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

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

Non-Technical Abstract Permanent magnets are key components in a diverse range of technologies, most notably in 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 the performance of magnets, new materials, new strategies, and new science are needed. One approach towards improving magnetic behavior is called exchange coupling, in which two different magnetic materials interact at the quantum level to create new properties not available from either material by itself. For this to be effective, however, requires that the materials be intertwined at the nanoscale, called a nanocomposite material. Most research on nanocomposite materials has explored two extremes. Extreme simplicity is provided by thin films, in which the two materials are simply thin, planar layers on top of one another. Extreme complexity is provided by bulk nanocomposites, where tiny grains with irregular shapes, sizes and interfaces, are mutually intertwined. This project investigates a very special nanocomposite, called the nanochessboard, where a regular tiling of the two materials is achieved by self-assembly, in specific combinations of materials, 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 study emphasizes varying the materials combinations in order to illuminate how the nature of the materials affects both their self-assembly into nanochessboards, and their ability to exchange couple. In turn, this will provide the Broader Impacts of the study - the important new understanding of the behaviors that can be used to define new strategies for creating high-performance exchange-coupled magnets. Technical Abstract This project will select and compete lengthscales, structural vs. magnetic, using advanced processing 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 in chessboards by pseudo-spinodal decomposition, if processing conditions are carefully controlled. The Intellectual Merit is framed by the specific choice of alloys to be used here, which dictates the uniaxial magnetocrystalline anisotropy associated with the L10 phase, which, in turn, selects the critical lengthscale for exchange coupling. Chessboards are produced by cooling continuously through the eutectoid, but to gain additional control over the periodicity, thermomechanical processing and cyclic annealing will be investigated. The degree of exchange coupling will be interrogated using First Order Reversal Curve analysis, which excels at revealing how magnetic phases interact. In addition to the growth of chessboards from an initially polycrystalline A1 starting material, monocrystalline A1 will also be grown, and then annealed under magnetic fields or biaxial strain, to create a single-alignment-variant chessboard. This will enable better understanding of the magnetization reversal mechanisms, as well as the chessboard transformation itself. Real-time x-ray diffraction will be employed during chessboard formation to better elucidate the kinetics and progression of the transformation. The Broader Impacts of success in the proposed research would be the contribution to our understanding of exchange-coupled ferromagnetism in bulk nanocomposite magnets, not only including hierarchical lengthscales, but the role of phase morphology, interfaces and strain. Improving permanent magnets is of ongoing major significance with regard to increasing the efficiency of electric motors.