# Symmetry Analysis of Magnetoelectric Effects in Honeycomb Antiferromagnet Co4Nb2O9

## TL;DR

This paper uses symmetry analysis to explain magnetoelectric effects in honeycomb antiferromagnet Co4Nb2O9, revealing how spin dependence and symmetry constraints influence electric polarization and optical properties.

## Contribution

It provides a detailed symmetry-based explanation of magnetoelectric effects and spin-dependent polarization in Co4Nb2O9, linking experimental observations with theoretical symmetry analysis.

## Key findings

- Electric polarization rotates opposite to magnetic field with twice the speed.
- Symmetry constraints explain the restricted spin-dependence of polarization.
- Discussion of optical properties and dichroism related to quadrupolar excitations.

## Abstract

Magnetoelectric effects in honeycomb antiferromagnet Co4Nb2O9 are investigated on the basis of symmetry analyses of Co ions in trigonal P-3c1 space group. For each Co ion, the possible spin dependence is classified by C3 point-group symmetry. This accounts for the observed main effect that an electric polarization rotates in the opposite direction at the twice speed relative to the rotation of the external magnetic field applied in the ab-plane. Inversion centers and twofold axes in the unit cell restrict the active spin-dependence of the electric polarization, which well explains the observed experimental results. Expected optical properties of quadrupolar excitation and various types of dichroism are also discussed.