, both the ZFC and FC curves are reproduced by numerical simulations. The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.

From the measurements of the ac susceptibility of SrRuO3, we determine the critical temperature TC=159.7 ± 0.2 K and critical exponents δ = 2.50 ± 0.15 (from the critical isotherm orδ= 3.17 ± 0.20 (from magnetization versus magnetic field at Curie temperature), γ+β = 1.50±0.05 (from the temperature dependence of the internal field of the crossover line), and γ = 0.96 ± 0.05 (from the temperature dependence of the susceptibility along the same line). These exponent values are very close to those predicted by the mean field model.

Zero-field-cooled (ZFC) magnetization, field-cooled (FC) magnetization, AC magnetic susceptibility and major hysteresis loops of sintered SrRuO3 have been measured over the range of magnetic ordering temperature. The data were analyzed within the framework of the Preisach model. The numerical simulations match the major features of ZFC/FC magnetization. The results indicate that various ZFC/FC behaviors originate from the applied field and the Barkhausen spectrum of the sample.