Magnetic Nanostructures through Metallic Dewetting

摘要：本研究项目将研究一种制备不同金属和合金纳米颗粒的新策略，并表征其尺寸相关性质。当材料的结构达到纳米长度尺度时，许多电子、磁性和光学性质都与尺寸有关。闪存、计算机硬盘中的磁性记录介质和数字视频播放器中的二极管激光器都是使用纳米结构材料技术的例子。本研究项目中感兴趣的材料是尚未制备成具有良好尺寸控制的纳米颗粒的金属和合金。重点将放在已知具有有趣的磁电子或磁光学特性的合金上，或者在节能应用中作为永久磁铁的潜在用途。该方法将使用带有规则凹坑阵列的纳米模板来控制颗粒大小。在沉积所需材料的薄膜后，将其加热，直到薄膜软化并填充凹坑。均匀的坑尺寸将导致均匀的颗粒尺寸。这种制造方法将使探索尺寸依赖行为成为可能。磁性、磁阻和磁光性质的表征将为工程纳米结构材料提供有价值的新数据。技术摘要：本研究项目将研究通过在纳米模板上润湿和脱湿金属薄膜来形成单分散磁性纳米颗粒，并研究其磁性、磁阻和磁光性质。虽然L10 FePt将成为目标材料之一，但由于其氧化敏感性或复杂的晶体结构，尚未通过化学方法制成的磁性金属合金将特别引起人们的兴趣。目前面临的挑战是克服对氧化的敏感性，这使得许多这些颗粒无法通过溶液化学方法制备，并实现晶体取向以测量它们的各向异性磁性。制备磁性合金纳米颗粒阵列有三种策略，每种策略都有其优点和局限性：1)在纳米颗粒单层上沉积阴影，2)在纳米孔阵列中播种后沉积，以及3)使用纳米柱阵列作为磁性合金薄膜的硬掩膜。在所有情况下，将使用快速热退火来探索合金纳米颗粒晶体取向的方法。模板将是介电材料（SiOx, SiNx, MgO）和导电TiNx。磁性合金将包括常见的L10合金（FePt）和较少研究的L10合金（FeNi, MnAl, MnBi），自旋电子学材料（Heusler合金Co2FeSi和Ni2MnGa，加上FeCoB）和磁光学材料（非晶材料GdFeCo和TbFeCo）。研究结果将有助于在纳米尺度上提高对金属润湿和脱湿的理解。纳米图像化工艺将适用于广泛的复杂材料，而不仅仅是磁性金属合金。单分散、钝化磁性金属合金纳米颗粒的制备将使定量尺寸依赖的测量成为可能。磁化行为将揭示表面化学的作用和粒子表面交换作用的减少。将对单个纳米粒子进行新型电阻、磁电阻测量。纳米粒子的光滑单层阵列将用光学和磁光谱学来表征。该项目将包括一名研究生的博士论文研究，以及几个本科生的研究项目。通过与磁学社区相关的许多教育活动，也将对广泛的受众产生影响。

Non-technical Abstract:This research program will investigate a new strategy for making nanoparticles of different metals and alloys, and characterize their size-dependent properties. Many electronic, magnetic, and optical properties become size-dependent when the material is structured on the nanometer length scale. Flash memory, magnetic recording media in computer hard disks, and diode lasers in digital video players, are examples of technology using nanostructured materials. The materials of interest in this research program are metals and alloys that have not yet been prepared as nanoparticles with good size control. The emphasis will be on alloys known to have interesting magneto-electronic or magneto-optical properties, or potential use as permanent magnets in energy-saving applications. The approach will use nanopatterned templates with regular arrays of pits to control the particle size. After depositing a thin film of the desired material, it will be heated until the film dewets and fills the pits. The uniform pit size will lead to a uniform particle size. This fabrication method will enable exploration of size-dependent behavior. Characterization of the magnetic, magnetoresistive, and magneto-optical properties will provide valuable new data that can be used in engineering nanostructured materials.Technical Abstract:This research program will investigate the formation of monodisperse magnetic nanoparticles created by the wetting and dewetting of thin metal films on nanopatterned templates, and to investigate their magnetic, magnetoresistive, and magneto-optical properties. While L10 FePt will be one of the target materials, there will be particular interest in magnetic metal alloys that have not yet been made by chemical methods, due to their oxidation sensitivity or complex crystal structures. The challenges are to overcome the sensitivity to oxidation that makes many of these particles impossible to prepare by solution chemistry methods, and to achieve crystallographic orientation to measure their anisotropic magnetic properties. There will be three strategies for preparing the magnetic alloy nanoparticle arrays, each with strengths and limitations: 1) shadow deposition on a nanoparticle monolayer, 2) deposition after seeding in nanohole arrays, and 3) using nanopillar arrays as a hard mask for a magnetic alloy thin film. In all cases rapid thermal annealing will be used to explore methods for crystallographically orienting the alloy nanoparticles. The templates will be dielectric materials (SiOx, SiNx, MgO) and conducting TiNx. The magnetic alloys will include L10 alloys, both familiar (FePt), and less studied (FeNi, MnAl, MnBi), materials for spintronics (the Heusler alloys Co2FeSi and Ni2MnGa, plus FeCoB), and for magneto-optics (the amorphous materials GdFeCo and TbFeCo). The results will lead to an improved understanding of metallic wetting and dewetting on the nanoscale. The nanopatterning process will be applicable to a wide range of complex materials, not only magnetic metal alloys. The preparation of monodisperse, passivated magnetic metal alloy nanoparticles will enable quantitative size-dependent measurements. The magnetization behavior will reveal the roles of surface chemistry and reduced exchange interactions at the particle surface. Novel resistance, magnetoresistance measurements will be made on individual nanoparticles. Smooth monolayer arrays of nanoparticles will be characterized by optical and magneto-optical spectroscopy. This project will involve the doctoral thesis research of a graduate student, along with several undergraduate research projects. There will also be impact to a broad audience through numerous educational activities associated with the magnetics community.