Behr, Dylan;
(2025)
On the phenomenology of competing instabilities in selected transition metal oxides.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Strongly correlated electron systems display a complex interplay of structural, magnetic, and electronic degrees of freedom, leading to a variety of emergent phenomena. Utilising neutron and X-ray scattering techniques alongside a group-theoretical framework, I have developed and applied a unified approach to characterise this interplay in a selection of non-model 3d transition metal oxides, with a particular focus on magneto-structural coupling. I first investigate a spin reorientation transition in BiCrO₃, solving its magnetic structures and determining Dzyaloshinskii-Moriya interactions responsible for weak ferromagnetism in each phase. The interactions are demonstrated to couple spin to respective octahedral rotations and antiferroelectric distortions by a phenomenological model. I then present a study on PrMn₇O₁₂, where magneto-elastic coupling and a canted ground state are elucidated by neutron powder diffraction. A mean field model that captures the influence of competing exchange interactions on the complex ferrimagnetic ordering is developed that, together with the results, extends our understanding of magnetism in the A³⁺Mn₇O₁₂ quadruple perovskite manganites. Turning to BiMn₇O₁₂, I determine a highly canted magnetic phase composed of polar E-type antiferromagnetic order superposed on a ferrimagnetic order. Modelling reveals the coupling of the polar antiferromagnetism to ferroelectric distortions, exemplifying a novel category of type I multiferroics with inverse exchange-striction. I then explore the Cu-doped BiMn₇O₁₂ systems in the half-metallic limit, employing a host of complementary experimental techniques to characterise their structural, magnetic order, disorder and transport properties. The study reveals a diverse set of behaviours across the solid solutions, marked by coexisting short and long range magnetic correlations, spin freezing transitions, near zero thermal expansion and magneto-resistance. Lastly, in a resonant X-ray study on CoTi₂O₅, I demonstrate near-complete antiferromagnetic domain switching under uniaxial stress, exploiting the spin Jahn-Teller to relieve its perfectly frustrated exchange topology.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | On the phenomenology of competing instabilities in selected transition metal oxides |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10203584 |
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