Unconventional magnetism in bulk and nanostructured semiconductors
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The goal of this thesis is to understand the unconventional magnetism (also known as sp/d 0 magnetism) exhibited by certain bulk and nano-structured semiconductors and insulators, that are not intentionally doped with magnetic elements. The common features in most experiments reporting d 0 magnetism are the presence of defects and 2 nd row elements. This indicates that defects play an important role in inducing and mediating magnetism in these materials. As the acceptor and acceptor-like defect levels (or example, those created by the cation vacancies) are derived from the fairly localized 2 p states of the 2 nd row elements, such defects can result in local moment formation. Depending on the range and strength of the exchange interactions between the localized moments, one might obtain macroscopic magnetism in such defective semiconductors and insulators. A microscopic understanding of the mechanism(s) responsible for the observed behavior is important, not only as an academic pursuit, but also for the use of this property in creating future dilute magnetic semiconductors. To this end, systematic first principle electronic-structure calculations were carried out in bulk Gallium Nitride, Boron Nitride and Zinc Oxide. Acceptor-like defects, such as cation vacancies, cation substitutional and larger defect-complexes were introduced in the otherwise perfect crystals. The magnetic properties of the resulting structure were investigated as a function of defect's charge state and inter-defect distances. The calculations for defects in semiconductors were carried out using supercell geometries within density functional theory. An exchange mechanism is proposed to explain the magnetic interaction between the defect-induced magnetic moments. The role of defects in inducing magnetism in nanostructures was also studied. The calculations were carried out for the GaN and ZnO nanowires and are presented in this thesis.