Of Fe <sub> 3 </ Sub> O <sub> 4 </ Sub> Nanoparticles Of Fept / Ta <sub>, And 5 </ Sub> Ta Multilayer Magnetic Aftereffect

The magnetic hysteresis and aftereffect have been measured and simulated by the Preisach hysteresis model of Fe₃O₄ nano-particles and [FePt/Ta]₅Ta multilayer films. The experimental data of hysteresis loops at different temperatures and magnetic aftereffect curves at different magnetic fields have been analyzed, and series of useful parameters of the samples were got. The results showed that the series of parameters for the same sample can be used to describe different magnetization processes, and the same series of parameters used in calculating hysteresis loop and magnetic aftereffect have unified the magnetic hysteresis and magnetic aftereffect phenomena. The fluctuation field and activation volume can also be got from the simulating, and we can deduce the mechanism of reversal magnetization and estimate the thermal stability of the materials.In the studies for Fe₃O₄ nano-particles, the Preisach distribution was the product of the interaction field and coercive filed distribution, and the interaction field distribution was logarithmic;the coercive filed distribution were Gaussian. A temperature dependent Preisach distribution has been proposed in which the influences of temperature were added to the Preisach parameters as a form of exponent. The hysteresis loops at different temperatures were reproduced and a series of useful parameters of the samples were got. Due to the wide distributing of interaction field the magnetization decay with time followed typical logarithmicbehaviors in single field process. The decay rates were different for different magnetic fields and it was larger for magnetic field closer to coercive force. The activation volume obtained from the simulating was larger than the mean size of the particle, which may arise from the large interaction between the particles. The magnetic moments of the particles may be aligned to the same direction and forming grain colonies with coincident magnetization direction. The fluctuation field was larger when the magnetic field was closer to the coercive filed which was determined by the distribution of activation energies arising from both a volume distribution of the particles and an anisotropy field distribution. The aftereffects under double-field process were also simulated, and the condition under which the time dependence of magnetization was non-monotonic was that the value of (Hz-H\)IHj was between 14 —25.In the studies for [FePt/Ta]sTa multilayer films, the Preisach distribution was got from the experimental hysteresis loops and remanence loops assuming the interaction field distribution and coercive filed distribution were both Gaussian. The Preiasch modeling reproduced excellently the hysteresis loops of the [FePt/Ta]5/Ta films with different time of Ta deposition. Series useful parameters were got and lay foundation for studying their other properties. The magnetization decay with time was logarithm dependent in single field process. The decay rates were different for different fields and it was larger for magnetic field closer to coercive field, since the energy barriers were in the minimum at this field. In the double-field process the magnetization decays with time deviated from the In / behavior were observed and reproduced by the Preisachhysteresis model. Moreover, we found that the viscosity coefficient gradually decreased as the sample underwent an increasing number of magnetization reversal cycles, because as the sample experiences more magnetization reversal, the sample is brought closer to the anhysteretic state, so the internal energy of the system is gradually decreased. The decay rate is lowered as the system is brought to lower energy metastable states.