The magnetic properties of a model two-dimensional dipolar thin film

MacIsaac, Allan B. (1997) The magnetic properties of a model two-dimensional dipolar thin film. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this thesis the magnetic properties of a model for a two-dimensional, dipolar thin film are determined using Monte Carlo simulations. Three different versions of the model have been considered and the results have been compared to previous theoretical results as well as to recent experiments on ultra-thin magnetic films. -- The first version of the model is a uniaxial model, with dipolar and exchange interactions. The ground states for this model are shown to be stripe phases for all J/g > 0.85, where J is the strength of the exchange interaction and g is the strength of the dipolar interaction. At finite temperature there is a phase transition from these stripe phases at low temperature to an orientationally disordered phase at higher temperatures. This orientational disordered phase appears to map continuously to the paramagnetic phase. The effect of an applied field on the stripe phases is also considered. -- The second version of the model is a planar model. In this model the magnetic moments interact only via the dipole-dipole interaction. The results of Monte Carlo simulations are interpreted within the context of a linearised spin wave calculation. The comparison indicates that the model system orders at a finite temperature in the thermodynamic limit. The nature of the transition from the paramagnetic state to the low temperature ordered state is studied by considering the decay of the two-point correlation function, both above and below the critical temperature. -- The third version of the model is a dipolar Heisenberg model, in which the magnetic moments interact via the dipole-dipole interaction. The moments are also subject to a magnetic surface anisotropy. This model is shown to exhibit a novel reorientation transition. The phase diagram for this model is developed and compared to that obtained for a similar system with a dominant ferromagnetic exchange interaction.

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