Studies Of Transport And Magnetoresistance Properties In TiO₂ And WO₃ Doped La_2/3Ba_1/3MnO₃

Due to the discovery of the colossal magnetoresistance effect, the influences on the CMR effect and related physical properties focused on perovskite manganite with different additions were extensively studied. In this thesis, a series of (1-x) La₂/3Ba₁/3MnO₃+xTiO₂(x=0⁵%) and La₂/3Ba₁/3MnO₃+xWO₃(x=0²0%)polycrystalline ceramic samples were prepared by solid-state-reaction method. The microstructure and surface morphology of the samples were characterized by X-ray diffraction (XRD) using Cu Ka radition and microscope at room temperature. The magnetoresistances of the samples were measured in different magnetic fields at 300K and 78K. The resistance measurements were performed by four-probe method at 30²90K. The results were as follows:(1) The magnetoresistances are obviously influenced by sintering temperature greatly in (1-x)La₂/3Ba₁/3MnO₃+xTiO₂(x=0⁵%) system. XRD pattens show a single phase with perovskite structure for the samples sintered at 1450℃. With increasing TiO₂ doping level, the resistivity increases quickly, and the metal-insulator transition temperature T_p decreases. The samples with 1% TiO₂ dopant can enhance the magnetoresistance obviously. The magnetoresistance at 300K in an applied magnetic field of 1T reaches 12%, which is 3 times larger than that of pure La₂/3Ba₁/3MnO₃ under the same condition.(2) The magnetoresistances at room temperature are influenced by the sintering temperature obviously in WO₃ doped La₂/3Ba₁/3MnO₃, when WO₃ doping level is less than 15%. The average grain size of the samples sintered at 1450℃ is about several microns. With increasing WO3 doping level, the resistivity increases, and the metal-insulator transition temperature T_p decreases gradually. When WO3 doping level is in the range of 1%¹5%, the magnetoresistance at 300K for the samples sintered at 1450℃ is obviously larger than...