Direct measurement of magnetic monopole charge and transport dynamics

Thesis

Magnetic monopoles are magnetic excitations with quantized magnetic charge whose existence in pyrochlore spin ice materials has been studied widely. As a platform to study the hypothetical magnetic monopoles, classical spin ices have aroused fundamental interest. Despite the extensive studies, the transport theory of the magnetic monopole fluid remains elusive, especially indicated by the recent investigation with noise spectroscopy. This thesis endeavors to elucidate the dynamics of the monopole fluid by developing advanced SQUID-based spectrometer and provides several unique pieces of evidence towards the dynamical mechanism.

In Chapter 1 of the thesis. I first briefly introduce spin ice materials and the emergence of the magnetic monopoles from a historical basis. Chapter 2 points out recent puzzles in magnetic monopole dynamics uncovered by the noise spectroscopy. Both the free monopole fluid theory and the advanced dynamical fractal theory on monopole motion are described in detail. In Chapter 3, I present a thorough description of the home built SQUID-based monopole current/noise spectrometer that I have been working on throughout my DPhil. The high precision spectrometer compatible with external magnetic field is designed to perform experiments mentioned in Chapter 2. In addition, the calibration process and introduction on types of measurement of SQUID spectroscopy are included. Starting from Chapter 4, I present the work using this newly-developed technique. In Chapter 4, I discuss the exploration of dichotomous monopole dynamics originated from another slower spin flipping rate in Dy2Ti2O7. By combining Monte Carlo simulation and monopole current measurements, signatures from dynamical fractal motion are identified. Chapter 5 further investigates the monopole noise towards lower temperatures, leading to the discovery of dynamical heterogeneity of the magnetic monopole fluid in its supercooled state. Monopole noise characterizes the gradual transition from the free monopole fluid to the supercooled fluid, and eventually towards the glass-forming state. In Chapter 6, I provide a direct measurement on the magnetic charge, which is the core of the monopole dynamics. The excellent agreement of the charge between theory and experiments marks the importance of entropic force in the monopole transport. Finally, the overall work and the potential future exploration is summarized in Chapter 7.

Authors: Hsu, C

• DOI: 10.5287/ora-ga6egdbb9
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: noise spectroscopy; frustrated magnetism; magnetic monopole; dysprosium titanate; SQUID spectrometer; spin ice; magnetic dynamics
• Subjects: Physics; Condensed matter