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Thermal and Magnetic Studies of Spin Ice Compounds

Bloxsom, J; (2016) Thermal and Magnetic Studies of Spin Ice Compounds. Doctoral thesis , UCL (University College London). Green open access

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Abstract

In recent years a great deal of interest has been attracted by the materials called ‘spin ices’, and the monopole-like quasiparticle excitations inside them. Spin ices are frustrated Ising ferromagnets with a high level of frustration arising from a spin configuration akin to the proton configuration of water ice. Excitations of the ground state configurations can produce local arrangements of spins which behave similarly to magnetic monopoles, including carrying an effective magnetic charge and experiencing Coulomb interactions with one another. By taking these ‘monopoles’ as the units of analysis, theories of charged particle interaction can be applied to magnetic spin ice crystals. This thesis will examine the applicability of a number of theories based on this model to experimental data of the real properties of spin ice, along with a novel ex-perimental method, and in turn report on what the results suggest about the phys¬ical nature of the spin ices in question. The main materials studied are dysprosium titanate (Dy2Ti2O7) and holmium titanate (Ho2Ti2O7), and additional investiga¬tions are performed on cadmium erbium selenide (CdEr2Se4) and praseodymium zirconate (Pr2Zr2O7). First, a new derivation of the Debye-H¨uckel theory of electrolytes adapted for spin ice is presented, incorporating a microscopically correct partition function and the effects of higher-energy excitations, called ‘double monopoles’. The theory is compared to specific heat experimental and simulation data for Ho2Ti2O7 and Dy2Ti2O7 and experimental data for CdEr2Se4. It is found that Debye-H¨uckel theory is an effective analytic theory of spin ice magnetic heat capacity even into high temperatures of 6 K or more, in contrast to earlier work which held that such temperatures are out of the effective region of the spin ice model. Extensions of the theory to account for lattice geometry, Bjerrum pairing and ‘entropic charge’ are considered. Second, several theories for describing the magnetic relaxation of spin ice are compared to experimental data from Dy2Ti2O7 at 0.4 to 0.6 K. The theories encompass the Wien effect seen in electrolytes, surface effects and the failure of the samples to equilibrate on experimental timescales. The results are inconclusive and suggest that multiple effects must be considered to form a complete theory of spin ice relaxation at low temperatures. Third, an absolute measurement of the entropy of the quantum spin ice Pr2Zr2O7 using a recently introduced method is reported and compared to previous work on the material, along with a prediction of its specific heat using Debye-H¨uckel theory. The results demonstrate that the method is effective at low temperatures and suggest that the low-temperature entropy of Pr2Zr2O7 is less than that of classical spin ices, and that its monopole dynamics are significantly different.

Type: Thesis (Doctoral)
Title: Thermal and Magnetic Studies of Spin Ice Compounds
Event: UCL
Open access status: An open access version is available from UCL Discovery
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1529383
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