| With the progress of science and technology, the development of the society, people are very active in researching and using all kinds of new energy. In recent years, the perovskite oxides have aroused attention because of their interesting physical phenomena such as the charge, spin, orbital ordering and phase separation. The perovskite oxides have colossal magnetoresistance (CMR) and possess important application value in industry for the development of magnetic recording read head and magnetic sensors. Studying the oversized magnetoresistance mechanism will play an important role in promoting and improving the development on many areas of condensed matter physics.In this dissertation, we select the perovskite oxides Sr1-xYxCoO3(0≤x≤0.30) and Sr1-xYxMnO3(0≤x≤0.30) as the subject. We study their crystal structure, microstructure magnetic properties, electrical transport properties and CMR effect, and we gain several valuable experiment and research results.This dissertation is divided into five chapters as following:The first chapter begins with the history and progress of the effect of magnetoresistance. We introduced the materials and characteristics of magnetoresistance, and we generalized the crystal structure, electron structure, magnetic structure, electrical transport properties, polaron theory and phase separation. Finally, we present the research purpose, ideas, and contents.In chapter two, we describe the preparation methods and processes of perovskite oxides. Then we introduce the polycrystalline sample testing methods and technologies, such as X-ray diffraction (XRD), scanning electronic microscope (SEM), vibrating sample magnetometer (VSM), the standard four-probe technique and superconducting quantum interference devices (SQUID) magnetic measurement system.In chapter three, we present the detailed results of the structure, surface topography, magnetic, magnetic phase transition, electrical transport properties and magnetoresistance effect of the polycrystalline sample Sr1-xYxCoO3(x=0,0.05,0.10,0.15,0.20,0.25,0.30) prepared by conventional solid state reaction method. The X-ray diffraction indicated that the system studied in the present investigation undergoes structure phase transition at room temperature from hexagonal to cubic and at last to tetragonal symmetry. For x=0, the sample shows a hexagonal structure with P63/mmc space group, while the samples with0.05≤x≤0.15reveal cubic with Pm3m group symmetry, and the samples with x≥0.20reveal tetragonal with I4/mmm group symmetry. The maximum coercive field is4.6kOe at x=0.20. The system shows magnetic properties at the applied field of lkOe:for x=0, the Curie temperature is163K and with increasing of Y-doping, the transition temperature increases, at the same time, the antiferromagnetism is enhanced. All the compositions show a semiconducting-like behavior and some complicated features of temperature dependence of magnetoresistance (MR) are observed: all the compositions show a feature that+MR and-MR coexist. For x=0.1, the sample shows a maximum absolute value of MR (about17%) at370K. The Sr1-xYxCoO3samples seemly to obey variable range hopping conduction model showing a linear ln(p) versus T-1/4dependence at the temperature range150K≤T≤400K.The forth chapter we study the structure, surface appearance and magnetic properties of the polycrystalline samples Sr1-xYxMnO3(x=0,0.05,0.10,0.15,0.20,0.25,0.30) prepared by conventional solid state reaction method. The experiment results show that Y doping didn’t change the original structure and all the samples have orthorhombic perovskite structure. The lattice constant a and b show a trend of decrease while c increase overall. According to the data of vibrating sample magnetometer (VSM), although the sample (x=0) can reach saturation in the given field scope, the saturation magnetization of samples increased with the increase of magnetic intensity without saturation. The magnetic intensity shows a trend of decreasing after increasing with Y substituting Sr, and the magnetic intensity reach a maximum when x=0.25. Surface morphology has been examined by SEM. With Y doping, the samples have good connectivity,high density and low porosity.The fifth chapter gives a brief summary and prospect of the research work. At the same time, we also put forward some questions awaited for further research. |
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