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

We propose a fundamental investigation of the effects of disorder and reduced symmetry on the equilibrium and dynamic magnetic properties of patterned thin film metamaterials that exhibit frustration and spin ice behavior. We focus our attention on several distinct types of artificial quasicrystals whose aperiodic, long-range translational symmetry makes them prototypes of intermediate disorder that lies 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 will be fabricated to investigate the effects of continuously variable aperiodicity on magnetic reversal, dynamics and spin ice behavior. Various types of random disorder will be systematically patterned into all classes of metamaterials under study for comparison to effects of aperiodicity and reduced symmetry. We will search for spin wave localization due to controlled lattice disorder, and for finite-size scaling behavior of physical observables in patterns having variable topology, size and order. We will utilize ferromagnetic resonance, static magnetization nanoscale imaging techniques and numerical simulations, to characterize magnetic textures, topological defects, spin waves, spin ice behavior and possible phase transitions in frustrated lattices. We will exploit the unique temporal coherence and phase sensitivity of X-ray photon coherent scattering (XPCS) to conduct studies of novel photonic effects and magnetic dynamics as a function of temperature and magnetic field. We will pursue XPCS speckle relaxation data and numerical simulations that indicate isothermal magnetic field quenching of square artificial spin ice yields magnetic textures that are very close to the predicted spin ice ground state. We will also investigate the initial equilibration of the spin ice, which proceeds via a "directed walk" of topological charge excitations ("Dirac monopoles"). Longer time measurements will characterize a slower relaxation regime dominated by monopole annihilation and trapping. We will study the conditions under which 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. Broader Impacts of the Proposed Program: Graduate and Undergraduate students will receive instruction within and integrated program involving in-house thin film deposition and patterning, micromagnetic and Monte Carlo simulations, and advanced physical and structural characterization, including BB FMR, static magnetization, X-ray reflectometry and MFM. Very few laboratories can provide such a broad program of research tools and skills. We will provide state-of-art patterned thin films for collaborative studies of soft X-ray scattering, SEMPA, PEEM, MFM, magnonics and other techniques at Argonne and Lawrence Berkeley National Labs, and NIST, Gaithersburg, which will provide state-of-art research opportunities for postdoctoral researchers and Staff. Our team of Senior Personnel and graduate research assistants will conduct a pilot program of 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. Department of Education and the Commonwealth of Kentucky. Most of the STEM topics involve fundamental physics concepts and scientific methods with which the average elementary teacher is very unfamiliar; yet very little support is currently provided by local school districts and state government towards implementing new guidelines in classrooms.