Selection of Lengthscales in Fe-based Nanochessboards to Enhance Exchange-Coupled Ferromagnetism

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Non-Technical AbstractPermanent magnets are key components in a diverse range of technologies, most notablyin electric motors and electric generators. They are also critical to magnetic recording,which still forms the foundation of large-scale data storage devices. To improve theperformance of magnets, new materials, new strategies, and new science are needed. Oneapproach towards improving magnetic behavior is called exchange coupling, in whichtwo different magnetic materials interact at the quantum level to create new propertiesnot available from either material by itself. For this to be effective, however, requires thatthe materials be intertwined at the nanoscale, called a nanocomposite material. Mostresearch on nanocomposite materials has explored two extremes. Extreme simplicity isprovided by thin films, in which the two materials are simply thin, planar layers on top ofone another. Extreme complexity is provided by bulk nanocomposites, where tiny grainswith irregular shapes, sizes and interfaces, are mutually intertwined. This projectinvestigates a very special nanocomposite, called the nanochessboard, where a regulartiling of the two materials is achieved by self-assembly, in specific combinations ofmaterials, simply by heating them. The resulting structure looks much like a chessboard,but where the individual tiles are nano-sized. The Intellectual Merit of this studyemphasizes varying the materials combinations in order to illuminate how the nature ofthe materials affects both their self-assembly into nanochessboards, and their ability toexchange couple. In turn, this will provide the Broader Impacts of the study - theimportant new understanding of the behaviors that can be used to define new strategiesfor creating high-performance exchange-coupled magnets.Technical AbstractThis project will select and compete lengthscales, structural vs. magnetic, using advancedprocessing techniques to create refined nanochessboards in the Fe-Pd and Fe-Pt systems,in order to better understand, control, and enhance exchange-coupled ferromagnetism.These binary systems contain eutectoids of the form A1-- L10+L12 that result inchessboards by pseudo-spinodal decomposition, if processing conditions are carefullycontrolled. The Intellectual Merit is framed by the specific choice of alloys to be usedhere, which dictates the uniaxial magnetocrystalline anisotropy associated with the L10phase, which, in turn, selects the critical lengthscale for exchange coupling. Chessboardsare produced by cooling continuously through the eutectoid, but to gain additional controlover the periodicity, thermomechanical processing and cyclic annealing will beinvestigated. The degree of exchange coupling will be interrogated using First OrderReversal Curve analysis, which excels at revealing how magnetic phases interact. Inaddition to the growth of chessboards from an initially polycrystalline A1 startingmaterial, monocrystalline A1 will also be grown, and then annealed under magnetic fieldsor biaxial strain, to create a single-alignment-variant chessboard. This will enable betterunderstanding of the magnetization reversal mechanisms, as well as the chessboardtransformation itself. Real-time x-ray diffraction will be employed during chessboardformation to better elucidate the kinetics and progression of the transformation. TheBroader Impacts of success in the proposed research would be the contribution to ourunderstanding of exchange-coupled ferromagnetism in bulk nanocomposite magnets, notonly including hierarchical lengthscales, but the role of phase morphology, interfaces andstrain. Improving permanent magnets is of ongoing major significance with regard toincreasing the efficiency of electric motors.