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Date of Award

5-1978

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Supervisor

Professor M. F. Collins

Abstract

This thesis describes studies of ordering in binary alloys, and spin waves in triangular antiferromagnets.

The order-disorder phase transition in iron-cobalt alloys was studied by neutron-powder diffraction techniques. Twelve different compositions in the range from 30 to 70 atomic percent cobalt were investigated. All the specimens exhibited an order-disorder phase transition and the long range order parameter in each specimen was determined by measuring the intensity of superlattice reflections. The plot of the transition temperature against composition shows a peak at 47.9:0.3 atomic percent cobalt; this is surprising since simple theories predict a peak at the stoichiometric 50% composition. To within error, the critical index B of the order parameter is independent of composition; its weighted mean value is 0.303:0.004. This is comparable to the value 0.312:0.005 predicted for the Ising model.

Spin wave theory was developed for triangular antiferromagnets. In these systems at low temperatures, the spins have a simple antiferromagnetic coupling along the hexogonal c-axis but lie in the basal plane where they form a triangular structure. By using trial values of the exchange constants along directions parallel and perpendicular to the hexagonal c-axis, a numerical calculation of the spin wave dispersion relations in the system was carried out. There are six branches, including degeneracies, in the dispersion relations along the symmetry directions (ε00) and (00ε). Due to the fact that there is a much stronger coupling of magnetic moments along the c-axis compared with that in the basal plane, the energies of spin waves propagating along the c-axis (00ε) are much larger than those of the waves propagating in the perpendicular direction (ε00). The effect of the dipole-dipole interactions was to remove some of the degeneracies in the dispersion relations. The spin wave theory developed is applicable to any hexagonal triangular antiferromagnet having exchange and dipole-dipole coupling; it is applied in this thesis to CsMnBr₃ which is known to have a triangular antiferromagnetic structure.

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