Landau theory of topological defects in multiferroic hexagonal manganites

TL;DR

This paper presents a Landau theory-based analysis of topological defects in hexagonal manganites, explaining their complex interactions and emergent magnetic properties resulting from coupled structural, ferroelectric, and magnetic orders.

Contribution

It introduces a theoretical framework for understanding topological defects in multiferroic hexagonal manganites, incorporating parameters from first-principles calculations.

Findings

Explanation of electric polarization flip at structural domain boundaries

Identification of discrete vortex structures in the material

Demonstration of magnetic moments on ferroelectric domain walls

Abstract

Topological defects in ordered states with spontaneously broken symmetry often have unusual physical properties, such as fractional electric charge or a quantised magnetic field-flux, originating from their non-trivial topology. Coupled topological defects in systems with several coexisting orders give rise to unconventional functionalities, such as the electric-field control of magnetisation in multiferroics resulting from the coupling between the ferroelectric and ferromagnetic domain walls. Hexagonal manganites provide an extra degree of freedom: In these materials, both ferroelectricity and magnetism are coupled to an additional, non-ferroelectric structural order parameter. Here we present a theoretical study of topological defects in hexagonal manganites based on Landau theory with parameters determined from first-principles calculations. We explain the observed flip of electric polarisation at the boundaries of structural domains, the origin of the observed discrete vortices, and the clamping between ferroelectric and antiferromagnetic domain walls. We show that structural vortices induce magnetic ones and that, consistent with a recent experimental report, ferroelectric domain walls can carry a magnetic moment.