| Manganese peroveskite oxides provide an ideal laboratory for studying the physics of strongly correlated electronic systems due to the complex interaction between electrons,lattices and spins, so they have been the subject of intense study in recent years. Specially, the discovery of the colossal manganetoresistance (CMR) effect in this oxide has activated the interesting on the materials recently. However, because the larger magnetoresistance in this oxide was only observed in low temperature and decreased rapidly with the increase of temperature. Thus, these aspects restrict the application of the magnetoresistance in electronic devices. In this thesis, we use the optimal hole-doping manganites of La2/3Ca1/3MnO3 as the mother material for our study, and compound it with the heterogeneous structure oxide of SiO2. And then we mainly investigate the manganese oxides and related low-field magnetoresistance by engineering of heterogeneous structures for grain-boundaries in manganese. Meanwhile, its effect of engineering of heterogeneous structures for grain-boundaries on electrical and magnetic transport behaviors was also investigated. The main investigation of my thesis as the following: In chapter one, a summary of the structure, properties and magnetoresistance in the manganese oxides has been given, the research intention and significance of my thesis were put forward on the basis of the summary. The chapter two describes the preparation methods of samples, such as the solid state reaction, the sol-gel method and the composite method of the sample. And we do detailed researches on the influences of the different processes of the prepared of the samples in order to find the best process of the samples preparation. In chapter three, we summarize the essential character of pure La2/3Ca1/3MnO3 and then the effect of SiO2 doping on transport properties and the magnetoresistance of La2/3Ca1/3MnO3 has been experimentally studied. Compared with the pure La2/3Ca1/3MnO3, experimental results show that the metal-insulator transition temperature (Tp) of all the doped composites obviously increases and the resistivity extremely decreases. With the increase of SiO2 content in the range of x < 0.02, the Tp shifts to a higher temperature range and the resistivity decreases. Nevertheless, with the increase of SiO2 content for x > 0.02, the Tp shifts to a lower temperature range and the resistivity increases. What is more, a lager magnetoresistance with a width temperature window near room temperature can be obtained for the composites. In order to analyse the distribution of SiO2 in the composites, the microstructure of some samples were examined by XRD patterns and scanning electron microscope (SEM). The chapter 4 investigates the microstructure and electrical transport properties of the La2/3Ca1/3Mn(1-x)SixO3 composites. Because the size of Si4+ (0.40 ?) is smaller than that of Mn4+ (0.54 ?), the Mn-O-Mn bond length becomes shorter and the Double-exchange interaction between Mn ions gets stronger when the Mn4+ in La2/3Ca1/3MnO3 was substituted by Si4+. Thus the resistivity decreases and Tp shifts to a higher temperature range. At last, we also analyses the effect of the heterogeneous structures oxides on the grain-boundaries of the Manganese peroveskite oxides by comparing the experimental results of (1-x)La2/3Ca1/3MnO3+xSiO2 and La2/3Ca1/3Mn(1-x)SixO3 composites. And it is showed that a low-field magnetoresistance effect near room temperature can be obtained by the engineering of heterogeneous structures for grain-boundaries of ferromagnetic oxides. The last chapter is the conclusion.
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