First-principles Study Of Magnetism And Electronic Structures In LaTi O₃,SmNiO₃ And SeXo₃（X=Co,Mn） Films

ABO₃ perovskite oxides have abundant lattice structures.Due to the very strong coupling between lattice,orbital,charge and spin,the system has many novel physical properties,including colossal magnetoresistance effect,high-temperature superconductivity,ferromagnetism,ferroelectricity,topology,etc.,play a very important role in material preparation and device design.With the development and evolution of miniaturization and multifunctionalization of devices,the modification and regulation of material properties is one of the hot topics in the field of materials science,such as the realization of magnetic phase transitions,charge transfer,metal-insulator transitions,and magnetoelectric coupling.On the one hand,epitaxial stress can be achieved through lattice mismatch between the substrate and the film,causing the oxygen octahedra in the material to rotate and tilt,thereby affecting the magnetism and electronic properties of the material.On the other hand,superlattices can also be used to couple perovskite oxide materials with different physical properties between layers,and obtain completely different physical properties from the parent materials through interface effects,thus showing more interesting physical phenomena.Therefore,in-depth exploration of the underlying mechanisms behind the interesting physical phenomena of perovskite transition metal oxides is central to moving towards practical applications.In this thesis,the physical properties of perovskite transition metal oxides,such as magnetism,electronic structure and orbital order,are studied by first-principles calculations.The main research contents are as follows:（1）Applying epitaxial stress to the film through the substrate and adopting different growth orientations are common methods to study perovskite oxides.In this paper,the influence of epitaxial stress on the magnetism and electronic structure of La Ti O₃ films with[110]-oriented is investigated.Research shows that by adjusting the magnitude of epitaxial stress,the material undergoes a magnetic phase transition.Lattice distortion（i.e.,Ti-O-Ti bond angle and Ti-O bond length）plays an important role in the material’s physical properties（e.g.,magnetism and electronic structure）.（2）The interface effect is one of the effective ways to control the physical properties of perovskite oxides,and new electronic states can be formed through charge transfer at the interface.In this paper,the effect of charge transfer at the（Sm Ni O₃）_n/（YTi O₃）_m superlattice interface on the electronic structure of Sm Ni O₃ is investigated.Research shows that the charge transfer at the interface greatly changes the electronic structure of the material.For（Sm Ni O₃）₁/（YTi O₃）₁ and（Sm Ni O₃）₂/（YTi O₃）₂ superlattices,the system exhibits insulating properties.For the（Sm Ni O₃）₃/（YTi O₃）₁ superlattice,the system exhibits metallic properties.（3）Generally speaking,the A-site ion of ABO₃ type perovskite oxides are+2 or+3 valence,and the B-site ion are+4 or+3 valence.However,for Se XO₃（X=Co,Mn）materials,the valence of Se ion is+4,and the valence of X ion is+2,which is rarely studied at present.In this paper,the effects of epitaxial stress and oxygen defects on the electronic structure and magnetic properties of Se XO₃（X=Co,Mn）are investigated.It is found that stress and oxygen defects have different effects on the magnetic ground state and band gap,which are closely related to the changes of X-O bond length and X-O-X bond angle.