Theoretical Study of Magnetoelectric Effects in Honeycomb Antiferromagnet Co4Nb2O9

TL;DR

This theoretical study investigates the magnetoelectric effects in honeycomb antiferromagnet Co4Nb2O9, identifying the spin-dependent electric dipole as the origin of observed polarization rotation phenomena.

Contribution

It provides a symmetry-based theoretical analysis distinguishing between two types of spin-dependent electric dipoles, confirming the latter as the cause of experimental observations.

Findings

Type-II spin-dependent electric dipole explains the polarization rotation.

Symmetry analysis supports the electric dipole origin of the magnetoelectric effect.

General description of electric polarization in honeycomb lattices with C3 symmetry.

Abstract

The honeycomb antiferromagnet Co4Nb2O9 is known to exhibit an interesting magnetoelectric effect that the electric polarization rotates at the twice speed in the opposite direction relative to the rotation of the external magnetic field applied in the basal ab-plane. The spin-dependent electric dipole can be an origin of the magnetoelectric effect. It is described by the product of spin operators at different sites (type-I theory) or at the same site (type-II theory). We examine the electric polarization for the two cases on the basis of the symmetry analysis of the crystal structure of Co4Nb2O9, and conclude that the latter is the origin of the observed result. This paper also gives a general description of the field-induced electric polarization on honeycomb lattices with the C3 point group symmetry on the basis of the type-I theory.