Magnetic Nanostructures through Metallic Dewetting

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Non-technical Abstract:This research program will investigate a new strategy for making nanoparticles of different metals and alloys, and characterize their size-dependent properties. Many electronic, magnetic, and optical properties become size-dependent when the material is structured on the nanometer length scale. Flash memory, magnetic recording media in computer hard disks, and diode lasers in digital video players, are examples of technology using nanostructured materials. The materials of interest in this research program are metals and alloys that have not yet been prepared as nanoparticles with good size control. The emphasis will be on alloys known to have interesting magneto-electronic or magneto-optical properties, or potential use as permanent magnets in energy-saving applications. The approach will use nanopatterned templates with regular arrays of pits to control the particle size. After depositing a thin film of the desired material, it will be heated until the film dewets and fills the pits. The uniform pit size will lead to a uniform particle size. This fabrication method will enable exploration of size-dependent behavior. Characterization of the magnetic, magnetoresistive, and magneto-optical properties will provide valuable new data that can be used in engineering nanostructured materials.Technical Abstract:This research program will investigate the formation of monodisperse magnetic nanoparticles created by the wetting and dewetting of thin metal films on nanopatterned templates, and to investigate their magnetic, magnetoresistive, and magneto-optical properties. While L10 FePt will be one of the target materials, there will be particular interest in magnetic metal alloys that have not yet been made by chemical methods, due to their oxidation sensitivity or complex crystal structures. The challenges are to overcome the sensitivity to oxidation that makes many of these particles impossible to prepare by solution chemistry methods, and to achieve crystallographic orientation to measure their anisotropic magnetic properties. There will be three strategies for preparing the magnetic alloy nanoparticle arrays, each with strengths and limitations: 1) shadow deposition on a nanoparticle monolayer, 2) deposition after seeding in nanohole arrays, and 3) using nanopillar arrays as a hard mask for a magnetic alloy thin film. In all cases rapid thermal annealing will be used to explore methods for crystallographically orienting the alloy nanoparticles. The templates will be dielectric materials (SiOx, SiNx, MgO) and conducting TiNx. The magnetic alloys will include L10 alloys, both familiar (FePt), and less studied (FeNi, MnAl, MnBi), materials for spintronics (the Heusler alloys Co2FeSi and Ni2MnGa, plus FeCoB), and for magneto-optics (the amorphous materials GdFeCo and TbFeCo). The results will lead to an improved understanding of metallic wetting and dewetting on the nanoscale. The nanopatterning process will be applicable to a wide range of complex materials, not only magnetic metal alloys. The preparation of monodisperse, passivated magnetic metal alloy nanoparticles will enable quantitative size-dependent measurements. The magnetization behavior will reveal the roles of surface chemistry and reduced exchange interactions at the particle surface. Novel resistance, magnetoresistance measurements will be made on individual nanoparticles. Smooth monolayer arrays of nanoparticles will be characterized by optical and magneto-optical spectroscopy. This project will involve the doctoral thesis research of a graduate student, along with several undergraduate research projects. There will also be impact to a broad audience through numerous educational activities associated with the magnetics community.