Magneto-structural correlations and the role of disorder in frustrated magnets

Graham, Jennifer Nadine ORCID: 0000-0002-3593-2479 (2023). Magneto-structural correlations and the role of disorder in frustrated magnets. University of Birmingham. Ph.D.

Preview

Graham2023PhD.pdf
Text - Accepted Version
Available under License All rights reserved.
Download (14MB) | Preview

Abstract

This thesis is an experimental study that explores the role correlated disorder plays in the ground state selection of quantum magnetic materials. Whilst magnetic materials with conventional long-range magnetic orderings give rise to magnetic Bragg peaks that can be understood through average structure methods, the unconventional correlated short-range orderings of quantum magnetic materials give rise to broad diffuse features that rely on local structure methods for their elucidation. One of the most powerful local structure methods is reverse Monte Carlo analysis, which has been employed extensively throughout this thesis.

In the first chapter, a discussion surrounding the complexities of realising different quantum magnetic models in real materials is presented. This chapter also includes an overview of the experimental methods used in this thesis, from both an average and local structure perspective, a formalism that is employed throughout the results chapters.

Chapter 2 explores the structural and magnetic evolution of two samples of the S = 1 cubic spinel, ZnV\(_2\)O\(_4\), one prepared via a conventional solid-state sintering synthesis route and the other via a novel, rapid microwave assisted method. The microwave sample was found to be highly crystalline and remained in the cubic symmetry down to 4 K, whereas strain caused the sintered sample to undergo a tetragonal distortion below 35 K. This leads both samples to unique magnetic ground states that were explored with magnetic susceptibility measurements and diffuse neutron scattering.

Chapter 3 presents LiYbO\(_2\) as the first experimental realisation of a spiral spin liquid ground state in a distorted diamond structure. Comprehensive neutron scattering measurements have been conducted and analysed using state-of-the-art reverse Monte Carlo analysis, allowing for the experimental signature of the spiral spin liquid—a continuous ring of magnetic diffuse scattering—to be reconstructed.

Chapter 4 involves a neutron scattering study on a quasi-two-dimensional layered honeycomb material, Mn\(_{0.5}\)Fe\(_{0.5}\)PS\(_3\), which was found to have a spin glass ground state. Measurements of a single-crystal confirmed the model used to calculate the spin correlations was correct, which could be understood through considering differences in the ordered magnetic structures of the parent compounds.

Finally, in Chapter 5, the conclusions and outlook of this thesis are presented, with a focus on how disorder can be used to an advantage in future materials design, and how reverse Monte Carlo methodologies are developing in order to characterise more complex partially ordered quantum materials.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):