Magnetic Nanoparticle Interactions: From Magnetostatics to Exchange

技术：表面活性剂包覆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。表面活性剂控制粒径分布并提供自组装所需的迁移率，但它也限制了可制备的有趣磁性纳米结构的范围。核之间典型的2-4纳米的距离意味着粒子间的耦合几乎是纯静磁的。表面活性剂至多为铁和钴等材料的氧化提供一个暂时的半透性屏障。组件具有蜡的机械一致性，使其不适合大多数设备应用。表面活性剂很难实现良好的电接触，并且不同的表面覆盖量会导致表观隧道屏障中颗粒与颗粒之间的差异。本项目探索制备有序排列单分散金属纳米柱的加工方法。柱子将通过反应离子蚀刻（RIE）纳米粒子掩膜制成。在更简单的形式中，掩膜将是非磁性的，磁性颗粒将通过溅射沉积在密集柱阵列的顶部来产生。用高密度甲醇基等离子体RIE直接图像化磁性多层膜也将被研究，以确定最小特征尺寸和这种干蚀刻工艺对磁响应的影响。研究的重点是磁成像和中子反射率技术将揭示温度和场相关的磁相关长度的系统，这提供了对磁性纳米颗粒相互作用的更深入的理解。磷化锰具有一级铁磁性到顺磁性的转变，因此纳米颗粒之间的静磁相互作用强度会随着温度的变化而急剧变化。由于交换偏置效应，铁磁性Co纳米颗粒耦合到反铁磁性IrMn层可能显示出集体相关性的差异，这取决于反铁磁性畴的大小。模型软纳米晶体材料，在FeBSi基体中有铁柱，也将用于测试阵列中纳米颗粒之间的交换耦合程度，这种交换耦合应该在基体的居里温度下突然改变，从而可以定量比较交换和静磁对复合材料的贡献。最后，磁性材料的RIE将用于制备一些多层纳米柱，这些纳米柱的小特征尺寸导致了新的量子约束和自旋积累效应。该项目的智力价值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。纳米尺度表征工具将揭示远程磁序发展的新特征，并将澄清宏观测量的解释。非技术：这项工作将在许多方面产生广泛的影响。它将形成一个研究生的论文项目和几个学生的本科研究项目。将为中学生和本科物理实验室开发与MnP中磁相变相关的动手演示和实验室实验。该项目由DMR？金属专业和凝聚态物理专业。

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.