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Microscopic mechanisms in multiferroic materials
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Clarke, Gabriel Richard Michael (2022) Microscopic mechanisms in multiferroic materials. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3853394
Abstract
Ferroic materials are of fundamental importance in the modern world with applications ranging from sensors to computational memory and beyond, and multiferroic materials displaying several ferroic properties simultaneously have a great deal of potential for future devices. The antiferromagnetic Sr7Mn4O15 phase was recently identified as a potential multiferroic material. Chapters 2 and 3 explore the synthetic conditions necessary to expand the solid solution Sr7-xAxMn4O15 for A: Ca2+ and Ba2+. The structure is investigated using X-ray and neutron diffraction methods under variation of chemical substitution, temperature and hydrostatic pressure. Magnetic analysis is also performed using direct current magnetic susceptibility measurements, and long-range spin orderings are identified using low-temperature neutron diffraction experiments. Several novel compositions are synthesised and characterised; in particular, the Ba7Mn4O15 composition described in Chapter 3 exhibits a magnetoelectric ground state below 50 K.
The second half of this thesis is concerned with the responses of hybrid improper ferroelectric n = 2 Ruddlesden-Popper phases to hydrostatic pressure and electric fields. Chapter 4 describes a high-pressure experiment on Ca3Mn2O7 and Ca3Ti2O7, with accompanying computational analysis showing that both phases undergo a ferroelectric phase transition to a non-polar phase above 1 GPa and 30 GPa, respectively. Despite this, the relative energies of the polar and non-polar phases unexpectedly show that the polar phase is actually stabilised by increased pressure, meaning that the polarisation itself may increase with pressure up to a limit, contrary to the trend normally observed in proper ferroelectrics.
Chapter 5 reports an in situ synchrotron X-ray diffraction experiment on the substituted phase Ca2.15Sr0.85Ti2O7 in which compressed powder samples were subjected to an applied electric field while diffraction patterns were measured. The output time-resolved data are analysed to attempt to identify how ferroelectric switching may proceed in this and related phases, with octahedral rotations being identified as a plausible pathway. Throughout all chapters, the symmetries and phase transitions of the various materials are analysed through representation analysis.
| Item Type: | Thesis (PhD) | ||||
|---|---|---|---|---|---|
| Subjects: | Q Science > QD Chemistry | ||||
| Library of Congress Subject Headings (LCSH): | Ferromagnetic materials, Ferroelectricity, Ferroelectric devices, Synchrotrons | ||||
| Official Date: | March 2022 | ||||
| Dates: |
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| Institution: | University of Warwick | ||||
| Theses Department: | Department of Chemistry | ||||
| Thesis Type: | PhD | ||||
| Publication Status: | Unpublished | ||||
| Supervisor(s)/Advisor: | Senn, Mark S. | ||||
| Format of File: | |||||
| Extent: | vi, 172 leaves : illustrations, charts | ||||
| Language: | eng |
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