Crystal fields and spin dynamics in frustrated quantum materials

Thesis

This thesis presents studies of the magnetism in a number of frustrated quantum materials including 1D spin chain compounds, 2D honeycomb and triangular lattice materials, as well as 3D pyrochlores and Weyl semimetals. First, it reveals the tunable magnetic frustration in the family of PbM₂Ni₆Te₃O₁₈ spin chain compounds through experimental methods such as µSR and magnetometry, combined with computational simulations including dipolar field calculations and DFT+µ muon site determination. This demonstrates how one can use a suite of techniques to investigate the various aspects of novel frustrated materials.

The main focus of this thesis is to study the spin dynamics and the underlying magnetism in frustrated magnets using the µSR technique. Starting from the fundamental principles of µSR, this thesis provides a brief mathematical treatment of how various magnetic structures and dynamics result in a number of signature muon polarisation asymmetries. Furthermore, through zero-, transverse-, and longitudinal-field µSR experiments, the studies on α-RuI₃ and DyTa₇O₁₉ illustrate strong evidence for persistent spin dynamics at base temperature and the mode of spin diffusion.

Finally, through the analysis of the rare-earth oxides, the thesis outlines how the CEF environments determine the magnetic anisotropy, and together with exchange interactions and crystal geometry, they can lead to the spin dynamics commonly observed in frustrated spin liquid materials. This thesis sets out the differences between the Kramers and non-Kramers rare earth ions and how the magnetic behaviour might change in each scenario as a result of chemical disorder or the injection of muons in µ⁺SR experiments. Therefore, this thesis provides insight into future developments on how new materials can be designed to strengthen the effect of magnetic frustration.

Authors: Wu, CH

• DOI: 10.5287/ora-qay6yyr86
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: spin dynamics; frustrated quantum materials; crystal electric fields; magnetism; strongly correlated electrons; quantum spin liquids; muon spin rotation spectroscopy
• Subjects: Condensed matter; Magnetism