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Spin-like polar textures in ferroelectrics
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Rusu, Dorin (2021) Spin-like polar textures in ferroelectrics. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3759539
Abstract
The bulk properties stemming from the polar nature of the ferroic materials makes them suited for a wide range of devices such as piezoelectric actuators, non-volatile memories, pyroelectric sensors. In thin film form, the physical properties of these materials (polarization, domain topologies, Curie temperature etc.) become highly susceptible to the electrical and mechanical boundary conditions imposed by the boundary layers. The interface coupling between the ferroelectric and its boundary layers can give rise to unique phenomena which is unobserved in bulk ferroelectrics. The objective of this thesis is to pursue a recently observed ability of ferroelectric materials to display, under specific boundary conditions, complex polar topologies that resemble magnetic spin textures induced by non-collinear spin interactions (i.e. Dzyaloshinskii{Moriya interaction).
First, several PTO/STO superlattice architectures were grown, in order to re-create the polar vortex topology. This allowed us to capture the ferroelectric domains, going from a monodomain state to polar vortices and to ux-closure domains. X-ray diffraction studies have revealed that the polar topologies inside the PTO layers are composed of two perpendicular modulations, co-existing in the sample. In the second part of this study we focus on the sample showing polar vortices, and follow the evolution with temperature of all periodicities. It is shown that any form of periodic arrangement dissolved at around 150 °C, with the exception of the vortex array, which disappears at approximately 350 °C. Because of this we were able to study the nature of the vortex phase transition and concluded that it can be described within the mean _eld theory as a second order phase transition.
Next, we performed DFT calculations on metal-ferroelectric-metal structures in order to find a conductive substitute for STO, which could allow for polar vortices to form within PTO. LSMO and SRO were chosen due to their good lattice match to PTO and because of their electrical properties. Here, atomic relaxations were performed and the interface physics was described. LSMO, depending on the interface termination, was shown to have either a screening or an enhancing effect on the ferroelectric polarization, depending on its orientation. This can be detrimental to the formation of polar vortices, as one interface termination favors a specific polarization direction. On the other hand, SRO was shown to sustain the vortex topology, by confining the polarization to PTO. Finally, the results of the DFT calculations done on the SRO/PTO/SRO systems were tested by producing such architectures using PLD. It is shown that polar vortices do indeed form inside PTO, however the XRD results hint towards a more complex polar topology. Using a combination of electron microscopy techniques such as DF-TEM, STEM, diffraction contrast simulations, complemented by XRD studies and circular dichroism measurements, it is shown the emergence of a complex polar topology which resembles a unique spin texture called incommensurate spin crystals, recently found in ferromagnets.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics Q Science > QD Chemistry T Technology > TK Electrical engineering. Electronics Nuclear engineering |
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Library of Congress Subject Headings (LCSH): | Ferroelectricity, Ferromagnetic materials, Superlattices as materials, Thin films -- Magnetic properties, Thin films -- Electric properties, Polar vortex, Polarization (Electricity), Spin waves | ||||
Official Date: | October 2021 | ||||
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, 157 leaves : illustrations | ||||
Language: | eng |
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