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Advanced electron microscopy study of charged ferroelectric domain walls in multiferroic BiFeO3
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Ge, Wanbing (2023) Advanced electron microscopy study of charged ferroelectric domain walls in multiferroic BiFeO3. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3985146
Abstract
Ferroelectric materials, characterized by the existence of non volatile polarization vector whose direction can be switched by external field, have formed a large functional applications and raised extended and profound research interests. As the result of achieving a lowest possible energy profile, ferroelectric materials are generally divided into smaller domains with different orientations of the polarization vector, separated by a thin interface known as the domain wall. Domain walls, where the polarization transition happens, take up only a small share of the sample volume and were used to regarded as topological defects. However, they are found to possess a different electrical property unobserved in the bulk domain, which boosted the study and design of domain-wall dedicated devices. The object of this thesis is to build a comprehensive understanding of the complex domain and domain wall structure inside the flux-grown single crystal BiFeO3, examine ferroelectricity at vicinity of the domain wall down to the atomic scale, and study the change of electrical/chemical properties at the domain wall by the help of advanced transmission electron microscopy techniques.
First, domain structure in the flux-grown BiFeO3 single crystals was reconstructed covering a broad range from the bulk size down to atomic scale. A dense array of stripe nanodomains separated by sawtooth and flat domain walls were found to repeat within the crystal. Moreover, both domain walls were found to be 180°- type, a very rare structure which was conventionally believed to have high energy cost. The domain structure indicates the head-to-head flat domain walls were highly negatively charged, while the tail-to-tail sawtooth domain walls were either neutral or positively charged.
Next, local polarization transition was studied at the vicinity of both domain walls by polarization mapping at atomic resoltuion. The neutral sawtooth domain walls are found to be Ising-type, while the charged sawtooth walls show a mixture of Ising/Bloch/Néel behaviour. On the other side, the flat domain walls were found to have a reconstructed atomic structure that is stoichimetriclly different from the bulk material, and there is no polar transition at the flat walls.
Finally, local bonding status of the flat domain walls were investigated by high resolution EELS. Reconstructed flat domain walls are revealed to consist of alternating Bi clusters and Fe+O clusters, and have a slightly increase bandgap compared to the bulk. Fine ELNES analysis indicates Fe atoms are under the vary similar octahedra condition to the bulk material.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics | ||||
Library of Congress Subject Headings (LCSH): | Ferroelectric crystals -- Materials, Domain structure, Transmission electron microscopy, Polarization (Electricity), Bismuth ores | ||||
Official Date: | July 2023 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Sańchez, Ana M. | ||||
Format of File: | |||||
Extent: | xvii, 108 pages : colour illustrations | ||||
Language: | eng |
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