Canonical magnetic insulators with isotropic magnetoelectric coupling

TL;DR

This paper identifies fundamental structures enabling isotropic magnetoelectric coupling, focusing on a canonical structure realized in substituted pyrochlore compounds with notable orbital magnetoelectric effects.

Contribution

It provides an exhaustive symmetry-based search for simple structures with isotropic magnetoelectric coupling and links one canonical structure to real pyrochlore materials with significant orbital effects.

Findings

30 structures with isotropic magnetoelectric coupling identified

Canonical structure realized in substituted pyrochlore compounds

Substantial Chern-Simons orbital magnetoelectric component found

Abstract

We have performed an exhaustive representation-theory-based search for the simplest structures allowing isotropic magnetoelectric coupling. We find 30 such structures, all sharing a common pattern of atomic displacements in the direction of atomic magnetic moments. We focus on one of these 30 canonical structures and find that it is generically realized in a class of fractionally substituted pyrochlore compounds with an all-in-all-out magnetic order. Furthermore, we find that these substituted pyrochlore compounds have a substantial Chern-Simons orbital magnetoelectric component (\theta=0.1--0.2). While this component is also formally present in strong Z_2 topological insulators (\theta=\pi), its effects are observable there only if time-reversal symmetry is broken at the surface.