Artificial kagome spin ice: dimensional reduction, avalanche control and emergent magnetic monopoles

Abstract

Artificial spin-ice systems consisting of nanolithographic arrays of isolated nanomagnets are model systems for the study of frustration-induced phenomena. We have recently demonstrated that monopoles and Dirac strings can be directly observed via synchrotronbased photoemission electron microscopy, where the magnetic state of individual nanoislands can be imaged in real space. These experimental results of Dirac string formation are in excellent agreement with Monte Carlo simulations of the hysteresis of an array of dipoles situated on a kagome lattice with randomized switching fields. This formation of one-dimensional avalanches in a two-dimensional system is in sharp contrast to disordered thin films, where avalanches associated with magnetization reversal are twodimensional. The self-organized restriction of avalanches to one dimension provides an example of dimensional reduction due to frustration. We give simple explanations for the origin of this dimensional reduction and discuss the disorder dependence of these avalanches. We conclude with the explicit demonstration of how these avalanches can be controlled via locally modified anisotropies. Such a controlled start and stop of avalanches will have potential applications in data storage and information processing.

Artificial ferroic systems: novel functionality from structure, interactions and dynamics

“…In order to confirm the presence of monopoles, one can consider the coarse grained charge density, ρ m , [64] which, as can be seen in figure 6b, is indeed consistent with the position of the charge defects ∆Q/q = ±2. Interestingly, the Dirac strings in this two dimensional system are one dimensional [29], which is a signature of 'dimensional reduction due to frustration' [70], and is a consequence of the fact that the strings do not branch which, along with the fact that they do not propagate around corners, can be understood by considering the local dipolar fields [71]. The direction of Dirac string propagation can be controlled, for example, by local chirality of the spins, given by the local curling of the spins at the ends of the islands interacting at a vertex [72] and the chirality of the moving domain wall [73] in a connected system.…”

“…For example, the extent of the strings is dependent on the amount of disorder given by the distribution in switching fields of the islands. A system with high disorder is characterized by short avalanches, whereas low disorder results in avalanches whose length approaches the sample size [71,74].…”

“…[29] and Ref. [71]). In small clusters, one can use this to control the field induced states and vortex chirality [87].…”

Angular-dependent magnetization reversal processes in artificial spin ice

“…15 Therefore, the magnetization reversal in one bar triggers the reversal in neighbouring bars and leads to chains of reversal following Dirac strings in avalanche-like behavior. [15][16][17] Furthermore, in isolated bar systems, monopole chirality may provide additional factors that influence the reversal paths in such systems. 18 Varying the angle of the applied field with respect to the geometrical structuring also offers a route to influence the magnetization reversal by biasing particular sub-lattice directions.…”

Emergent magnetic monopoles in frustrated magnetic systems

“…Hügli et al [4] study artificial spin ice systems consisting of nanolithographic arrays of isolated nanomagnets. These are model systems for the observation of frustration-induced phenomena.…”