Magnetic Nanoparticle Interactions: From Magnetostatics to Exchange

技术：表面活性剂包裹的纳米颗粒由于其单分散性和形成有序阵列的能力而具有巨大的优势。纳米尺度的磁性测量表明，颗粒大小和间距的均匀性影响集体动力学响应和磁序的长度尺度。表面活性剂控制颗粒尺寸分布并提供自组装所需的迁移率，但它也限制了可以制备的有趣的磁性纳米结构的范围。典型的磁芯间距为2-4 nm，这意味着粒子间的耦合几乎是纯静磁的。表面活性剂充其量为铁和钴等材料的氧化提供了临时的、半透水的屏障。组件具有蜡的机械一致性，这使得它们不适合大多数设备应用。表面活性剂使得很难实现良好的电接触，而不同数量的表面覆盖会导致表观隧道势垒中颗粒对颗粒的差异。该项目探索了在有序阵列中制备单分散金属纳米柱的加工方法。柱子将由反应离子刻蚀(RIE)纳米颗粒掩膜制成。在更简单的形式中，掩模将是非磁性的，并且磁性颗粒将通过溅射沉积在密集柱阵列的顶部而产生。还将研究高密度甲醇基等离子体RIE对磁性多层膜的直接图案化，以确定最小特征尺寸以及这种干法刻蚀工艺对磁响应的影响。这项研究的重点是磁成像和中子反射率技术将揭示与温度和场有关的磁关联长度的系统，这提供了对磁性纳米粒子相互作用的更深层次的理解。磷化锰具有一级铁磁向顺磁的转变，因此纳米MnP之间的静磁相互作用强度随着温度的变化而急剧变化。由于交换偏置效应，耦合到反铁磁(AF)IrMn层的铁磁性Co纳米颗粒可能会显示出不同的集体关联，这取决于AF磁区的大小。还将使用在FeBSi基质中具有铁柱的模型纳米晶材料来测试阵列中纳米颗粒之间的交换耦合程度，该程度应在基质的居里温度处突然变化，从而能够定量比较对复合材料的交换和静磁贡献。最后，磁性材料的RIE将被用来制备一些多层纳米微管，其中较小的特征尺寸导致了新的量子限制和自旋积累效应。该项目的智能优势在于开发了一种通用的、可扩展的工艺，以在无机基质中制备均匀的磁性纳米颗粒和纳米颗粒阵列。纳米尺度的表征工具将揭示有关长程磁序发展的新特征，并将澄清对宏观测量的解释。非技术性：这项工作将在许多方面产生广泛影响。它将形成一个研究生的毕业论文项目和几个学生的本科生研究项目。将为中学生和本科物理实验室开展与MNP中的磁性相变相关的动手演示和实验室实验。本项目得到了S金属计划和凝聚态物理计划的联合资助。

TECHNICAL: Surfactant-coated nanoparticles have tremendous advantages due to their monodispersity and ability to form ordered arrays. Nanoscale magnetic measurements have demonstrated that uniformity in the particle size and spacing affects the collective dynamical response and the length scale of magnetic order. The surfactant controls the particle size distribution and provides mobility needed for self-assembly, but it also limits the range of interesting magnetic nanostructures that can be prepared. The typical 2-4 nm separation between cores means that interparticle coupling is almost purely magnetostatic. The surfactant provides at best a temporary, semi-permeable barrier to oxidation of materials such as iron and cobalt. Assemblies have the mechanical consistency of wax, making them unsuitable for most device applications. The surfactant makes it difficult to achieve good electrical contact, and varying amounts of surface coverage causes particle-to-particle differences in the apparent tunneling barriers. The project explores processing methods to prepare monodisperse metallic nanopillars in ordered arrays. The pillars will be made by reactive ion etching (RIE) a nanoparticle mask. In the simpler form the mask will be nonmagnetic and the magnetic particles will be created by sputter deposition on top of the dense pillar array. Direct patterning of magnetic multilayers with high density methanol-based plasma RIE will also be investigated to determine the minimum feature size and the effect of this dry etching process on magnetic response. The research focuses on systems where magnetic imaging and neutron reflectivity techniques will reveal temperature and field-dependent magnetic correlation lengths, which provides a deeper understanding of magnetic nanoparticle interactions. Manganese phosphide has a first order ferromagnetic to paramagnetic transition, so that the strength of magnetostatic interactions between MnP nanoparticles should change sharply with temperature. Ferromagnetic Co nanoparticles coupled to an antiferromagnetic (AF) IrMn layer may show differences in collective correlations due to exchange bias effects, depending on the AF domain size. Model soft nanocrystalline materials, with Fe pillars in a FeBSi matrix, will also be used to test the degree of exchange coupling between the nanoparticles in the array, which should change abruptly at the Curie temperature of the matrix, enabling quantitative comparison of exchange and magnetostatic contributions to the composite material. Finally the RIE of magnetic materials will be used to prepare some multilayer nanopillars where the small feature size leads to novel quantum confinement and spin accumulation effects. The intellectual merit of this project is in the development of a versatile, scalable process to prepare uniform magnetic nanoparticles and nanoparticle arrays in an inorganic matrix. The nanoscale characterization tools will reveal new features concerning the development of long-range magnetic order, and will clarify the interpretation of macroscopic measurements. NON-TECHNICAL: The work will have broad impact in numerous ways. It will form the thesis project for a graduate student and undergraduate research projects for several students. Hands-on demonstrations and laboratory experiments related to the magnetic phase transition in MnP will be developed for middle school students, and undergraduate physics laboratories. This project is jointly supported by DMR?s Metals program and Condensed Matter Physics program.