The samples Sr1–xCaxRuO3 (x = 0.0, 0.2, 0.6) have been synthesized by a solid state reaction, and characterized by the zero field cooled (ZFC), field cooled (FC) magnetic moments and the major hysteresis loops. The data have been analyzed within the framework of a generalized Preisach model, which includes thermal fluctuations, critical phenomenon, and temperature dependent distribution of free energy barriers. The model provides an excellent description of the experimental data, and the simulating results are in good agreement with the experiment results. All the ZFC/FC curves at different applied fields and all the major hysteresis loops at different temperatures can be replicated by the model with one set of parameters.

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.

A sample of nano-particle Fe3O4 was synthesized using coprecipitation, and characterized by zero field cooling (ZFC) and field cooling (FC) magnetic moments and the major hysteresis loops. The data are analysed within the framework of a generalized Preisach model, which provides an excellent description of the experimental data. The simulated results are in good agreement with the experimental data. Using the model, we obtain that the blocking temperature of Fe3O4 nano-particles with diameter of 8.4nm is 105K, and the interaction field distribution between the particles is 550Oe.

Single-phase polycrystalline samples of La0.8-x Ndx Na0.2 MnO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) were prepared by the sol–gel method. Investigations of Curie temperature and magnetic entropy change were carried out in the temperature range 240–340 K and magnetic field range 0–1 T. It was found that the Curie temperature TC decreases with increasing x, and that the maximum magnetic entropy change △S M , for x = 0.20, is 0.3692 J/mol K near the temperature 295 K. The results suggested that these perovskite manganites have some appropriate properties for a good candidate as magnetic refrigeration in the room temperature range.

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.