Transmission Electron Microscopy Studies On 3d And 5d Perovskite-like Transition Metal Oxides

Transition metal oxides show novel physical phenomena due to a variety of complex interactions among lattice,charge,spin,and orbital.Some of the materials with excellent properties have already been widely used in life,which has changed the way of human life and greatly promoted the development of modern society.Therefore,this kind of material has been and will be the target of scientists’explorations to search for materials with better performance.The crystal structures and physical properties of3d transition metal oxide La_0.67Sr_0.33Mn O₃（LSMO）and Sr Co O_2.5（SCO）films as well as 5d transition metal Ir oxide polycrystalline materials are studied in this dissertation.The main contents are as follows:1、LSMO is a ferromagnetic metal with room temperature Curie temperature and nearly 100%spin polarization,which has potential applications in spintronic devices.LSMO single layer and La_0.67Sr_0.33Mn O₃/Sr Co O_2.5（LSMO/SCO）double layers were grown on La_0.3Sr_0.7Al_0.65Ta_0.35O₃（LSAT）single crystal substrates by using pulsed laser deposition.The magnetic measurements show that the easy magnetization axis is along the[001]direction in the film plane in single layer LSMO,while the easy magnetization axis is along the[1 1?0]direction in the film plane in double layer LSMO.The ferromagnetism originates from Mn ions in LSMO.Using transmission electron microscopy,we statistically analyzed the distances between adjacent Mn ions in different directions.It is found that in single layer LSMO,the distance between neighbor Mn ions in[001]direction has the largest value and the Mn O₆ octahedron is elongated along[001],thus the 3z²-r² orbital energy level is lower than the x²-y² orbital energy level and the e_g electron of Mn³⁺prefers to occupy the 3z²-r² orbital,so the macroscopic easy magnetization axis is along the in-plane[001]direction.In the double layer LSMO,the distance between neighbor Mn ions in[001]direction has the smallest value and the Mn O₆ octahedron is compressed along[001].In this case,the energy level of x²-y² orbital is lower than that of 3z²-r² orbital and the e_g electron of Mn³⁺prefers to occupy the x²-y² orbital.The x²-y² orbital direction is along the crystallographic[100]and[010]directions.The shape anisotropy makes the magnetic moment direction prefer to incline along the film plane.So,the magnetic moment direction inclines along the[11?0]direction in the plane and thus the macroscopic easy magnetization axis is along the[11?0]direction.Through the analysis of electron energy-loss spectra,we observed that there is no obvious change valence state of Mn has no obvious change at LSMO/SCO interface,while the valence state of Co has a slight decrease.We think that the change of valence state of Co is caused by the change of local oxygen content,and there is no obvious charge transfer between Mn and Co ions.2、In LSAT/LSMO/SCO double layers,the microstructure of SCO was analyzed by transmission electron microscopy.It is revealed that both LSAT and LSMO layers have perovskite structures,while SCO has a brownmillerite structure.By comparing the experimental and simulated results of selected area electron diffraction patterns and high-angle annular dark-field images,we confirmed that the space group of SCO is Ima2（No.46）.In addition,there are perpendicular domain structures and stacking-fault structures in SCO.Based on quantitative analysis,it is found that the Co O₄ tetrahedron and Co O₆ octahedron in SCO are directly connected to form a vertical domain structure,and the oxygen content in the domain knot region of the octahedral layer remains almost unchanged,while the oxygen content in the domain knot region of the tetrahedral layer increases slightly.A stacking-fault structure is observed near the domain knot area.In the case that the Co O₄ tetrahedron and Co O₆ octahedron keep in the same layer,the oxygen content in the transition region changes nearly continuously.We also observed Sr Co O_2.5-δ（0<δ<0.5）nanocrystals with reduced oxygen content at the LSMO/SCO interface.3、Sr₃Ir₂O₇ polycrystalline samples were successfully synthesized by using high temperature and high pressure apparatus.Combined with the experimental and simulated results of transmission electron microscopy images,we found that there are both tetragonal and orthorhombic phases in the polycrystalline Sr₃Ir₂O₇.It is confirmed that there is a 90°crystallographic domain structure of the orthorhombic phase keeping the same long axis.We consider that this may be related to the reported 90°antiferromagnetic domain structures in Sr₃Ir₂O₇.In addition,the measured magnetic and transport properties show consistent transition characteristics,indicating that Sr₃Ir₂O₇ has a complex and interesting physical connotation.4、Polycrystalline Sr₄Ir₃O₁₀ was synthesized by high temperature and high pressure method.XRD result shows that Sr₃Ir₂O₇ and Sr Ir O₃ phases also coexist besides Sr₄Ir₃O₁₀.Combined with the experimental and simulated results of transmission electron microscopy,it is found that there is not only a tetragonal phase but also an orthorhombic phase in the synthesized polycrystalline Sr₄Ir₃O₁₀.This is the first time to report experimental evidence.