Magnetic order in disordered dipolar nanostructures

该项目由凝聚态物理和既定的激励竞争研究计划(EPSCoR)共同资助。非技术摘要本项目旨在确定影响纳米粒子组件磁性的因素，包括铁磁性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。铁磁性液滴是一种液态物体，在液体环境中几乎可以呈现并保持任何形状，并且可以在非磁性物体和永久磁体之间过渡。这些独特的性质可以在药物输送、微流控通道和执行器中得到应用。智能水凝胶传感器由于其生物兼容性和对水酸度的敏感性而被设想用于持续的健康监测，例如，水的酸度可以扩大或缩小水凝胶。嵌入的磁性纳米颗粒经历相变，无需电线进入或离开人体即可检测到。该项目为研究生和本科生提供有指导的研究培训和职业发展，并利用机构服务来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、地方会议和国家科学基金会资助的推广项目，让好奇科学的公众和高中生参与到拟议的研究中，并将他的发现整合到本科和研究生课程中。技术摘要无序磁序是一种复杂的非平凡现象，它控制着从消失(超顺磁性)到稳定(铁磁)磁化之间的转变，具有长程、准长程或短程有序。这种复杂性在液体体系中甚至更大，这些液体体系通过超顺磁性纳米颗粒在弯曲的液-液界面上的组装和堵塞而经历顺磁性铁流体和铁磁性液体之间的可逆转变。界面组装是由相反电荷的配体介导的，这些配体锚定纳米颗粒，减少它们的静电电荷和间距。该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，这种纳米结构将超顺磁性和铁磁性之间的可逆转换与硬磁性质结合在一起。主要研究人员通过使用具有不同无序度、对称性、层厚度和间距的纳米颗粒组件和光刻图案纳米结构来研究干燥状态下结构短程有序性和磁性之间的关系，并将这些发现应用于铁磁性液滴，从而实现了这一点。通过使用微磁蒙特卡罗模拟、磁学、铁磁共振光谱以及先进的X射线和电子显微镜进行的协同实验和数值研究，主要研究者证实或驳斥了以下三个假设：(1)无序度增加的材料倾向于从铁磁性到非共线磁化到超顺磁性的转变，而超顺磁性在各向异性排列中被延迟；(2)从宏观性质可以推断出控制磁序的结构短程有序；以及(3)纳米粒子在液-液界面的现场组装和堵塞使表面活性剂的纳米粒子具有增强的剩余磁化和矫顽场。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.