Effect of structural disorder and inhomogeneities on the magnetic phase transitions in α-Li₂IrO₃

Abstract

Quantum spin liquids are crystalline materials whose ground state is defined by long-range quantum entanglement, and an excitation spectrum described by novel quasiparticles. Generating such a state requires magnetic frustration - the inability to satisfy all magnetic exchange interactions simultaneously, preventing long-range magnetic order - which can be achieved, for example, through bond-dependent exchange as for the Kitaev model. α-Li₂IrO₃ in theory satisfies the Kitaev model, but in reality has been found to have low-temperature magnetic phase transitions, signifying that it does not fully adhere to the Kitaev model. Instead, structural distortions introduce competing interactions, which result in a magnetic ground state. In this work, we outline in detail the interplay of structural disorder, off-stoichiometry, and physical properties in α-Li₂IrO₃. We combine extensive structural- and physical property characterisation to show that the sample is best described as consisting of two phases, differing by the amount of stacking faults (X-ray diffraction, TEM, mathematical modeling), and that the transition at 15 K can directly be correlated with the phase fraction of the 'unfaulted' phase, suggesting that the different phase transitions occur in different phases, and are independent. We also show that stacking faults are intimately linked to the redox behaviour of the material (study of synthesis environments, TGA, neutron diffraction), and that the magnetic transition temperatures can be suppressed (heat capacity, μSR, magnetic susceptibility) through the combination of Li-vacancies (electrochemical charging) & oxygen redox (NEXAFS). Through surface-sensitive spectroscopic techniques (XPS, AES) we highlight clear signatures of strong oxidation at the grain surface in all samples, suggesting variations in the sample properties throughout the grains. Finally, using single crystals we demonstrate that disorder alone cannot achieve the same suppression of the transition temperature (diffuse scattering, magnetic torque), but likely accounts for variations in reported physical property data.

Date of Award	3 Jul 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Alexandra Gibbs (Supervisor)