Micromagnetic simulations of magnetization reversal in kagome artificial spin ice

Abstract

Artificial spin ice (ASI) is a magnetic metamaterial designed to exhibit frustration [1]. It consists of an array of magnetostatically interacting nanomagnets arranged in a geometry that prevents these interactions to be all satisfied simultaneously. They were originally conceived to replicate, in an artificial manner, interesting frustration induced phenomena found in natural frustrated magnets known as spin ices [2].One of those most remarkable phenomena is the emergence of quasiparticles similar to magnetic monopoles when such materials are excited above the ground state [3]. In ASI, this behavior is experimentally reproduced by subjecting the system to a magnetization cycle [4]. The magnetization reversal proceeds by creation of one-dimensional strings of flipped nanomagnets, referred as Dirac strings, that host the magnetic monopoles at their ends. This reversal mechanism has been already reproduced by Monte Carlo-based simulations, where the nanomagnets are treated as Ising variables and a phenomenological random switching field distribution is employed [5].In this work, we reproduced the same behavior using a more fundamental approach, that was lacking in literature, based on micromagnetic simulations, applied particularly to a large scale (more than a thousand magnets) kagome lattice arrangement of ASI [6]. We regarded a more realistic description of the shape of each nanomagnet, including its finite size and roughness at the edges, in such a way that no a priori switching field distribution was required but that came out naturally. Furthermore, our simulations predict a critical angle between the applied field and the direction of a sublattice that separates different reversal regimes: the process is mostly 2D for angles above the critical value, and it is 1D (i.e., via Dirac strings and monopoles) otherwise. **