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.

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 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.

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.

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.