First-Principles Study On The Structures And Properties Of Perovskite Manganese Oxides

Along with the rapid development of computational methods and computer technology, theory computation has become more and more important in modern science. Due to its moderate computational consumeand high precision, density functional theory (DFT) has become one of the most important methods in condensed matter physics, computational chemistry and material science.The discovery of magnetoresistance in metal multiplayer, magnetic tunneling junction and manganese perovskite oxides has aroused much attention recent years. Many works focus on the doped rare-earth manganate for their huge value of magnetoresistance and various magnetic and electric properties. However, there are still a lot of problems about the magnetoresistance mechanism which need to be clarified.In order to understand the states and properties of the material in nature, and direct its synthesis and improvement, SrMnO₃ system has been investigated by first-principles calculations with ultra-soft pseudo potentials plane-wave method based on the density functional theory. Two different crystal structures, i.e. cubic and hexagonal symmetries as well as three different chemical compositions, i.e. pure, and K-doped phases have been considered.In Thesis, we introduce the basic concept and progress of DFT. Development of quantum chemistry promotes the establishment of DFT. Theorem of Hohenberg-Kohn is the fundament of DFT and is developed to Kohn-Sham equation, which can be used to perform real calculations. Now, new corrections and extensions, together with developed exchange-correlation functionals, have made DFT more accurate and suitable for more systems. At the end of this chapter, we introduce some widely used simulation packages,in which we pay more attention to the Gaussian Software and the Gaussianbasis set.The calculated results indicate that the h-SrMnO₃ is antiferromagnetic (AFM) magnetic insulator at 0K, and the band gap is about 1.6 eV. The most stable magnetic ground state of hexagonal layers of SrMnO₃ displays the spin configuration of AFM coupling both within the face-sharing Mn2O9 dimer entity and between the corner-sharing octahedral. There are very strong hybridizations between the Mn 3d and O 2p states around Fermi level. h-SrMnO₃ belongs to covalent insulator, and these strong covalent interactions lead to the deviation of the ideal spin magnetic moment of the Mn4+. The spin exchange coupling constants are fitted within the Noodleman's broken symmetry methods through the calculated total energy for the various spin ordered states of h-SrMnO₃. The local microstructures (Mn-O-Mn bond) of h-SrMnO₃ determine the special magnetic exchange interaction. There are AFM exchange interactions both within the Mn2O9 entities and between the Mn ions in the corner-sharing octahedron of h-SrMnO₃, and the latter AFM exchange interaction is stronger than the former one.For c-SrMnO₃, we confirm that it takes the G-type AFM state using the first-principles method. And experiments give the same results. The calculated results indicate that the h-SrMnO₃ is AFM magnetic insulator at 0K, and the band gap is about 0.26eV. Here the calculated spin magnetic moment of the Mn4+ is 2.62μB, which got along with the value of experiments and deviated from the ideal spin magnetic moment of the Mn4+.Here we studied the stability of h-SrMnO₃.The free energy calculations showed that SrMnO₃ transforms from the hexagonal (P63/mmc) structure to the cubic (Pm-3m) structure at ³ GPa in agreement with experiment. During the transition, the discontinuous changes of the lattice parameters indicate the first-order phase transition characteristic.At last, The electronic structures of K-doped h-SrMnO₃ were studied also by first-principles calculations with plane wave pseudo potential method. The most stable magnetic ground state of Sr0.5K0.5MnO₃ displays the spin configuration of FM coupling within the face-sharing Mn2O9 dimer entity and AFM coupling between the corner-sharing octahedral. We found that there are energy levels in the band gap at 0K, which belongs to the hole-doped compound. And there are very strong hybridizations between the Mn 3d and O 2p states around Fermi level. The O2 of around Fermi level is more important. Analyzing Mulliken populations of Sr0.5K0.5MnO₃, the O of this system has strong covalent-bond character.