| Magneto-electronics lies in the area of current scientific interest in condensed matter physics and material sciences, due to abundant physical conceptions and applied potentials. It is a base of magnetic devices with high density and sensitivity. Recently, non-volatile magnetic computer memory (MRAM) is expected to have a large economic impact, and has been significantly focused on its experimenatal research. The MRAM needs to be of a large magnetoresistance (MR) effect at room temperature and lower magnetic field, in order to demands of practical application. However, the large tunneling MR effect is strongly dependent upon electrode material, which is of high spin polarization. On the other hand, half-metallic material is theoretically predicted to be perfect spin polarization, nearly 100%. Indeed, in recent years, some half-metallic oxides have been fabricated and measured, such as Fe3O4 (P=-80%), CrO2 (P=98.4%), LaCaMnO3 (P=78%), etc.. The spinel structure ferrimagnetism FeaO4 provides with not only strong magnetism, but also 858K Curie temperature. Material of FeaO4 has been applied widely in magnetic type, biologic medicine, and other fields. As one of half-metallic materials, in nanometer point contacts experiment, the MR FeaO4 of is -84% at 60 Oe and at room temperature. At the same time, the enhanced MR effect can be observed in FeaO4Ag, FesO4 Y Fe2O3, and Fe3O4/polymer etc. composites. Therefore, it is an efficient way to add the nanometer graphite particles to Fe3O4 granules for enhancing extrinsic MR, due to the formation of new microstructure, which adjusts surface of particle and interface of particles.In the other hand, with the development of material science, many researchers pay much attention to studies on composites, which exhibit not only the properties of single component but also some astonishing properties in composite. Therefore, it is very important to study magnetic properties of composite with the variation of microstructure.The thesis was originally intended to study two kinds of novel composites: (1) cubic Fe3O4 / spherical conducting graphite composite; (2) MnZn ferrite / SiO2 non-magnetism composite. The emphasis is placed on the magnetotransport of the composites that isconcerned with the relation between microstructure and MR effect, and discussion of magnetotransport mechanism in composites. Particular interest is to study influence of component on microstructure and magnetic properties.The main work involves:1. Fe3O4 / C granular compositeThe composite consists of cubic, ferrimagnetism Fe3O4 particles and spherical, conducting, diamagnetic graphite particles. Work is focused on the magnetotransport properties in terms of microscopic mechanisms, in the binary composite with different size, electrical, and spin polarization. (1) Magnetoresistance and percolation effect:There are 6 orders of magnitude conductance difference between Fe3O4 powder and graphite powder at room temperature. Therefore, it may exhibit a transition from high conductance to low conductance with the increasing of FeaO4 particles. The experiment indicates that the percolation threshold is pc=0.2l2 and a maximum MR value reaches -32% at T=77K when approaches increasingly . We mainly study the influence of microstructure on magnetoresistance near percolation region and discuss the mechanisms which lead to enhanced magnetoresistance. (2) Percolation threshold and critical exponentPercolation threshold and critical behavior of FesCVC composite have been studied at different temperature. The experiment indicates that percolation threshold is dependent not only size of particle but also the different of conductance of two components. At the same time, we explain the non-universal critical exponent in composites and variation of critical exponent at different temperature.2. MnZn ferrite /SiCh magnetic composite:Permeability , coercivity and the specific magnetization with the variation of magnetic particles content have been studied. The experiment indicates thatStudy...
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