Local indirect magnetoelectric coupling at twin walls in CaMnO$_3$

TL;DR

This study uses density functional theory to reveal that twin walls in CaMnO$_3$ induce local polar distortions and magnetization, enabling magnetoelectric effects absent in the bulk material.

Contribution

It demonstrates that ferroelastic twin walls in CaMnO$_3$ can host emergent polar and magnetic properties through local symmetry breaking, a novel insight into domain wall functionalities.

Findings

Enhanced out-of-plane magnetic moments at twin walls

Local magnetization confined to the domain wall

Coexistence of polarization and magnetization at twin walls

Abstract

Ferroelastic twin walls in centrosymmetric perovskites can host emergent polar and magnetic properties forbidden in the bulk. We use density functional theory calculations to study the geometry and magnetic properties of ferroelastic domain walls in orthorhombic CaMnO$_3$, which belongs to the most common perovskite space group, $Pnma$. At the wall, the inherent inversion symmetry-breaking induces local polar distortions dependent on the wall geometry, which couple to the magnetic order through the octahedral distortions. Noncollinear calculations reveal enhanced out-of-plane magnetic moments on the Mn atoms and a local, finite magnetization confined to the wall. Strain fields across twin walls thus give rise to coexistence of polarization and magnetization as well as magnetoelectric response that is absent and symmetry-forbidden in bulk CaMnO$_3$. We propose that magnetoelectric coupling and coexisting polarization and magnetization can emerge at twin walls in bulk centrosymmetric antiferromagnets.