Entropy-driven order in an array of nanomagnets

TL;DR

This paper demonstrates that artificial spin ice arrays can exhibit entropy-driven magnetic order, where increased entropy in one subset of moments induces order in another, expanding understanding of entropy's role in magnetic systems.

Contribution

It introduces a novel entropy-driven ordering mechanism in tetris artificial spin ice, with designable binary degrees of freedom and measurable entropy effects.

Findings

Two-dimensional magnetic order induced by entropy in one subset of moments.

Discrete binary degrees of freedom are directly observable and precisely calculable.

System's interactions and entropy are well-defined, enabling controlled studies.

Abstract

Long-range ordering is typically associated with a decrease in entropy. Yet, it can also be driven by increasing entropy in certain special cases. We demonstrate that artificial spin ice arrays of single-domain nanomagnets can be designed to produce entropy-driven order. We focus on the tetris artificial spin ice structure, a highly frustrated array geometry with a zero-point Pauli entropy, which is formed by selectively creating regular vacancies on the canonical square ice lattice. We probe thermally active tetris artificial spin ice both experimentally and through simulations, measuring the magnetic moments of the individual nanomagnets. We find two-dimensional magnetic ordering in one subset of these moments, which we demonstrate to be induced by disorder (i.e., increased entropy) in another subset of the moments. In contrast with other entropy-driven systems, the discrete degrees of freedom in tetris artificial spin ice are binary and are both designable and directly observable at the microscale, and the entropy of the system is precisely calculable in simulations. This example, in which the system's interactions and ground state entropy are well-defined, expands the experimental landscape for the study of entropy-driven ordering.