The Studies On The Synthesis And Physics Properties Of Colossal Magnetoresistive Manganite Perovskites

The synthesis methods for manganese perovskite film and ceramic are systematically studied. The properties are greatly influenced by different preparing conditions. Based on the researches, new synthesis approaches are put forward for manganese perovskites. Two parts are included in this thesis.The first part is about the deposition of manganese perovskite films. We have studied the oxygen pressure during the deposition of La0.67Sr0.33MnO3 films with magnetron sputtering and annealing effect on the properties. Results show that raising the oxygen pressure will substantially increase the metal-to-insulator transition temperature and reduce the peak resistivity. After annealing, with a more effective supply of oxygen, the transition temperature shifts much higher, and grains grow larger to achieve more compact structure and lower resistivity. On the attempt to deposition La0.67Sr0.33MnO3 films with electron beam evaporation, we have found out that when at the substrate temperature of 500oC, epitaxial films can be deposited on LaAlO3 (001) substrate. However, with the absence of oxygen supply during deposition, films show insulator property on the measurable range. With annealed at 650oC in air for 10hours, films show metal-to-insulator transition at 185K. The other part is focus on synthesis of manganese perovskite ceramics. We have compared sol-gel method and solid state reaction, the two popular approaches for preparing perovskite ceramics, from the microstructure and electric transportation. A new method has been put forward. By mechanical ball milling, ingredient powders are refined to nano-scale, and differ from the mechanical alloying, we add ethanol to avoid phase transformation and amorphousity during milling. By post sintering the as-milled powders, polycrystalline La2/3Ca1/3MnO3 has been successfully synthesized. The microstructure and transport properties have been studied. Significant grain boundary effect was observed when the crystallite size was 170nm at milling time of 10hours and sintering temperature of 850oC. The microstructure and transportation were substantially affected by variation of milling time and sintering temperature. As both factors increase, grains grow rapidly, and grain boundary effect is reduced. Large MR of 23.5% is observed at 78K in an applied field of 3kOe with the average grain size of 170nm. While with average grain size of 4.5μm, MR of 22% was recorded near the metal-to-insulator temperature. A clear demonstration of the competition between intrinsic (from the crystal lattice) magnetoresistance (MR) and extrinsic (from the grain boundaries) MR had been observed.