Coercivity of Magnetic Nanoparticles and Nanocomposites

使用洛伦兹显微镜的福柯方法在纳米尺度上研究了纳米颗粒和纳米复合材料的磁化反转或矫顽力。研究了磁晶各向异性值非常大或非常小的纳米粒子，以扩展现有的矫顽力模型。在高各向异性球磨SmCo5纳米颗粒中，重点是了解应变，晶界和颗粒尺寸的作用，以最大化开关场。在低各向异性Fe50Co50纳米颗粒中，需要最小的矫顽力，但尚不清楚软磁纳米复合材料的最佳前驱体是超顺磁性的，还是仅仅具有低矫顽力。加工方法是多种多样的，以优化纳米颗粒的矫顽力，从而使它们被压实成纳米复合材料。研究了冷等静压、磁压实和等离子体压实，以确定在不显着晶粒生长的情况下获得最高密度的最佳方法。研究了两种具有潜在应用前景的纳米复合材料，一种是由SmCo5和Fe50Co50纳米颗粒压实制成的交换弹簧磁铁，另一种是由Fe50Co50纳米颗粒压实制成的软磁纳米复合材料，它们具有非常薄的富碳涂层。洛伦兹显微镜技术用于观察亚微米长度尺度上的磁化反转。显微结果，结合标准的方法来分析微观结构和化学成分，找出在软磁材料中降低永磁体矫顽力或增加其矫顽力的薄弱环节。该反馈指导了优化纳米复合材料磁性能所需的加工修改。该项目研究交换弹簧磁体和软磁纳米复合材料，前者能以更低的成本获得比当前高性能永磁体更高的能量产品，后者能以更低的成本获得比现有材料更低的功率损耗、更高的磁导率和更高的稳定性，适用于高温或高频环境。***

9900550MajetichThe magnetization reversal, or coercivity, of nanoparticles and nanocomposites is investigated on a nanometer scale using the Foucault method of Lorentz microscopy. Nanoparticles with extremely large or very small values of magnetocrystalline anisotropy are studied to extend existing models of coercivity. In high anisotropy ball milled SmCo5 nanoparticles the emphasis is on understanding the roles of strain, grain boundaries, and particle sizes in order to maximize the switching field. In low anisotropy Fe50Co50 nanoparticles minimal coercivity is desired, but it is unclear whether the best precursors to soft magnetic nanocomposites are superparamagnetic, or merely have low coercivity. The processing methods are varied to optimize the coercivities of the nanoparticles, whereupon they are compacted into nanocomposites. Cold isostatic pressing, magnetic compaction, and plasma pressure compaction are examined to determine the best method to yield the highest density without significant grain growth. Two types of nanocomposites with potential applications are examined, exchange spring magnets made by compaction of SmCo5 and Fe50Co50 nanoparticles and soft magnetic nanocomposites made by compaction of Fe50Co50 nanoparticles that have very thin carbon-rich coatings. Lorentz microscopy techniques are used to observe magnetization reversals on a submicron length scale. The microscopy results, in combination with standard methods to analyze the microstructure and chemical composition, identify the weak links that reduce the coercivity of a permanent magnet, or increase it in a soft magnetic material. The feedback guides the processing modifications required to optimize the magnetic properties of the nanocomposites.%%%This program examines exchange spring magnets, which could have higher energy products at lower cost than current high performance permanent magnets, and soft magnetic nanocomposites, which could have lower power losses, higher permeabilities, and greater stability than existing materials in niche applications at high temperatures or high frequencies.***