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Computer simulations invoking the Metropolis Algorithm and Monte Carlo techniques were constructed to model the behavior ofvarious systems of magnetic semiconductors. Among the systems modeled were bulk type II FCC antiferromagnets and simple cubic thin films and superlattices. Studies of bulk antiferromagnets revealed dependence of Neel Temperature on nearest neighbor interactions as well as next nearest neighbor contributions in contradiction to mean field theory. Thin film studies revealed the effects of film thickness on critical temperature as well as differing magnetization curves for different portions of the fIlm. Superlattice simulations showed agreement with experimental data regarding anisotropy studies and also investigated the correlation beween interlayer and intralayer ordering in a superlattice. These simulations predict that interlayer coupling begins either at a temperature equal to that of the onset of intralayer ordering or at some higher temperature. / Graduation date: 2004

Graduation date: 1996

Graduation date: 1997

Graduation date: 1996

Mechanical behavior of Nb-Ti superconducting composites with copper matrices have been studied experimentally and theoretically. Experimental work includes extensive measurements of Cu/Nb-Ti composite system. Techniques for fine fiber testing and composite wire measurement have been developed. Experimental parameters examined in this research include geometry, hardness, Young's modulus, Poisson's ratio, yield strength and ultimate strength. Three theoretical models have been developed to study the mechanics of the Cu/Nb-Ti composite system. The influence of several design parameters on the mechanics of the Nb-Ti composites was studied and provides some insight on superconducting composite design for improvements in processing and performance. The mechanical behavior of the Cu/Nb-Ti composite system are found to be functions of geometry, composition and processing. Geometry of Nb-Ti superconducting composites is different from most engineering composites and there are two factors affecting sample geometry: the variability of fiber geometry and the placement of fibers within the composite. The strength distribution of Nb-Ti fibers is closely related to the distribution of fiber geometry and the composite strength increases as the scatter of fiber strength decreases. Heat treatment reduces the hardness of the bulk copper dramatically. The first heat treatment increases the strength and hardness of the Nb-Ti fibers, further heat treatments reduce the strength and hardness while increasing Young's modulus of the fibers. As the extent of cold work increases, the strength of Nb-Ti fibers and that of the composite wires increases. Cold work effects on the Young's modulus of the composites and the Nb-Ti fibers are not significant. For a constant global Cu/SC ratio, the lower the local Cu/SC ratio, the lower the micro-in-plane stresses. From this point of view, the fibers should be packed as close as possible to one another. For a constant local Cu/SC ratio, when the inner radius of the Nb-Ti assembly increases (the fibers are packed further from center), the macro-in-plane stresses increase. From this point of view, the fibers should be packed as close to the center of the wire as possible. For a constant geometry, the higher the difference between E[subscript f] and E[subscript m], the lower the in-plane stresses, and the higher the difference of the Poisson's ratio between the components, the higher the in-plane stresses. / Graduation date: 1995

Graduation date: 1993