Polycrystalline samples of La0.67-x Ca0.33 MnO3 (x = 0.00, 0.02, 0.06, and 0.10) with different La site vacancies have been prepared by the sol–gel method. The grain sizes of the samples are observed by SEM. XRD results show that the samples are single phase of perovskite-type oxides. The Curie temperatures and the magnetic entropy changes of the samples are measured on a vibrating sample magnetometer (VSM). The influence of the vacancy concentration in the La site on their magnetocaloric properties has been investigated. A large magnetic entropy change (～2.78 J/kg K) is obtained in the sample La0.67-x Ca0.33 MnO3 (x = 0.02) at its Curie temperature (277 K) in an applied magnetic field of 1 T. Results of magnetic measurement show that these materials with La site vacancies can be used as candidates for magnetic refrigerants near room temperature.

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.