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Local investigations of coupled multiferroic order parameters
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Seddon, Samuel (2022) Local investigations of coupled multiferroic order parameters. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3884896
Abstract
This thesis aims to examine several material systems in which multiple ordering parameters are present. The origins of the multiferrocity are explored by observing the interaction of the different ferroic orderings at the nanoscale, using various scanning probe microscopies. Three distinct material systems are employed as they host different degrees of multiferroic coupling.
The first is magnetite, Fe3O4, humankind’s oldest-known multiferroic material. Ferrimagnetic below ∼860 K, magnetite becomes ferroelastic as it is cooled below the Verwey transition at 120 K. Magnetic force microscopy (MFM) data reveals three distinct magnetic switching behaviours in the material, regimes that are matched with bulk magnetometry data, and indicate a strong degree of magnetoelastic coupling. Below 38 K magnetite also hosts a ferroelectric ordering, where Piezo force microscopy (PFM) is used to visualise the ferroelectric domain patterns for the first time. PFM is also used to probe the nature of the ferroelectric phase transition locally.
The second is pyrrhotite, Fe7S8, a natural mineral exhibiting a strong magnetoelastic coupling. Here MFM is used to directly correlate local magnetic switching behaviours with various features within the bulk magnetic hysteresis; A magnetic hysteresis as acquired from MFM field dependent measurements is visualised. Local magnetic domains are correlated directly with expected pinning from ferroelastic domains, and the domains responsible for the difference in saturation magnetisation across samples are identified. Application of in-plane fields confirm the hard and easy magnetic axes of the material, again visualised by MFM for the first time. Finally, a constructed thin film heterostructure SrRuO3-PbTiO3 (ferromagnetic – ferroelectric) provides a system where a ferromagnetic layer has a forced polarisation at its interface. The system is shown to exhibit electrical measurements known to be a signature of Skyrmions - namely the topological Hall effect. Here field dependent MFM measurements acquired throughout the key fields were used to reveal the emergence of two periodic, non-topological, chiral spin textures. Scanning transmission electron microscopy and density functional theory are used to observe and analyse surface inversion symmetry breaking and confirm the role of an interfacial Dzyaloshinskii–Moriya interaction at the heart of the system. Classification of the types of multiferrocity is not the principle aim of this thesis, but rather a result apparent from the degree of coupling observed. At its core, this thesis looks to push the boundaries of what has so far been possible in scanning probe microscopy, as an exploratory tool for these complex systems. These boundaries include the physical measurement in high magnetic fields, and low temperatures, but also addressing the development of the research field, as data sets grow increasingly larger.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics T Technology > TK Electrical engineering. Electronics Nuclear engineering |
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Library of Congress Subject Headings (LCSH): | Ferromagnetic materials, Ferroelectric crystals, Ferroelectricity, Magnetic couplings, Pyrrhotite, Magnetic force microscopy, Skyrme model, Spintronics | ||||
Official Date: | 2022 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Alexe, M. (Marin) | ||||
Format of File: | |||||
Extent: | xi, 115 pages : illustrations. | ||||
Language: | eng |
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