Chiral switching and dynamic barrier reductions in artificial square ice

TL;DR

This paper reveals how chiral magnetic effects and local environment influence switching barriers in artificial spin ice, significantly affecting transition rates and relaxation dynamics, which are crucial for accurate kinetic Monte Carlo simulations.

Contribution

It introduces the concept of barrier splitting due to chiral reversal channels and demonstrates its impact on transition rates in artificial spin ice systems.

Findings

Barrier splitting can be substantial in nanomagnetic switching.

Transition rates can be exponentially increased due to barrier reductions.

Faster relaxation times and altered correlations are observed.

Abstract

Collective dynamics in lithographically-defined artificial spin ices offer profound insights into emergent correlations and phase transitions of geometrically-frustrated Ising spin systems. Their temporal and spatial evolution are often simulated using kinetic Monte Carlo simulations, which rely on the precise knowledge of the switching barriers to obtain predictive results in agreement with experimental observations. In many cases, however, the barriers are derived from simplified assumptions only, and do not take into account the full physical picture of nanomagnetic switching. Here we describe how the immediate magnetic environment of a nanomagnet reversing via quasi-coherent rotation can induce clockwise and counter-clockwise switching channels with different barrier energies. This barrier splitting for chiral reversal channels can be sizeable and, as string-method micromagnetic simulations show, is relevant for artificial spin ice systems made of both exchange -- as well as magnetostatically --dominated units. Due to the barrier splitting (and further reductions due to non-uniform reversal) transition rates can be exponentially enhanced by several orders of magnitude compared to mean-field predictions, especially in the limit of rare switching events where thermal excitation is less likely. This leads to significantly faster relaxation time scales and modified spatial correlations. Our findings are thus of integral importance to achieve realistic kinetic Monte Carlo simulations of emergent correlations in artificial spin systems, magnonic crystals, or the evolution of nanomagnetic logic circuits.