Collaborative Research: Controlled Disorder and Topological Defects in Magnetically Frustrated Thin Film Metamaterials

摘要：薄膜的纳米尺度图案化的现代技术产生的超材料的行为不同于传统的化学技术制造的；它们的基本性质可以从体行为显著改变，这涉及到有限大小系统中单粒子相互作用的集体行为的出现。薄膜尺寸缩小到纳米范围跨越了自然长度尺度，如铁磁畴壁宽（7-100纳米），并跨越了以强波动为特征的介观体系，这种波动破坏了有序基态的稳定，必须在现代设备中加以控制。另外，几何约束或竞争相互作用带来的挫败感也会阻止系统达到平衡。人工自旋冰是由细“线”形成的典型超材料，其大的长宽比使其表现为经典的伊辛自旋，由于挫折和能量障碍而无法平衡成磁有序。对远离平衡状态的系统中的非热动力学的理解很少，并且相对较少的工作涉及无序对自旋冰动力学的影响。人工自旋冰的优势在于，它们的空间顺序和拓扑结构可以被仔细控制，它们的波动可以被设计成覆盖一定范围的时间尺度。我们的计划研究如何控制周期性平移和点对称的导线网络导致磁场反转和自旋波动力学的强烈变化。然而，随机无序可以产生“自旋玻璃”状态，只有短程有序，受挫的系统可能“有序脱离无序”，打破周期对称，减少低能量简并。块状磁性准晶体表现出惊人的物理特性和挫败感，因为它们具有标志性的长程取向顺序，没有周期性的平移对称性，使它们处于周期性晶体和非晶态材料之间的独特位置。已知的大块准晶体经历自旋玻璃，而不是远程磁序，很难在实验室中生长和表征。我们在纳米制造方面的进展已经产生了“人工准晶体”，这些准晶体的弛豫动力学、反转、磁相关性和平衡基态的实现通过图案设计被系统地控制，以揭示周期性、准周期性或随机无序的固有结果。我们的学生接受薄膜沉积和图像化，先进的数值模拟，铁磁共振，静态磁化，x射线反射和原子力显微镜的内部指导。很少有实验室能提供如此广泛的研究工具和培训计划。在阿贡、劳伦斯伯克利国家实验室和盖瑟斯堡的NIST，薄膜在纳米尺度上被用于相干软x射线散射、扫描电子显微镜与极化分析、磁力显微镜和其他实验。高级调查人员和研究生研究助理举办研讨会，为肯塔基州费耶特县的小学教师提供教学辅助、课程和专业发展，以满足美国和肯塔基州教育部颁布的新的STEM教育目标。来自西北大学附近学区的高中生学习如何对薄膜材料进行先进的微波测量和数据分析。技术摘要：研究了无序化和约化对称性对具有挫折和自旋冰行为的图图化磁性薄膜超材料的平衡和动态磁性能的影响。人们的注意力集中在人工准晶体上，这些准晶体的非周期性、长程位置顺序使它们处于周期性Bravais晶格和随机无序玻璃之间的独特位置。基于周期性Bravais晶格的Fibonacci畸变，一组相关的非周期、长程有序晶格揭示了连续可变非周期对磁反转、动力学和自旋冰行为的影响。各种类型的随机无序可以被图像化成所研究的所有类别的超材料，以系统地研究非周期性和变点对称性的影响。由于受控晶格无序，以及具有可变拓扑、大小和无序的模式中的物理可观察物的有限尺寸缩放行为，自旋波定位的搜索正在进行中。铁磁共振、静态磁化、纳米成像技术和数值模拟用于表征人工受挫晶格中的磁性织构、拓扑缺陷、自旋波、自旋冰行为和可能的相变。我们的团队打算通过极化分析和数值模拟来验证和扩展扫描电子显微镜的初步结果，这些结果表明，人工准晶自旋冰的生长样品具有非常接近长期寻找的铁磁基态的磁性结构。利用x射线光子相干散射独特的时间相干性和相灵敏度作为温度和磁场的函数，研究了自旋冰的平衡和非热动力学。初始x射线散射结果表明，适度的外加磁场可以控制轨道角动量向从方形人工自旋冰磁性织体中产生的拓扑相位奇点共振散射的软x射线“漩涡束”的转移。后续实验试图确定控制涡旋光束特性的自旋冰结构的确切拓扑特征。

Nontechnical Abstract:Modern techniques for nanoscale patterning of thin films yield metamaterials that behave differently from those fabricated via traditional chemical techniques; and their fundamental properties can be markedly altered from bulk behavior, which involves the emergence of collective behavior from single-particle interactions in finite-size systems. The reduction of film dimensions into the nanometer range traverses natural length scales such as ferromagnetic domain wall widths (7-100 nm), and crosses a mesoscopic regime characterized by strong fluctuations that destabilize ordered ground states and must be controlled in modern devices. Alternatively, frustration imposed by geometrical constraints or competing interactions also prevents systems from reaching equilibrium. Artificial spin ices are exemplary metamaterials formed from thin "wires" whose large length-to-width ratio makes them behave as classical Ising spins that resist equilibration into magnetic order due to frustration and energy barriers. Understanding athermal dynamics in systems far from equilibrium is poorly understood, and relatively little work has addressed effects of disorder on spin ice dynamics. Artificial spin ices offer the advantage that their spatial order and topology can be carefully controlled, and their fluctuations engineered to cover a range of time scales. Our program investigates how control of periodic translational and point symmetries of wire networks results in strong changes in magnetic reversal and spin wave dynamics. Whereas random disorder can yield "spin glass" states with only short-range order, frustrated systems may "order out of disorder" that breaks periodic symmetry and reduces low-energy degeneracies. Bulk magnetic quasicrystals exhibit striking physical properties and frustration due to their signature long-range orientational order without periodic translational symmetry, placing them in a unique niche between periodic crystals and amorphous materials. Known bulk quasicrystals undergo spin-glass, rather than long-range magnetic order, and are difficult to grow and characterize in the laboratory. Our advances in nanofabrication have produced "artificial quasicrystals" whose relaxation dynamics, reversal, magnetic correlations and attainment of an equilibrium groundstate are systematically controlled via pattern design, in order to reveal what magnetic behaviors are inherent consequences of periodicity, quasiperiodicity or random disorder.Our students receive in-house instruction in thin film deposition and patterning, advanced numerical simulations, ferromagnetic resonance, static magnetization, X-ray reflectometry and atomic force microscopy. Very few laboratories can provide such a broad program of research tools and training. Thin films are patterned at the nanoscale for use in coherent soft X-ray scattering, scanning electron microscopy with polarization analysis, magnetic force microscopy and other experiments performed at Argonne, Lawrence Berkeley National Labs, and NIST, Gaithersburg. Senior investigators and graduate research assistants conduct workshops to provide elementary school teachers in Fayette County, Kentucky with instructional aids, curriculum and professional development needed to meet new STEM education goals promulgated by the U.S. and Kentucky Departments of Education. High school students from school districts neighboring Northwestern U. learn how to carry out advanced microwave measurements and data analyses of thin-film materials.Technical Abstract:The effects of disorder and reduced symmetry on the equilibrium and dynamic magnetic properties of patterned magnetic thin-film metamaterials that exhibit frustration and spin ice behavior are studied. Attention is focused on artificial quasicrystals whose aperiodic, long-range positional order places them in a unique niche between periodic Bravais lattices and randomly disordered glasses. A related set of aperiodic, long-range-ordered lattices based on Fibonacci distortions of periodic Bravais lattices reveal the effects of continuously variable aperiodicity on magnetic reversal, dynamics and spin ice behavior. Various types of random disorder can be patterned into all classes of metamaterials under study to systematically study effects of aperiodicity and variable point symmetry. Searches are underway for spin wave localization due to controlled lattice disorder, as well as for finite-size scaling behavior of physical observables in patterns having variable topology, size and disorder.Ferromagnetic resonance, static magnetization, nanoscale imaging techniques and numerical simulations are used to characterize magnetic textures, topological defects, spin waves, spin ice behavior and possible phase transitions in artificial frustrated lattices. Our Team intends to verify and expand on initial results of scanning electron microscopy with polarization analysis and numerical simulations that indicate as-grown samples of artificial quasicrystalline spin ice have magnetic textures that are very close to a long-sought ferromagnetic ground state. The equilibration and athermal dynamics of spin ices are investigated using the unique temporal coherence and phase sensitivity of X-ray photon coherent scattering as a function of temperature and magnetic field. Initial X-ray scattering results show a modest applied magnetic field can be used to control the transfer of orbital angular momentum to a soft X-ray "vortex beam" resonantly scattered from topological phase singularities generated in the magnetic texture of square artificial spin ice. Follow-up experiments seek to identify the exact topological features of spin ice textures that control vortex beam characteristics.