Defect-Controlled Magnetic Anisotropy in Alloys

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TECHNICAL: This project describes a research program to elucidate the conditions and mechanisms under which disorder effects can dominate the anisotropy of magnetic materials. Traditionally, the magnetic anisotropy is understood to be determined by the crystal structure, and disorder plays little role in the microscopic theory. However, recent results indicate that inhomogeneities naturally present in a large class of alloys may conspire to completely eliminate the magneto-crystalline anisotropy. If this proves correct, it will likely have a large impact on the future development of the field, and this project seeks to test this model throughout its range of applicability using the following techniques: (1) The structure of alloys will be studied by neutron diffraction; (2) The local chemical nature of the materials will be studied by Mossbauer spectroscopy; (3) The crystal and magnetic structure will be studied simultaneously by conventional and Lorentz transmission electron microscopy To systematically establish the range of applicability of the model, the following material systems will be studied: The high-temperature (non-ferroelastic) phase of ferromagnetic shape-memory alloys (FSMAs), e.g. NiMnGa and FePd; FeCo alloys - A model system with similar magnetic properties and poorly understood structural disorder. NON-TECHNICAL: Materials with low anisotropy and large magnetostriction have become increasingly important for applications as sensors or transducers. The results of these studies could have large impact on the economy through such applications. Additionally, many students will be trained in materials synthesis and characterization techniques through this research. The cooperation with Morgan State University (a historically black institution) and the outreach programs with River Hill High School, Howard County, MD and The Holton-Arms School (a girls-only school), Montgomery County, MD will ensure that the program connects with HBCU Universities and the potentially interested segment of the high school population. The following impacts will result from this work: (1) the clear hypotheses will provide equally clear themes for Ph.D. theses, (2) the educational benefits will extend to undergraduate (HBCU) and high-school students that will cooperate in the studies/serve as interns in PIs' laboratories, (3) the national and international dissemination of the results will contribute to the general intellectual discourse, and, finally, (4) the project is of fundamental and technological interest. The latter is based on the low anisotropy/high magnetostriction combination needed for magnetic solid state actuators.