Magnetic monopole and string excitations in a two-dimensional spin ice

TL;DR

This paper investigates magnetic monopole-like excitations in a two-dimensional artificial spin ice, revealing Coulombic and linear confining interactions, and discusses how entropic effects influence monopole behavior in lower dimensions.

Contribution

It provides the first detailed analysis of monopole excitations in 2D artificial spin ice, highlighting the presence of a confining string potential and contrasting it with 3D systems.

Findings

Monopole-like excitations interact via Coulombic plus linear potential.

The linear string tension may vanish due to entropy, allowing free monopoles.

Entropic effects can either confine or free magnetic monopoles depending on conditions.

Abstract

We study the magnetic excitations of a square lattice spin-ice recently produced in an artificial form, as an array of nanoscale magnets. Our analysis, based upon the dipolar interaction between the nanomagnetic islands, correctly reproduces the ground-state observed experimentally. In addition, we find magnetic monopole-like excitations effectively interacting by means of the usual Coulombic plus a linear confining potential, the latter being related to a string-like excitation binding the monopoles pairs, what indicates that the fractionalization of magnetic dipoles may not be so easy in two dimensions. These findings contrast this material with the three-dimensional analogue, where such monopoles experience only the Coulombic interaction. We discuss, however, two entropic effects that affect the monopole interactions: firstly, the string configurational entropy may loose the string tension and then, free magnetic monopoles should also be found in lower dimensional spin ices; secondly, in contrast to the string configurational entropy, an entropically driven Coulomb force, which increases with temperature, has the opposite effect of confining the magnetic defects.