Ground state lost but degeneracy found: the effective thermodynamics of artificial spin ice

TL;DR

This paper investigates the non-thermal behavior of artificial spin ice, revealing an effective degeneracy and presenting a formalism to predict local state populations without adjustable parameters.

Contribution

It introduces a novel formalism to predict state populations in artificial spin ice, demonstrating effective degeneracy despite the system not reaching its ground state.

Findings

Demagnetization does not lead to the ground state.

The system exhibits effective extensive degeneracy.

A formalism predicts local state populations without adjustable parameters.

Abstract

We analyze the rotational demagnetization of artificial spin ice, a recently realized array of nanoscale single-domain ferromagnetic islands. Demagnetization does not anneal this model system into its anti-ferromagnetic ground state: the moments have a static disordered configuration similar to the frozen state of the spin ice materials. We demonstrate that this athermal system has an effective extensive degeneracy and we introduce a formalism that can predict the populations of local states in this ice-like system with no adjustable parameters.