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

非技术摘要：现代纳米尺度薄膜图案化技术产生的超材料表现出与传统化学技术不同的行为；它们的基本性质可以从整体行为显著改变，这涉及到有限尺寸系统中单个粒子相互作用产生集体行为。薄膜尺寸减小到纳米范围，跨越了自然长度的尺度，如铁磁磁区壁宽(7-100 nm)，并跨越了以强烈波动为特征的介观区域，该区域破坏了有序基态的稳定，必须在现代器件中进行控制。或者，几何约束或竞争相互作用造成的挫折感也会阻碍系统达到平衡。人造自旋冰是由细丝形成的典型超材料，其大的长宽比使其表现为经典的伊辛自旋，由于受挫和能量障碍，伊辛自旋抵制平衡进入磁性秩序。对远离平衡的系统中的非热动力学了解得很少，相对较少的工作涉及无序对自旋冰动力学的影响。人造自旋冰的优势在于，它们的空间顺序和拓扑结构可以被仔细控制，它们的波动可以覆盖一系列时间尺度。我们的程序研究了控制导线网络的周期性平移对称性和点对称性如何导致磁反转和自旋波动力学的强烈变化。随机无序可以产生只有短程有序的“自旋玻璃态”，而受挫的系统可能会“无序无序”，从而破坏周期对称性，减少低能简并。块状磁性准晶由于其特征的长程取向有序而不具有周期性的平移对称性，显示出惊人的物理性质和挫折性，使其处于周期晶体和非晶态材料之间的独特位置。已知的大块准晶经历的是自旋玻璃化，而不是长程磁序，并且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“人造准晶”，它们的弛豫动力学、反转、磁关联和达到平衡基态都是通过图案设计来系统地控制的，以揭示什么是周期性、准周期性或随机无序的固有结果。我们的学生接受关于薄膜沉积和图案化、高级数值模拟、铁磁共振、静态磁化、X射线反射术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方案。薄膜在纳米尺度上被图案化，用于相干软X射线散射、带有极化分析的扫描电子显微镜、磁力显微镜和其他在Argonne，Lawrence Berkeley National Labs和NIST，Gaithersburg进行的实验。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工具、课程和专业发展，以满足美国和肯塔基州教育部颁布的新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：研究了无序和对称性降低对具有受挫和自旋冰态的图案化磁性薄膜超材料的平衡和动态磁性质的影响。人们的注意力集中在人造准晶上，它们的非周期、长程位置顺序使它们处于周期性布拉维晶格和随机无序玻璃之间的独特利基位置。基于周期Bravais晶格的斐波那契扭曲，建立了一组相关的非周期长程有序晶格，揭示了连续变化的非周期性对磁反转、动力学和自旋冰行为的影响。各种类型的随机无序可以被模式化成所有类别的超材料，以系统地研究非周期性和变点对称性的影响。人们正在寻找由于受控晶格无序引起的自旋波局域化，以及具有不同拓扑、大小和无序的图案中物理可观测到的有限大小标度行为。铁磁共振、静态磁化、纳米尺度成像技术和数值模拟被用来表征人工受阻晶格中的磁性织构、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，随着生长，人造准晶自旋冰的样品具有非常接近人们长期寻找的铁磁基态的磁性织构。利用X射线光子相干散射的独特的时间相干性和位相敏感性，研究了自旋冰的平衡和非热动力学。初步的X射线散射结果表明，一个适度的外加磁场可以用来控制轨道角动量向软X射线“涡旋束”的转移，该“涡旋束”是从正方形人造自旋冰的磁性结构中产生的拓扑相位奇点共振散射而来的。后续实验试图确定控制涡流光束特征的自旋冰纹理的确切拓扑特征。

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.