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

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

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.