Magnetic order in disordered dipolar nanostructures

该项目是由凝聚态物理和建立计划，以刺激竞争研究（EPSCoR）共同资助。该项目旨在确定影响纳米颗粒组件磁性质的因素，包括铁磁液滴和水凝胶传感器，以便它们可以用于关键的健康和技术应用。铁磁液滴是一种液体体，在液体环境中可以采取并保持几乎任何形状，并且可以在非磁性物体和永磁体之间转换。这些独特的特性可以在药物输送、微流体通道和致动器中找到应用。智能水凝胶传感器由于其生物相容性和对水酸度的敏感性，例如水凝胶的放大或缩小，被设想用于持续的健康监测。嵌入的磁性纳米颗粒经历相变，可以在不需要电线进出人体的情况下进行检测。该项目为研究生和本科生提供指导研究培训和职业发展，并利用机构服务招收代表性不足的学生。首席研究员通过现有的大学服务、社交媒体、当地会议和国家科学基金会资助的外展项目，吸引对科学好奇的公众和高中生参与拟议的研究，并将他的发现整合到本科和研究生课程中。摘要磁性失序是一种复杂的非平凡现象，它支配着消失磁化（超顺磁）和稳定磁化（铁磁）之间的过渡，具有长程、准长程或短程有序。在液体系统中，通过在弯曲的液-液界面上组装和干扰超顺磁性纳米颗粒，在顺磁性铁磁流体和铁磁液体之间进行可逆转换，这种复杂性甚至更大。界面组装是由带相反电荷的配体介导的，这些配体锚定纳米粒子并减少它们的静电电荷和间距。本项目的目标是确定超顺磁性纳米结构无序系综的设计策略，该超顺磁性纳米结构结合了超顺磁性和铁磁性之间的可逆转变以及硬磁性。主要研究者通过研究干燥状态下结构短程有序和磁性之间的关系来实现这一目标，使用纳米颗粒组件和具有不同程度的无序、对称、层厚度和间距的光刻图案纳米结构，并将这些发现应用于铁磁性液滴。通过使用微磁蒙特卡罗模拟、磁强计、铁磁共振光谱、先进的x射线和电子显微镜进行实验和数值研究，主要研究者证实或驳斥了以下三个假设：(1)无序度增加的材料有利于从铁磁性到非共线磁化到超顺磁性的转变，这种转变在各向异性排列中被延迟；(2)由宏观性质可以推断出支配磁序的结构短程有序；(3)纳米颗粒在液-液界面的场内组装和干扰使表面活性剂的纳米图案具有增强的剩余磁化和矫顽力场。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project is jointly funded by Condensed Matter Physics and the Established Program to Stimulate Competitive Research (EPSCoR).Non-technical abstractThis project aims to identify factors that affect magnetic properties of nanoparticle assemblies, including ferromagnetic liquid droplets and hydrogel sensors, so that they can be used for critical health and technology applications. Ferromagnetic liquid droplets are liquid bodies that can take and sustain virtually any shape while in liquid environment and can transition between a non-magnetic object and a permanent magnet. These unique properties could find application in drug delivery, microfluidic channels, and actuators. Smart hydrogel sensors are envisioned for continuous health monitoring due to their biocompatibility and sensitivity to, e.g., water acidity, that enlarges or shrinks the hydrogel. Embedded magnetic nanoparticles undergo a phase transition that can be detected without the need for electric wires into or out of the human body. This project provides mentored research training and career development to graduate and undergraduate students and leverages institutional services to recruit underrepresented students. The principal investigator engages the science-curious public and high school students in the proposed research via existing university services, social media, local conferences, and National Science Foundation-funded outreach programs and integrates his findings into undergraduate and graduate courses. Technical abstractMagnetic order by disorder is a complex non-trivial phenomenon that governs the transition between a vanishing (superparamagnetic) and a stable (ferromagnetic) magnetization with long-range, quasi-long-range, or short-range order. This complexity is even larger in liquid systems that undergo a reversible transformation between paramagnetic ferrofluid and ferromagnetic liquid by the assembly and jamming of superparamagnetic nanoparticles on curved liquid-liquid interfaces. The interfacial assembly is mediated by oppositely charged ligands that anchor the nanoparticles and reduce their electrostatic charge and spacing. The goal of this project is to determine design strategies for disordered ensembles of superparamagnetic nanostructures that combine reversible transformation between superparamagnetism and ferromagnetism with hard-magnetic properties. The principal investigator accomplishes this by studying the relationship between structural short-range order and magnetic properties in the dried state using nanoparticle assemblies and lithographically patterned nanostructures with different degrees of disorder, symmetry, layer thickness, and spacing and applying these findings to ferromagnetic liquid droplets. Through coordinated experimental and numerical studies using micromagnetic Monte Carlo simulations, magnetometry, ferromagnetic resonance spectroscopy, and advanced x-ray and electron microscopies, the principal investigator corroborates or refutes the following three hypotheses: (1) Materials with increasing disorder favor a transition from ferromagnetism to non-collinear magnetization to superparamagnetism that is delayed in anisotropic arrangements; (2) Structural short-range order governing magnetic order can be inferred from macroscopic properties; and (3) In-field assembly and jamming of nanoparticles at liquid-liquid interfaces enable nanopatterning of the surfactants with an enhanced remanent magnetization and coercive field.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.