Thermal activation, long-range ordering, and topological frustration of artificial spin ice

Frustrated systems, typically characterized by competing interactions that cannot all be simultaneously satisfied, are ubiquitous in nature and display many rich phenomena and novel physics. Artificial spin ices (ASIs), arrays of lithographically patterned Ising-like single-domain magnetic nanostructures, are highly tunable systems that have proven to be a novel method for studying the effects of frustration and associated properties. The strength and nature of the frustrated interactions between individual magnets are readily tuned by design and the exact microstate of the system can be determined by a variety of characterization techniques. Recently, thermal activation of ASI systems has been demonstrated, introducing the spontaneous reversal of individual magnets and allowing for new explorations of novel phase transitions and phenomena using these systems. In this work, we introduce a new, robust material with favorable magnetic properties for studying thermally active ASI and use it to investigate a variety of ASI geometries. We reproduce previously reported perfect ground-state ordering in the square geometry and present studies of the kagome lattice showing the highest yet degree of ordering observed in this fully frustrated system. We consider theoretical predictions of long-range order in ASI and use both our experimental studies and kinetic Monte Carlo simulations to evaluate these predictions. Next, we introduce controlled topological defects into our square ASI samples and observe a new, extended frustration effect of the system. When we introduce a dislocation into the lattice, we still see large domains of ground-state order, but, in every sample, a domain wall containing higher energy spin arrangements originates from the dislocation, resolving a discontinuity in the ground-state order parameter. Locally, the magnets are unfrustrated, but frustration of the lattice persists due to its topology. We demonstrate the first direct imaging of spin configurations resulting from topological frustration in any system and make predictions on how dislocations could affect properties in numerous materials systems.

SCHUBERT’S WINTERREISE AND THE PIANO TRIO IN Eb; TWO REMARKABLE WORKS FROM ONE REMARKABLE YEAR – 1827: A RECORDING PROJECT

During Franz Schubert’s penultimate year of 1827, he produced two profoundly important and mature works that are the focus of this recording project. The works are, in chronological order: • Winterreise (cycle of 24 songs on the poetry of Wilhelm Müller, 1794-1827) • Piano Trio in Eb Major, Op. 100, D. 929 A unique feature of the project is to present Winterreise in two poetic orders: as traditionally performed and published by Schubert, and in the final ordering published by the poet. The program notes accompanying the dissertation’s three compact discs have extensive information as well as comparative tables of Müller’s and Schubert’s final ordering of the cycle. There are significant differences in ordering, and ultimately the listener will determine which is more dramatically satisfying. Dark melancholy is the central emotion in Winterreise, which Schubert composed at various times throughout 1827 in a mood of corresponding gloom and distress. By contrast, the summer and fall of that year produced, in quick succession, the two glowing and remarkable Piano Trios in Bb and Eb, the second of which is included on these compact discs. The contrast between the trios and Winterreise follows the outward circumstances of Schubert’s life and health, a pattern of sorrow and later consolation and elation. The sound recordings for this dissertation recording project are available on three compact discs that can be found in the Digital Repository at the University of Maryland (DRUM). Winterreise was recorded in August 2009, at the University of Baltimore recital hall in Baltimore, Maryland with University of Maryland Professor François Loup. The trio, recorded in live performance in Baltimore in the spring of 2010, features two members of the Baltimore Symphony Orchestra: Qing Li, B.S.O. principal second violin, and Bo Li, B.S.O. section cellist.