Experimental and Theoretical Studies of Anisotropy of Magnetic Susceptibility and Remanence

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Measurable magnetic anisotropy results from a large and complex set of mechanisms and physical effects on both the grain scale and the bulk rock scale. These include: nonhydrostatic stress and magnetostriction; crystal defects and microstructures and their interactions with magnetic domain walls; reorientation of mineral grains by particulate flow, plastic deformation or other mechanisms; and magnetostatic interactions among nonuniformly-distributed ferrimagnetic particles. In our research, we are targeting particular aspects of this complex set of processes, through a combination of controlled high-temperature deformation experiments using synthetic rock analogs with prescribed compositions and particle characteristics; numerical models of the anhysteretic magnetization process for anisotropically-interacting magnetic particles; and experimental determination of the anhysteretic anisotropy of synthetic samples produced by electron-beam lithography. Specifically our experiments involve the systematic investigation into the effects of high-temperature deformation on magnetic remanence, bulk magnetic properties and magnetic anisotropy in synthetic samples containing magnetite in a matrix of calcite, fayalite, or biotite. With our experimental protocols, we are studying the interrelationships between strain, reorientation of magnetic grains, development of deformation microstructures within the magnetites, changes in bulk magnetic properties, and development of magnetic fabrics as measured by anisotropy of magnetic susceptibility and remanence. We are also investigating, using fundamental experimental and theoretical approaches, the nature of anhysteretic remanent magnetization and its anisotropy, a property that is widely measured but not very well understood. We are especially focusing on the role of interparticle magnetostatic interactions. The effects of magnetic interactions are being studied using a series of nanofabricated arrays of magnetite particles produced by electron beam lithography. The nanofabrication process allows us to produce particle arrays with controlled particle sizes, shapes, orientations and interparticle spacings. Our results of these investigations are helping us better understand the significance of magnetic fabrics in naturally-deformed rocks, in terms of deformation history and mechanisms. Broader impacts include the training and education of a graduate student and a postdoc, and the potential of this work to allow rock deformation to be characterized by magnetic anisotropy.