Transmission Electron Microscopy Study Of Transition Metal Oxide Thin Film

Recently,as the high developing speed in information,it is very important to achieve device miniaturization.Expitaxially grown single crystallization thin film has become a good approach.Exploring the microscopic mechanism of novel physical properties of materials at atomic scale plays a guiding role for the designing of the next generation device.Based on this purpose,we have conducted the following works by scanning transmission electron microscopy（STEM）and first-principles calculations.1. An abnormal suppression in saturation magnetization with increasing the thickness of LaCoO₃ was observed in（LaCoO₃/LaMnO₃）₅ superlattices with fixed LaMnO₃ thickness and varied LaCoO₃ thickness grown on SrTiO₃（001）substrates.In this work,the superlattices were studied by STEM at the atomic scale.Based on the results of selected area electron diffraction and high-angle annular dark-filed images,we found that the c axis of LaMnO₃ sandwiched by LaCoO₃ layers experiences a transition from the in-plane direction to the out-of-plane direction as the LaCoO₃ layer thickness increases due to the oxygen octahedral（tetrahedral）coupling with LaCoO₃layers.Utilizing peak pairs analysis on annular bright-field（ABF）images,the precise atomic positions and the degree of octahedral rotation are obtained.It shows that the degree of MnO₆ octahedral rotation increases with increasing the thickness of LaCoO₃,leading to the suppression in saturation magnetization.Our work provides a way to tune the magnetic properties of epitaxially grown thin films via interfacial octahedral engineering.2. Grouping different transition metal oxides together by interfacial engineering is an important route toward novel phenomena.The magnetic easy axis of single layer La_2/3Sr_1/3MnO₃ thin film is along the in-plane direction,while we measured a perpendicular magnetic anisotropy in LaCoO_2.5/La_2/3Sr_1/3MnO₃/LaCoO_2.5 trilayer.Using ABF-STEM,we found a coupling between MnO₆ octahedra and CoO₄ tetrahedra occurs at interface,which leads to the elongation of MnO₆ octahedra.Furthermore,the result of first-principles calculations shows that the elongation of MnO₆ octahedra at interface causes a prefer occupation of the d_3z²-r² orbital for e_g electron.Through spin-orbital coupling,the total magnetic moment of the La_2/3Sr_1/3MnO₃ thin film prefers to arrange along the out-of-plane direction and thus has the perpendicular anisotropy.The present work demonstrates the great potential of symmetry engineering in designing artificial materials on demand.3. Lanthanum cobalt oxides have attracted inclusive attentions because of the stripelike structures related to magnetism.In this study,we tuned two metastable states in three kinds of lanthanum cobalt oxide thin films by electron beam irradiation and recorded their dynamic transition process in situ in a transmission electron microscope.The lanthanum cobalt oxide thin films commonly exhibit a homogeneous microstructure in the initial state and then transfer to a stripelike superstructure with 3a₀ periodicity（a₀ is the simple perovskite lattice parameter）,further develop into a superstructure with 2a₀ periodicity in dark stripes（brownmillerite structure）.To explore the forming energy discrepancy within the two metastable states,we performed first-principles calculations on the LaCoO_3-δ（0≤δ≤0.5）thin film system by geometry optimization.The calculation results suggest that the forming energy of the 3a₀periodicity stripelike structure is a little lower than that of the 2a₀ periodicity in the LaCoO_3-δthin film.Our work explains why the two stripelike structures often coexist in lanthanum cobalt oxide thin films and extends prospective applications related to oxygen vacancies in thin films.4. Imaging of atomic-scale mechanisms behind ferroelectric switching in new multiferroic materials is critical for gauging their potential use in nanoelectronic devices.In this realm,ε-Fe₂O₃ films combining ferrimagnetism and ferroelectricity at room temperature are of exemplary interest.Here,a high-resolution in-situ STEM study on ionic transpositions and domain wall dynamics inε-Fe₂O₃ films under an applied electric field is combined with first-principles calculations.The experiments demonstrate that ferroelectric switching proceeds locally via the motion of non-polar domain walls in an electric field of 250-500 kV cm^-1.Calculations indicate that the activation barrier for switching at domain walls is nearly a quarter of that corresponding to the most likely transition paths in bulkε-Fe₂O₃.Moreover,domain walls provide symmetry lowering,which we show is necessary for ferroelectric switching.