Heisenberg pseudo-exchange and emergent anisotropies in field-driven pinwheel artificial spin ice

Abstract

Rotating all islands in square artificial spin ice (ASI) uniformly about their centres gives rise to the recently reported pinwheel ASI. At angles around 45 o , the antiferromagnetic ordering changes to ferromagnetic and the magnetic configurations of the system exhibit near-degeneracy, making it particularly sensitive to small perturbations. We investigate through micromagnetic modelling the influence of dipolar fields produced by physically extended islands in field-driven magnetisation processes in pinwheel arrays, and compare the results to hysteresis experiments performed in-situ using Lorentz transmission electron microscopy. We find that magnetisation end-states induce a Heisenberg pseudo-exchange interaction that governs both the inter-island coupling and the resultant array reversal process. Symmetry reduction gives rise to anisotropies and array-corner mediated avalanche reversals through a cascade of nearest-neighbour (NN) islands. The symmetries of the anisotropy axes are related to those of the geometrical array but are misaligned to the array axes as a result of the correlated interactions between neighbouring islands. The NN dipolar coupling is reduced by decreasing the island size and, using this property, we track the transition from the strongly coupled regime towards the pure point dipole one and observe modification of the ferromagnetic array reversal process. Our results shed light on important aspects of the interactions in pinwheel ASI, and demonstrate a mechanism by which their properties may be tuned for use in a range of fundamental research and spintronic applications.

Stray-Field Imaging of a Chiral Artificial Spin Ice during Magnetization Reversal

“…While these topologies were found to be dependent on the angle of the applied reversal eld, we expect these to additionally be inuenced by the predicted intrinsic chirality of the system, in particular when the eld angle is close to zero. Moreover, the chiral boundaries of arrays with a similar geometry have been shown to play a role in the magnetization reversal process 31 and may compete with the intrinsic chirality studied here which occurs in the bulk of the array.…”

Field‐Driven Reversal Models in Artificial Spin Ice

“…Here, we use islands of length 470 nm, width 170 nm, and thickness 10 nm, arranged in a lattice with a nearest neighbor distance of 420 nm, in order to match our earlier work on this system. [ 31,37 ] A set of geometries is defined by island rotation angle, α, from pinwheel at to square at , as shown in Figure 1b. The models are, in part, defined by the island reversal barrier, which we quantify by the angular dependence of the island's coercive field.…”

“…In a previous work, we found that the experimentally observed reversal of pinwheel arrays [ 31 ] could not be replicated using a simple PD model, but that the main features could be reproduced through MM modeling. [ 37 ] It was recently reported that the use of the SW‐PD model also allows the magnetization reversal of pinwheel ASI to be reproduced. [ 41 ] While the SW‐PD model is a substantial improvement on the PD model, we will show that not all magnetization properties are reproduced in this model.…”

“…Importantly, the reversal process in pinwheel ASI [ 31 ] is not purely due to changes in the distribution of dipolar coupling strengths, [ 29 ] but it has also been attributed to a breaking of the Ising nature of the island magnetization. [ 37 ] In particular, both the bending of island magnetization in “end‐states” that form at the point of reversal in field‐driven processes, and the incorporation of an angle dependent energy barrier significantly influence the collective properties. Along with the finite island size, the non‐Ising aspects of the island properties in the pinwheel arrangement creates a strong coupling between nearest neighbor islands that gives rise to pseudo‐exchange effect and emergent anisotropies which are offset from the geometrical axes of the array.…”

Fast Pixelated Detectors in Scanning Transmission Electron Microscopy. Part II: Post-Acquisition Data Processing, Visualization, and Structural Characterization

“…In addition to the structural analysis of the type discussed above, the ability to inspect the 4D dataset in this manner is particularly useful in magnetic imaging, where the source, either magnetic or crystalline, of apparent beam-shifts can be interrogated by navigating the 4D dataset using a color vector magnitude image produced from the analysis [examples of vector magnitude plots of this type may be seen in Paterson et al (2019)]. This type of beam-shift processing will be discussed further in Part III.…”