Microstructure Characterization Of Perovskite Oxide Films And Electron Beam Irradiation Effect Investigation

In strongly electron correlated systems,the interactions between lattice,spin,orbital and charge have attracted enormous attention.The rising of two-dimensional（2D）materials makes it possible to tune these interactions.Introducing external strains,chemical doping can adjust and improve the physical properties in the 2D materials.Electron microscopy is one of the powerful methods to study micro structures and electronic structures.In addition,with the rapid development of corrections of sphere aberration and chromatic aberration,spatial resolution and energy resolution have been dramatically improved.In this dissertation,we mainly focused on the 2D transition metal oxides with perovskite structures using transmission electron microscopy.We also studied the electron irradiation effects on the materials.The main parts are as follows:1.Scanning transmission electron microscopy（STEM）was used to study the microstructures of the defect structures in LaCoO₃（LCO）/SrRuO₃（SRO）multilayer films grown on the SrTiO₃ substrate.According to the STEM-high-angle annular dark-field（HAADF）images,several types of Ruddlesden-Popper（R.P.）stacking faults were found:The directions of these stacking faults are perpendicular or parallel to the interface;the horizontal stacking faults locate either at the interface,or in the LCO layers;the vertical stacking faults are in the SRO layers.The formation of these stacking faults is correlated with chemical environment and external strains.In addition,we proposed 3D atomic structural models for these R.P.stacking faults,in order to provide a better understanding.2.To study the transform of the magnetic easy axis in La_1-xSr_xCoO_3-δ（LSCO）/La_2/3Sr_1/3MnO₃（LSMO）/La_1-xSr_xCoO_3-δ-SrTiO₃ substrate（x=0.2,0.5）thin films at low temperatures,we focused on the study of microstructures and electronic structures.The STEM images with atomic resolution demonstrate the LSCO layers provide different strain impacts on the LSMO layers.The dark stripes with horizontal and vertical directions induce the elongation and compression of the MnO₆ octahedra,hence the 3d electron orbital occupation is randomly distributed at room temperature（RT）.According to the EELS spectra at low temperature and RT,we found that the shrinking of lattice parameters of LSCO has a compressive strain to the lattice structure of LSMO,which makes MnO₆ octahedra elongate along c axis.Hence,dz²-r² orbit is preferentially occupied.Due to the spin-orbital coupling in transition metals,the orbital moment favors the direction of spin moment along c axis.Hence,the total magnetic moment is in the out-of-plane direction at low temperature.This study provides some evidence for the magnetic easy axis transition at low temperatures.In addition,considering the different strains from nanodomains in LSCO,the in-plane strain of LSMO should be unequal in both orthogonal directions.This anisotropy effect on the structure of the thin film reduces the symmetry of LSMO,which enhances the magnetic anositropy energy.3.Rear earth element cuprate（R₂CuO₄）compounds have attracted much attention due to the interesting physical properties,such as high temperature superconductivity.The distortion of Cu-O bonding is closely related with the electronic properties in cuprates.In this dissertation,we studied the incommensurately modulated structures in Tb₂Cu_0.83Pd_0.17O₄ compound.The electron diffraction patterns along different zone axes show the out-of-plane displacement on Cu/Pd,which is a unique modulated structure in R₂CuO₄.Under the electron beam illumination,a reversible and repeatable smectic–nematic phase transition occurs by tuning the electron dose rate.To figure out the phase transition mechanism,we carried out in situ heating,cooling and low voltage TEM experiments.The results reveal that electron radiolysis is the dominant factor to the phase transition.By quantitatively analyzing the modulated structures,we proposed that the origin of the superlattice structure is caused by the charge ordering on Cu sites.4.Manganese bromide（MnBr₂）is widely used in the syntheses of organic-inorganic hybrid compounds.It is necessary to understand its crystal structure and chemical stability.We studied the irradiation resistance of MnBr₂ under the electron beam illumination in TEM.Based on the experimental observations,we found that after adsorbing the vapor from air,the MnBr₂ powder is decomposed under continuous exposure to the electron beam.We recorded the evolution process,analyzed the chemical compositions and proposed a chemical transform path.