****NON-TECHNICAL ABSTRACT**** Magnetism is one of the oldest phenomena known to men, yet one of the most difficult to understand. Magnetic materials, typically made from metallic ions such as nickel or cobalt, have been known and used for centuries. This project will pursue a series of experiments to test whether magnetism can arise in materials where none of the constituents are magnetic by themselves. This goal will be pursued by modifying oxygen-based semiconductors through the introduction of atomic scale point defects, specifically making holes in the ordered network of oxygen atoms. These defects are expected to lead to an increase in the electrical conductivity of these oxide semiconductors, which can trigger the development of ferromagnetism. A combination of imaging and analytical techniques, the interplay among mobile electrons, electrical gating, and optical excitations, will be used to elucidate how magnetism develops in these semiconducting systems. This award will substantially advance the fundamental understanding of how ferromagnetism develops in materials that are intermediate between metals and insulators as well as provide important insight into materials for making new magnetic memory and logic devices, which could be used for advanced computing. The students participating in this project will be trained in advanced analytical techniques applicable to careers in the semiconductor/nanotechnology industries, as well as in academia.
This project will pursue a series of experiments to test a number of mechanisms that have been proposed for the development of room temperature ferromagnetism in semiconducting transition metal oxides, rationalizing the properties of materials intermediate between local moment magnetism and itinerant magnetism. These properties include the interplay between localized moments, mobile charge carriers, and point defects, specifically oxygen vacancies. This will be accomplished by correlating the emergence of an increased electrical conductivity in oxygen deficient oxide semiconductors with the development of ferromagnetism. Using a unique combination of imaging and analytical techniques it is expected to be possible to elucidate how the shift in carrier concentration with the inclusion of non-magnetic ions and point defects, electrical gating, and optical excitations affects these magnetic properties and how local moments from transition metals interact with the oxygen vacancy to induce ferromagnetism. This award will substantially advance the fundamental understanding of ferromagnetism by probing in a controlled manner the magnetism in systems intermediate between insulators and conductors. The two Ph.D. students and multiple undergraduates participating in this project will be trained in advanced analytical techniques applicable to careers in the semiconductor/nanotechnology industries, as well as in academia.
Magnets have been known to man for thousands of years. Yet, magnetism is one of the least well understood phenomena in solid state physics. We often think of magnetic materials of being at least partially comprised of ions that are magnetic, such as iron or nickel. These metals typically have d-electrons, which are believed to be responsible for their magnetic properties. We are interested in a new type of magnetism that has recently been discovered in various semiconducting magnetic oxides, which have no magnetic ions and hence no d-electrons all: the so-called d0 magnetism. The development of a comprehensive understanding of the origin of magnetism in these complex systems has been referred to as one of the most significant challenges in modern magnetism. The research supported by this award was designed to study the possible connection between localized magnetic moments, charge carriers, and ferromagnetism. The results of our work gave us some new insights into fundamental questions about the origin of magnetism and the interplay between the electron spin and charge in these materials. In particular, we learned that in a number of oxide semiconductors, such as in indium oxynitride, as well as in indium oxide, ferromagnetism can only develop in the presence of oxygen vacancy defects and high charge carrier density. Moreover, we found that in this case spins of charge carriers are aligned, which results in spin polarization. We emphasize that the role of oxygen defects can be further enhanced by the interaction with other defects. For example, in magnesium oxide, all the samples which were annealed in a vacuum, thus attaining a reduced oxygen defect concentration, became magnetic; at the same time, co-doping with aluminum increased the magnetic signal even further. We also learned that indium oxide exhibits so-called persistent photoconductivity upon irradiation with UV light, and examined the connection between structural defects, transport, and magnetic properties. Also, by analyzing the approach of zinc oxide to magnetic saturation, we learned that it has exceedingly small magnetic anisotropy, another strong argument in favor of unconventional magnetism in magnetic oxides. As these materials are magnetic at room temperatures, these results are important for spintronic device applications. Under the auspices of this award, we also established international student exchange programs with the Indian Institute of Technology - Madras and the Research Center on Nanomaterials and Nanotechnology at the University of Oviedo, Spain.
