Direct observation of magnetic monopole freedom in two-dimensional artificial spin ice

TL;DR

This study demonstrates the free movement of magnetic monopoles in a specially designed two-dimensional artificial spin ice, showing their creation, transport, and annihilation without string attachments, enabled by ground-state degeneracy.

Contribution

It provides the first direct observation of free magnetic monopoles in 2D artificial spin ice, highlighting the role of ground-state degeneracy in enabling monopole freedom.

Findings

Magnetic monopoles are created, transported, and annihilated freely in the system.

Ground-state degeneracy reduces string energy, allowing monopole freedom.

Spin correlation transitions from antiferromagnetic to ferromagnetic through an ice regime.

Abstract

Magnetic monopole unpairing as a function of external magnetic fields is presented as a fingerprint of this emergent quasiparticles freedom in a two-dimensional artificial spin ice system. Such freedom, required for example for further application in magnetricity, is only possible due to ground-state degeneracy, which causes a decreasing of the string energy in rectangular geometries, designed to allow highest equidistance among nanomagnets. We show by simulations that spin correlation in different rectangular artificial spin ices evolves from antiferromagnetic ordered magnetic structure to a ferromagnetic one, passing through an ice regime were pinch points related to Coulomb phase are observed. By measurements of magnetic force microscopy, we observe magnetic monopole creation, transport and annihilation in such systems with free monopoles created and transported throughout the sample without strings attached, as is commonly observed in conventional artificial spin ice systems.