Artificial spin ice has become a valuable tool for understanding magnetic interactions on a microscopic level. The strength in the approach lies in the ability of a synthetic array of nanoscale magnets to mimic crystalline materials, composed of atomic magnetic moments. Unfortunately, these nanoscale magnets, patterned from metal alloys, can show substantial variation in relevant quantities such as coercive field, with deviations up to 16%. By carefully studying the reversal process of artificial kagome ice, we can directly measure the distribution of coercivities, and by switching from disconnected islands to a connected structure, we find that the coercivity distribution can achieve a deviation of only 3.3%. These narrow deviations should allow the observation of behavior that mimics canonical spin-ice materials more closely.Water ice and spin ice are classic examples of geometrically frustrated systems [1,2], both with residual low-T entropy [3]. In water ice, thermodynamic phases with ordered protons were discovered after decades of experiments [4]. In contrast, no dipole-ordered phase has been observed in spin ice even at the lowest accessible temperatures, contrary to a theoretical prediction [5]. Divergent relaxation times and quenched disorder in samples have been cited as possible explanations. Artificial spin ice has been proposed to help address these questions [6], as it allows the direct control of the geometry of the lattice, with the combined ability to directly image the resulting microstate. Here, samples are composed of lattices of nanoscale ferromagnetic islands, where the magnetization of each element points along its longitudinal axis. At the vertices of the lattice, the ferromagnetic elements interact, and because of the geometry of the system, their magnetic configurations are frustrated [6][7][8][9][10][11]. This allows the study of frustration in systems where crystalline imperfections can be completely removed by design, or introduced in a controlled way. Unfortunately, current lithographic techniques are limited by unintended roughness at edges and interfaces, creating inadvertent disorder. This diminishes the ability to compare observations from artificial spin ice materials with studies of spin ice oxides, where magnetic atoms are presumed to be identical.Edge roughness of nanomagnetic elements is known to substantially influence the coercive field, by creating nucleation sites that can initiate the magnetic reversal [12]. In some artificial spin ice geometries, this edge roughness can create a large variability in the behavior of the artificial "atoms" (magnetic nano-islands). In recent studies of artificial kagome ice, the variations in coercivity were found to be substantial-up to 16% of the average coercive value [8,9,13]. This variability can easily be reduced by choosing materials with low crystal anisotropy [9], but we here show substantial further reduction with a geometry with connected magnetic islands. In a connected geometry, nontrivial spin textures (domain walls) alread...
Abstract:We report FePd 3 as a material for studying thermally active artificial spin ice (ASI) systems and use it to investigate both the square and kagome ice geometries. We readily achieve perfect ground state ordering in the square lattice and demonstrate the highest yet degree of monopole charge-ordering in the kagome lattice. We find that smaller lattice constants in the kagome system generally produce larger domains of charge order. Monte Carlo simulations show excellent agreement with our data when a small amount of disorder is included in the simulation. Main text:Frustrated systems have emerged as an important topic of condensed matter physics, and geometric frustration is of particular prominence, where the frustration arises from an ordered structure rather than crystalline imperfections [1]. In such systems, an apparent degeneracy of ground states prevents long range order, often when detailed analysis of perturbations predict that an ordered state nevertheless should occur [2]. Despite decades of intense interest, frustrated systems still pose fundamental problems, with many unanswered questions, due in part to the tendency of these systems to inefficiently explore their configuration spaces and to lose ergodicity [3]. Monte Carlo simulations can address some of these issues, through the introduction of more complicated basic excitations [4,5], but questions about the specific
We model the dynamics of magnetization in an artificial analogue of spin ice specializing to the case of a honeycomb network of connected magnetic nanowires. The inherently dissipative dynamics is mediated by the emission and absorption of domain walls in the sites of the lattice, and their propagation in its links. These domain walls carry two natural units of magnetic charge, whereas sites of the lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between these charges play a major role in the physics of the system, as does quenched disorder caused by imperfections of the lattice. We identify and describe different regimes of magnetization reversal in an applied magnetic field determined by the orientation of the applied field with respect to the initial magnetization. One of the regimes is characterized by magnetic avalanches with a 1/n distribution of lengths.
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