A Group-theoretical Approach to Enumerating Magnetoelectric and Multiferroic Couplings in Perovskites

TL;DR

This paper employs group theory to systematically identify and classify magnetoelectric couplings in perovskites, providing a framework to guide the design of novel multiferroic materials with specific magnetic and electric properties.

Contribution

It introduces a group-theoretical method to enumerate possible magnetoelectric couplings in perovskites, reducing the search space for new multiferroic materials and proposing schemes for engineering weak ferromagnetism.

Findings

Third order couplings involve A and B-site magnetic order with specific irreps.

Fourth order couplings can involve multiple irreps conserving crystal momentum.

DFT calculations support the proposed coupling schemes.

Abstract

We use a group theoretical approach to enumerate the possible couplings between magnetism and ferroelectric polarisation in the parent $Pm\bar{3}m$ perovskite structure. We show that third order magnetoelectric coupling terms must always involve magnetic ordering at the A and B-site which either transforms both as R-point or both as X-point time odd irreducible representations (irreps). For fourth order couplings we demonstrate that this criterion may be relaxed allowing couplings involving irreps at X, M and R-points which collectively conserve crystal momentum, producing a magnetoelectric effect arising from only B-site magnetic order. In this case, exactly two of the three irreps entering the order parameter must be time-odd irreps and either one or all must be even with respect to inversion symmetry. We are able to show that the time-even irreps in this triad must transform as one of: X$_{1}^{-}$, M$_{3,5}^{-}$ and R$_{5}^{+}$, corresponding to A-site cation order, A-site anti-polar displacements or anion rock-salt ordering. This greatly reduces the search-space for (type-II multiferroic) perovskites. We use similar arguments to demonstrate how weak ferromagnetism may be engineered, and we propose a variety of schemes for coupling this to ferroelectric polarisation. We illustrate our approach with DFT calculations on magnetoelectric couplings, and by considering the literature we suggest which avenues of research are likely to be most promising in the design of novel magnetoelectric materials.