A quantum-mechanical potential barrier model for estimating the number ratio between different valence cations in multiatom compounds is proposed. It is supposed that there is a potential barrier between a cation-anion pair. The height of the potential barrier is proportional to the ionization energy of the cation, and the width of the potential barrier is related to the distance between neighboring cations and anions. As examples for using this model, the distribution of cations with different valences in some ABO3 lanthanum manganites is explained satisfactorily.

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.

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.

The magnetic aftereffects of Fe3O4 nano-particles were simulated by Preisach hysteresis model, the results showed excellent agreement with the experiment data. The simulating results showed that due to the wide distribution of interaction field, magnetization decay followed typical logarithmic time dependence, and the decay occurred more rapidly for field closer to the coercivity. The thermal fluctuation field and activation volume were also obtained, which are the main characteristics of the thermal fluctuations in magnetic recording medium. From the fact that activation volume is larger than the mean particle size, we deduced the reversal magnetization mechanism. One set of parameters used in simulating hysteresis loops and magnetic aftereffects has unified the hysteresis and magnetic aftereffect phenomena. The aftereffects under double-field were calculated, and the condition under which the time dependence of magnetization was non-monotonic was given.