Domain Walls And Structures Study On Multiferroic Compound YMnO₃

In this thesis, we applied the diffraction contrast image technique of transmission electron microscopy(TEM) and the high-angle annular-dark-field(HAADF) imaging technique with the Cs-corrected STEM to investigate the domain walls and structures in the hexagonal multiferroic material YMnO3. The main contents can be summarized as follows:1. we successfully applied high-angle annular-dark-field(HAADF) imaging technique with the Cs-corrected STEM to map the atomic shift in the hexagonal multiferroic material YMnO3. We have observed two types of interlocked ferroelectric domain walls and translation domain walls, which are accompanied with the1/6[210] and1/3[210] displacements respectively. These results verify the interlocked nature of the domain walls in hexgonal YMnO3. We also owed the origin of the interlocked domain walls to the randomness when generating the six different domains(α+、β-、γ+、α-、β+、γ-) and the least-energy principle. For the later HAADF imaging study, we also observed the lateral transformation between different ferroelectric translation domain walls(typel and type2) with distinct displacements.2. we applied the diffraction contrast image technique in the JEOL2010TEM to study the six-state vortex domain, four-state vortex domain and the ferroelectric annular domain in the hexagonal multiferroic material YMnO3. We created all the possible seven kinds of schematic diagram of the six-state vortex domain according to the number of different ferroelectric translation domain walls(typel and type2) along the b axis. We have speculated that the four-state vortex domain in the dark field image is formed according to the translation of edge dislocation. We also made a simple analysis of the ferroelectric translation domain walls in the ferroelectric annular domains, and we finally draw a conclusion that the annular domain wall is composed of two types of different domain walls(typel and type2) along the b axis. Subsequently, we applied the HAADF imaging technique with the Cs-corrected STEM to map the polarizations of the four-state vortex-like domain which is similar to the six-state vortex-like domain.