Emergent surface multiferroicity

TL;DR

This paper demonstrates that the surface of certain antiferromagnets can exhibit multiferroic properties like magnetoelectric effects and net polarization, even when the bulk material does not, highlighting a novel surface-bulk relationship.

Contribution

It reveals that centrosymmetric antiferromagnets can host surface multiferroicity driven by magnetic octupoles, independent of spin-orbit coupling, supported by first-principles calculations on FeF2.

Findings

Surface exhibits linear magnetoelectric effect.

Surface has a net magnetization and electric dipole moment.

Bulk remains non-polar with no net magnetization.

Abstract

We show that the surface of a centrosymmetric, collinear, compensated antiferromagnet, which hosts bulk ferroically ordered magnetic octupoles, exhibits a linear magnetoelectric effect, a net magnetization, and a net electric dipole moment. Thus, the surface satisfies all the conditions of a multiferroic, in striking contrast to the bulk, which is neither polar nor exhibits any net magnetization or linear magnetoelectric response. Of particular interest is the case of non-relativistic $d$-wave spin split antiferromagnets, in which the bulk magnetic octupoles and consequently the surface multiferroicity exist even without spin-orbit interaction. We illustrate our findings using first-principles calculations, taking FeF$_2$ as an example material. Our work underscores the bulk-boundary correspondence in these unconventional antiferromagnets.