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

TL;DR

This study explores how dipolar interactions and anisotropies influence magnetization processes in pinwheel artificial spin ice, revealing a Heisenberg pseudo-exchange mechanism and pathways to tune array properties for applications.

Contribution

It introduces the concept of Heisenberg pseudo-exchange interactions in pinwheel ASI and analyzes how array geometry and island size affect magnetic reversal behaviors.

Findings

Magnetization end-states induce a Heisenberg pseudo-exchange interaction.

Symmetry reduction leads to anisotropies and avalanche reversals.

Reducing island size transitions the system from strong coupling to point dipole regime.

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$^\mathrm{o}$, 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.