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.

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.

Ionic size is considered to be an important factor influencing the unit cell volume of manganites with an ABO3 structure. For La1-xSrxMnO3, however, although the average effective ion radius of all cations taken together increases with increasing x, the unit cell volume decreases for x<0.5. In the case of La1-xNaxMnO3, the unit cell volume reaches a minimum when x = 0.3. Up to now, no satisfactory explanation of these phenomena has been found. In this letter, an explanation based on the ionic cohesive energy with a small additional metallic cohesive energy is described.

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.

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.