Coercivity of Magnetic Nanoparticles and Nanocomposites

9900550不可抗力纳米粒子和纳米复合材料的磁化反转，或磁化率，在纳米尺度上使用洛仑兹显微镜的福柯方法进行研究。 研究了具有极大或极小磁晶各向异性值的纳米颗粒，以扩展现有的磁晶各向异性模型。 在高各向异性球磨SmCo5纳米粒子的重点是了解应变，晶界和颗粒尺寸的作用，以最大限度地提高开关场。 在低各向异性的Fe50Co50纳米颗粒中，需要最小的磁性，但目前还不清楚软磁纳米复合材料的最佳前体是超顺磁性的，还是仅仅具有低磁性。 改变加工方法以优化纳米颗粒的结晶度，从而将它们压实成纳米复合材料。 冷等静压，磁压实，等离子体压力压实检查，以确定最好的方法，以产生最高的密度没有显着的晶粒生长。 两种类型的纳米复合材料与潜在的应用进行了研究，交换弹簧磁铁的SmCo5和Fe50Co50纳米粒子的压实和软磁纳米复合材料的压实Fe50Co50纳米粒子，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。 显微镜结果与分析微观结构和化学成分的标准方法相结合，确定了降低永磁体磁性或增加软磁材料磁性的薄弱环节。 反馈指导优化纳米复合材料磁性所需的工艺修改。%该计划研究交换弹簧磁体，它可以比当前的高性能永磁体以更低的成本具有更高的能量积，以及软磁纳米复合材料，它可以具有更低的功率损耗，更高的磁导率，以及比现有材料更高的稳定性在高温或高频下的利基应用。

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.***