Studies On The Preparation And Transport Properties In Bulks And Thin Films Of La_1-xNa_xMnO₃

The colossal magnetoresistance (CMR) behaviors in perovskite manganese oxides have attracted great attention due to both fundamental theory of condensed matter physics and potential applications of CMR materials in magnetic devices, e.g., read and write heads for magnetic disk drivers, magnetic random access memories as well as magnetic field sensors. Therefore, the study on CMR effect has become a focus of recent condensed physics and material science. In this thesis, the author has investigated the physics and transport properties in bulks and thin films of Na-doped manganese oxide LaMnO₃.In chapter 1, we briefly review the progress and physics properties of different kinds of magnetoresistance materials. Finally, the potential applications about these kinds of materials are introduced.In chapter 2, we detailedly introduced plenty of physics properties and the research process in manganese oxides, including the crystalline & electronic structure, electromagnetic transport and some special physics phenomenons. Furthermore, we discussed some probably theoretical models and influence factors about CMR in manganese oxides.In chapter 3, the structure and electromagnetic properties of polycrystalline bulk La_1-xNa_xMnO₃ (x = 0.10, 1/6, 0.20) are investigated. The results indicate that the structure of all these samples is rhombohedral. The Curie temperature T_c is near or above room temperature. A bifurcation occurs between both Field-Cooled and Zero-Field-Cooled M^T curves. An apparent metal to insulator transition is observed in the ρ-T curve and the transition temperature corresponds to the Curie temperature T_c A largest MR ratio (17.4%) has been observed at 302K under a 0.5T magnetic field in x=0.10 sample. Meanwhile, an unusual electronic transport is observed at the low temperature. Correspond to the bifurcation, we think the unusual electronic transport due to the electronic and magnetic imhomogeneous distributions.In chapter 4, we studied the transport properties and magnetoresistance of La_1-xNa_xMnO₃(x = 0.10, 1/6, 0.20) thin films deposited onto LaAlO0₃ (001) single crystal substrates with RF magnetron sputtering. The results indicate that anapparent metal to insulator transition occurs in all thin films and the transition temperature Tp is much lower than that of bulks. Near the Tp, a large magnetoresistance is observed. The largest MR ratio (26.4%) has been observed at 236K under a 0.5T magnetic field in x=0.20 sample. According to the linear fitting results of resistivtiy data, the transport principles of thin films are combination effect of electron-electron, electron-magnetron scattering (low temperature) and small polaron hoping modal (high temperature).In the chapter 5, we studied the transport properties and magnetoresistance of Lao.8Nao.2Mn03/Lao.75MnC>3/ Lao.sNao^MnCh tri-layer film deposited onto LaAlO3(001) single crystal substrate with RF and DC magnetron sputtering. The structure of tri-layer film is oriented as examined by X-ray diffraction. The temperature dependence of resistance shows that the resistance of tri-layer film is larger than that of LNMO single film. A enhancement in the MR ratio (32.2%) has been observed at 200K under a 0.5T magnetic field. The enhancement in MR is probably due to a spin scattering of eletctrons at the layer interfaces and an interaction between the LNMO and the L0.75MO layer as well as a lattice distortion caused by an internal stress in the LNMO/L0.75MO/LNMO tri-layer.