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

非技术摘要永磁体是各种技术中的关键部件，尤其是电动马达和发电机。它们对磁记录也至关重要，磁记录仍然是大规模数据存储设备的基础。为了提高磁体的性能，需要新的材料、新的策略和新的科学。一种改善磁性行为的方法被称为交换耦合，在这种方法中，两种不同的磁性材料在量子水平上相互作用，创造出任何一种材料本身无法获得的新性质。然而，要做到这一点，就需要这些材料在纳米尺度上相互交织在一起，称为纳米复合材料。大多数对纳米复合材料的研究探索了两个极端。薄膜提供了极端的简单性，在薄膜中，两种材料只是相互叠加的薄的平面层。块状纳米复合材料提供了极端的复杂性，其中具有不规则形状、大小和界面的微小颗粒相互交织在一起。这个项目研究了一种非常特殊的纳米复合材料，称为纳米棋盘，其中两种材料的规律性是通过自组装实现的，在特定的材料组合中，只需加热即可。由此产生的结构看起来很像一个棋盘，但单个瓷砖是纳米大小的。这项研究的智力价值在于强调不同的材料组合，以阐明材料的性质如何影响它们在纳米棋盘中的自组装，以及它们交换耦合的能力。这将反过来提供研究的更广泛的影响--对可用于定义用于创建高性能交换耦合磁体的新策略的行为的重要新理解。技术摘要本项目将选择并竞争长度尺度，结构与磁性，使用先进的加工技术在Fe-Pd和Fe-PT系统中创建精细的纳米黑板，以便更好地理解、控制和增强交换耦合铁磁性。这些二进制系统包含A1-L10+L12形式的共析，如果仔细控制加工条件，则通过伪旋节点分解产生黑板。这里使用的合金的具体选择决定了与L10相相关的单轴磁晶各向异性，进而选择了交换耦合的临界长度标度，从而确定了智能优点。棋盘是通过共析连续冷却生产的，但为了获得额外的周期性控制，将研究热机械处理和循环退火。交换耦合的程度将使用一阶反向曲线分析来询问，该分析擅长揭示磁相如何相互作用。除了从最初的多晶A1起始材料生长棋盘外，还将生长单晶A1，然后在磁场或双轴应变下进行热处理，以创建单排列变化的棋盘。这将使我们能够更好地理解磁化反转机制，以及棋盘变换本身。在棋盘形成过程中将使用实时X射线衍射，以更好地阐明转变的动力学和过程。这项研究的成功将有助于我们理解块体纳米复合磁体中的交换耦合铁磁性，不仅包括层级长度尺度，而且还包括相形态、界面和应变的作用。改进永磁体对于提高电机效率具有持续的重大意义。

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.