The Effect Of La-Doping On Electronic Structure And Magnetic Properties Of Ca₄Mn₃O₁₀

Since the discovery of colossal magnetoresistance (CMR) in perovskites, there have been extensive experimental studies on crystal structures, electronic, magnetic and transport properties, etc. of perovskites, the aim is to find the origin of the magnetoresistance. Up to now, there are two main viewpoints on the mechanisms of CMR: one is the magnetic scattering origin based on the double-exchange model; another is the role of the Jahn-Teller distortion effect. The central focus of those viewpoints is to explain the increase of the resistance and the consequent matal-to-insulator transition around the transition temperature.Meanwhile, CMR is also found in the layered perovskite oxide with a lower external field, which will makes the application more fesible. The quasi-two dimensional feature exhibited by the layered perovskites has a crucial effect on the lowered external field. In order to get a overall picture on the evolution of the electronic and magnetic properties in the layered perovskite oxide as well as the quasi-two dimensional feature in CMR, we carry out a theoretical study on the electronic and magnetic properties of triple-layer perovskite oxide, and compare the results with the experimental observations. Hoping to get a further understanding about the abundant features exhibited by perovskite oxide.We employ the multiband Hubbard model to describe the perovskite oxide. Using the unrestricted Hartree-Fock approximation, we get a linearized Hamiltonian. We apply the real space recursion method to calculate the energy band structure and some pertinent physical quantities. The five metastable states we take into consideration including ferromagnetically ordered state (FM); nearest neighbor (G-type) an-tiferromagnetically ordered state; chain (C-type) antiferromagnetically ordered state; and the two types of layer antiferromagnetically ordered state, including Al-type antiferromagnetically ordered state and A2-type ferromagnetically ordered state. We finally compare our study with the experimental results and give an explanation.The whole thesis consists of six chapters.Chapter one presents a review of the history and progress of perovskites, introduces some of the experimental observations about the perovskite La_1-xCa_xMnO₃ and gives the definition of magnetoresistance.Chapter two is mainly about the physical properties and theoretic explanation of perovskite oxide. In the physical properties, we introduce the crystal structure, electronic structure and some of the ordered phases; in the theoretic explanation, we discuss the super-exchange, double-exchange model, Jahn-Teller distortion effect and their explanation of the experimental observations.Chapter three presents the experimental observations of layered perovskite oxides, including the double-layer and triple-layer perovskite oxides.Chapter four introduces the theoretic model and the numerical method used to study the perovskite oxide. Including the multiband Hubbard model, the unrestricted Hartree-Fock approximation, and the real space recursion method.In chapter five we display our calculated results, make a comparison with the experimental observations, and give an explanation.In the final chapter, we give a summarize and the prospect of furture work.