The Study Of The Correlation Between The Magnetism And Microstructures Of Epitaxial Cobalt Oxide Thin Films

The low-dimension functional materials have attracted great attention in recent years because of their complex microstructures and mysterious physical properties.A deep study of the correlation between the microstructure and physical properties is of great significance to the construction of novel micro-nano devices using materials.In this thesis,we systematically studied correlation of microstructure and magnetism of strained cobalt oxide thin films,by using combined transmission electron microscopy(TEM)analysis,the traditional characterization methods and the first-principles calculation.Great efforts have been devoted to spin-state and magnetism modification through changing temperatures,applying external pressures,introducing divalent ions,irradiating by electron beam,induced interface by constructing superlattices.Moreover,we further investigated the effects of electron beam irradiation on the microstructure of the low-dimensional materials in in situ TEM.The main research contents are summarized as follows:1.This thesis presents a systematic analysis for the atomic lattice of the strained La1-xSrxCoO3(0?x?0.1)epitaxial films grown on SrTiO3 and LaAlO3 substrates.Superstructures characterized by dark stripes are observed in the lattice image,evolving with combined Sr-doping and lattice strains.Interestingly,we found a close relation between the proportion of the Co ions in dark stripes and the saturation magnetization of the films: the latter grows linearly with the former.This result implies that magnetism could be exclusively ascribed to the Co ions in dark stripes.2.The correlation of microstructure,electronic structure and magnetism of La0.9Ca0.1CoO3 thin films grown on SrTi O3 substrates was studied using the Scanning Transmission Electron Microscopy and Electron Energy Loss Spectrum techniques.The results show that saturation magnetization of the pristine La0.9Ca0.1CoO3 thin films is measured to be ~0.2 ?B/Co,which is significantly lower than that of LaCoO3 thin film(~0.8 ?B/Co).The theoretical calculations indicate that smaller radius Ca2+ doping in LaCoO3 thin films transforms high spin(HS)Co3+ ions in the dark Co-O layers to low spin(LS)Co ions,decreases the number of magnetic Co ions and the pseudo superexchange interaction between HS-LS-HS Co3+ ions.3.We demonstrated the modification of the magnetism of La0.9Ca0.1CoO3 thin films by introducing stripe-like superstructure in a controllable manner using the electron beam irradiation in a transmission electron microscope.The results indicate that Electron Beam Irradiation(EBI)induced unit cell volume expansion accompanies the formation of oxygen vacancies which leads to the spin state transition of Co ions.The double exchange interaction between high spin Co2+ and intermediate spin Co3+ in the dark Co-O layers increases.Our results not only pave the way for tuning the magnetization of La0.9Ca0.1CoO3 thin films artificially by EBI,but also a deep understanding the origin of magnetism of La0.9Ca0.1CoO3 thin films,which is of great significance for the designing of the future devices using La0.9Ca0.1CoO3 related materials.4.The effects of SrTiO3 interlayers on the microstructure and magnetic properties of strained LaCoO3-SrTiO3 multilayers were investigated.On one hand,the SrTiO3 interlayers in the multilayers can keep the strain on LaCoO3 layers,which is conductive to the formation of stripe-like superstructure in ultra thin LaCoO3 layers.The ordering of the high spin Co3+ in the dark layers increases the magnetization of multilayers.On the other hand,there is a deadlayer without any dark stripes in the interface between SrTiO3 and LaCoO3 layers,due to the elemental diffusion.High spin state Co ions transforms into low spin state in the deadlayer,which suppresses the magnetization of multilayers.5.The La CoO3-SrTiO3 multilayers on LaAlO3 substrate shows a larger magnetization than that of LaCoO3 thin film.Dark stripes perpendicular to interface were appeared in the multilayers,while only few ones parallel to interface were observed in thin films.The huge lattice mismatch between LaAlO3 substrate and SrTiO3 interlayers of multilayers is quickly released by forming a new superstructure in the first LaCoO3 layer.The lattice constant of LaCoO3 layers is mainly affected by SrTiO3 interlayers instead of the LaAlO3 substrate,which explains the appearance of stripe-like superstructure and increase the magnetization of LaCoO3-SrTiO3 multilayers on LaAlO3 substrate.6.Individual Au85Sn15 nanowires with low melting point have been synthesized.The evolution of their morphology,microstructure,chemistry,and phase change induced by EBI in In Situ TEM were studied for the first time.The wire-shaped nanosolders melted and reflowed to spherality,the multi-phases can be transformed into a nearly homogeneous Au5 Sn phase under irradiation of electron beam.The melting process is a combined transformation of chemical and crystal phases.Our work is believed not only provide a new glance at the intrinsic melting mechanism of 1D Au85Sn15 nanoscale solders under EBI,but also a deep understanding the effects of electron beam irradiation on low-dimensional materials.