Microstructural Study Of Multiferroic Materials By Electron Microscopy

Multiferroics is a hot topic in both materials science and condensed matterphysics, which has extensive applications in storages, sensors, and spintronicdevices. As materials’ properties are determined by their structures, in order tounderstand the novel properties of multiferroics and the mechanisms involved, itis of vital importance to study the microstructures of multiferroics. In this thesis,the microstructures of several important multiferroics are studied by usingdifferent kinds of techniques of electron microscopy.Firstly, atomic-scale study of topological vortex-like domain pattern inhexagonal manganite YMnO₃single crystals was carried out by usingCS-corrected transmission electron microscopy. The domain configurationshown here was confirmed to be different from a real vortex and thevortex-related issue was discussed. We pointed out that distinguishing of si_xferroelectric domains in the vortex-like pattern was not the sufficient conditionto determine whether this pattern was a real vortex or not. The antiphaserelationship must be carefully checked. Local ferroelectric dipole distributionwas provided by quantitative image analysis. Moreover, atomic configurationsof two kinds of interlocked domain walls were revealed with the help ofatomistic simulation.Secondly, the microstructures of hexagonal and orthorhombic manganiteepitaxial thin films were studied, respectively. The microstructural study ofhexagonal HoMnO₃thin films indicated that major defects in the films wereout-of-phase boundaries and their formation mechanisms were ascribed to thesurface step mechanism and the nucleation layer mechanism. The defects wereoff-stoichiometric and electronic structure of HoMnO₃was affected byhigh-energy electron beam irradiation. On the other hand, microstructural studyof epitaxial stabilized orthorhombic TmMnO₃thin films indicated that majordefects in the films were interfacial misfit dislocations. Epitaxial strain in thefilms was relaxed by misfit dislocations as well as surface fluctuations.Thirdly, the microstructure of La-and Ti-codoped BiFeO₃epitaxial thinfilms was characterized. It was found that La-and Ti-codoping can reduce the leakage current and improve ferroelectric behavior of BiFeO₃without changingits crystal structure. The correlation of microstructure with leakage current wasdiscussed. The leakage problem was found to be caused by different kinds ofdefects and chemical inhomogeneities in the films and the leakage mechanismwas dominated by bulk-limited Poole-Frenkel emission.Finally, the microstructures of Pb(Mg_1/3Nb_2/3)_1-xTi_xO₃（PMN-PT） basedmultiferroic composites were revealed. The microstructural study ofYBa₂Cu₃O_7-δ/PMN-PT indicated that major defects in the films were interfacial misfitdislocations and planar defects. Moreover, a small amount of a-axis oriented domainsexisted and in some cases tiny precipitates were observed. The formation mechanism ofthe inclined domain walls was discussed. It was a consequence of competition betweenthe surface elastic energy and the domain wall energy. The microstructural study ofPr_0.6Ca_0.4MnO₃/PMN-PT indicated that many oriented domains existed in the films.Atomic-scale characterization of the anti-phase boundaries pointed out that all thedomain walls were consist of single-line anti-phase boundary units regardless of theirdifferent apparent morphologies. In addition, the number of the oriented domainsincreased with the increase of thickness of the thin films, which was in favor of themanipulation of phase separation in Pr_0.6Ca_0.4MnO₃.