Transmission Electron Microscopy Study Of Perovskite Functional Oxide Films

Perovskite functional oxide films have received extensive attention due to their unique physical properties such as colossal magnetoresistance,ferroelectrics and multi-ferroics.As a strongly-correlated electron system,they demonstrate abundant physical phenomena through the coupling among electron spin,orbital and crystal lattice.In this thesis,the microstructure of perovskite function oxide films was investigated using transmission electron microscopy(TEM)in details,and was further correlated with their physical properties.In chapter 1,the preparation,microstructure and physical properties of perovskite functional oxide films are reviewed.Based on the crystal,electronic and magnetic structures of perovskite oxides,the preparation,microstructure characterization,and strain state of the epitaxial films are introduced.Furthermore,recent research developments on perovskite epitaxial films are summarized.In chapter 2,the surface morphology and microstructural defects of SrRuO3 epitaxial films were investigated by high-resolution TEM(HRTEM),and the growth mechanism of films was further explored in details.TEM observations demonstrate that the free surface of the epitaxial film is not smooth,and several V-grooves are unevenly distributed on the flat surface.The dominant angle between the sidewalls of these V-grooves is 90 o,and threading dislocations usually end at the apexes of the V-grooves.During the film growth process,a two-dimensional growth mode transforms into a three-dimensional one,along with the formation of V-grooves on the film surface.In chapter 3,the microstructure of LaCoO3 films with different substrate was investigated by HRTEM,and was further correlated with their magnetic properties.Nanoscale domains with either horizontally or vertically modulated dark stripes are observed in the LaCoO3 films on SrTiO3 and(La,Sr)(Al,Ta)O3 substrates,whereas only horizontally-modulated domains are found in the films on LaAlO3 substrate.These modulated dark stripes stem from oxygen-vacancy ordering.Magnetic measurements show that the saturation magnetization of the film on SrTiO3 is larger than those on other substrates.The magnetic properties of the films strongly depend on the strain state,the content of oxygen vacancies and the orientation of domain structures.In chapter 4,HRTEM,electron energy-loss spectroscopy coupled with scanning-transmission electron microscopy were used to investigate the interfacial structure and oxygen chemical environments of BaTiO3/SrTiO3 superlattice films.In the O-K edge spectra of BaTiO3,the energy-loss peak at ~ 547 eV splits into two peaks,whereas no splitting was observed in the spectra of SrTiO3.The multiple-scattering calculations of O-K edge spectra indicate that the splitting of BaTiO3 was attributed to the complex crystal structure including two unequal oxygen sites and low site symmetry.