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Title: Domains and functionality in multiferroic BiFeO3 films
Author: Waterfield Price, Noah
ISNI:       0000 0004 6501 1368
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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For over half a century, the technological promise of spins manipulable by a small voltage has captivated the interest of experimental and theoretical researchers alike. However, if thin-film multiferroics are to be incorporated into future data storage devices, a much greater understanding of their behaviour and how they differ from their bulk counterparts is required. In this thesis, we probe the fundamental multiferroic properties of BiFeO3 films through a combination of state-of-the-art diffraction and microscopy techniques. We investigate the coupling between magnetic, ferroelectric, and structural order, with a focus on domains, and how the domain structure may be manipulated in order to tailor the multiferroic properties of the material. Using non-resonant magnetic x-ray scattering (NXMS) and neutron diffraction, we study the magnetic and structural properties of (111)pc-oriented BiFeO3 films. Contrary to the general belief that to they grow as a rhombohedral monodomain, we find that they comprise a sub-micron texture of monoclinic domains. The magnetic structure is found to be intimately coupled to the structure, resulting in the propagation vector being locked to the monoclinic b-axis. This magnetoelastic coupling opens up a route to strain-engineer the magnetic domains via epitaxial strain. By growing BiFeO3 films on a lower-symmetry, TbScO3 substrate, we are able to engineer a magnetic, structural and ferroelectric monodomain, coherent over the entire film, constituting an increase in the domain size by over five orders of magnitude. We directly demonstrate the coupling between ferroelectric and magnetic order parameters of the cycloidal magnetic structure. Using NXMS polarimetry to measure directly the magnetic polarity, we show that upon switching the ferroelectric polarisation, the magnetic polarity switches accordingly---a major rearrangement of the magnetic structure, with each spin rotating by 90 degrees on average. This goes counter to idea that magnetic and ferroelectric order parameters are only weakly coupled in type-I multiferroics. Finally, using photoemission electron microscopy we are able to directly image the sub-micron magnetostructural domain structure. We further show that there is a strong interfacial coupling between the magnetostructural domains of BiFeO3 with a ferromagnetic overlayer. The BiFeO3 domains are found to impose a uniaxial anisotropy in the overlayer, opening up a route to control ferromagnetic domains.
Supervisor: Dhesi, Sarnjeet ; Radaelli, Paolo Sponsor: Engineering and Physical Sciences Research Council ; Diamond Light Source
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Thin Films ; Magnetism ; Ferroelectricity ; Magnetic x-ray diffraction ; Thin films ; X-ray photoemission electron microscopy ; BiFeO3 ; Ferroelectrics ; Magnetic neutron diffraction ; Multiferroics