Magnetocapacitance In Half-Metallic Granular Composites

Due to the complete spin-polarization of conductive electron in "half-metals", the resistance in their heterogeneous structure is extremely sensitive to the magnetic field and the great magnetoresistance effects are observed. Through experiments, it has be found that magnetoresistance effects would be enhanced when half-metallic oxides were doped by dielectric oxides,metals,polymers etc.. In the last few years, the magnetoresistance effects and other important magnetic transport properties in half-metallic granular composites had been widely researched. Recently, magnetocapacitance effect has been found in granular composite, unrelated to true magnetoelectric coupling, and the mechanism is a new subject needed be explored. In this thesis, we tried to find giant magnetocapacitance effects in half-metallic granular composites of great magnetoresistance effects and investigated the apparently large magnetodielectric effect given rise by magnetoresistance effect, concerning the resistance of half-metallic granular composites obviously being changed by small magnetic field.Through a combination of magnetoresistance and Maxwell-Wagner effect, Catalan achieved resistive magnetocapacitance which was approved in experiment by Lawes,Tackett et ah. Following the work of Catalan, we discussed resistive magnetocapacitance, concerning the microstructure of granular composite. Based on magnetic particle composite systems as well as magnetoresistance mechanism and distinguished two scatting mechanism (bulk and interface scatting), the frequency dependent relative dielectric constant and magnetocapacitance as a function of the volume or area fraction of the inclusions f were investigated, employing the effective-medium theory for asymmetry (MGT) microscopic structure. Generally speaking, the particle sizes of granular composite aren't well proportioned and exists a distribution. Calling the dipolar factor of the concentric sphere and the function of distribution of particle sizes, we generalized the Maxwell-Garnett theory to a system with a distribution of particle sizes, discussed the magnetic field dependent the magnetocapacitance and magnetocapacitance as a function of the volume or area fraction of the inclusions f with different distribution width, and concerned the influence of the ratio of the inclusions conductivity on magnetocapacitance of systems. Besides the asymmetry microstructure, we researched the resistive magnetocapacitance for a symmetry microstructure, employing the effective-medium approximation (EMA). With the EMA, we discussed the relationship with magnetic field and magnetocapacitance, the frequency dependent magnetocapacitance and magnetoloss, and the influence of percolations on magnetocapacitance.