Brooks-Bartlett, ME; (2015) Numerical investigations of classical spin models: the impact of lattice geometry, confinement and long-range interactions. Doctoral thesis , UCL (University College London).

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Abstract

This thesis addresses the nature of magnetic phases and phase transitions in a number of classical spin systems in which the magnetic behaviour is governed by lattice geometry, the range of interactions and dimensional confinement. The work presented here is underpinned by the use of classical Monte Carlo methods incorporating both long-range exchange and dipolar contributions to the magnetic Hamiltonian. These numerical simulations are used not only to test the theoretical concepts presented in this thesis, but also as a means for arriving at a more detailed understanding of the results of previous experimental studies. Results are presented for a frustrated Ising model on a pyrochlore lattice (closely related to spin ice), along with studies of proximity effects in magnetic multilayers. The first of these systems reveal a previously unknown magnetic phase – a monopole crystal – which is shown to exist against a background spin liquid phase. The coexistence of these apparently mutually exclusive phases is shown to result from magnetic moment fragmentation. Studies of the proximity effect demonstrate the effects of magnetic induction at the interface between a single layer of a strong magnet and a thin layer of a weaker magnet. It is shown here that the transition temperature of both layers is enhanced by this interaction, with the effect in the stronger magnetic layer being a result of an increased effective thickness. Similar proximity effects are examined in the context of a magnetic trilayer exhibiting the exchange spring effect.

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