# Magnetically induced Ferroelectricity in Bi$_2$CuO$_4$

## TL;DR

This study reveals that Bi$_2$CuO$_4$ exhibits magnetically induced ferroelectricity due to spin ordering, with experimental evidence from neutron scattering and dielectric measurements showing inversion symmetry breaking and magnetoelectric effects.

## Contribution

First detailed experimental investigation demonstrating magnetically induced ferroelectricity in Bi$_2$CuO$_4$, linking magnetic order to electric polarization and symmetry breaking.

## Key findings

- Cu spins are aligned parallel to the $ab$ plane.
- Electric polarization appears below antiferromagnetic transition under magnetic field.
- Magnetoelectric effect explains spin-induced ferroelectricity in the material.

## Abstract

The tetragonal copper oxide Bi$_2$CuO$_4$ has an unusual crystal structure with a three-dimensional network of well separated CuO$_4$ plaquettes. This material was recently predicted to host electronic excitations with an unconventional spectrum and the spin structure of its magnetically ordered state appearing at T$_N$ $\sim$43 K remains controversial. Here we present the results of detailed studies of specific heat, magnetic and dielectric properties of Bi$_2$CuO$_4$ single crystals grown by the floating zone technique, combined with the polarized neutron scattering and high-resolution X-ray measurements. Our polarized neutron scattering data show Cu spins are parallel to the $ab$ plane. Below the onset of the long range antiferromagnetic ordering we observe an electric polarization induced by an applied magnetic field, which indicates inversion symmetry breaking by the ordered state of Cu spins. For the magnetic field applied perpendicular to the tetragonal axis, the spin-induced ferroelectricity is explained in terms of the linear magnetoelectric effect that occurs in a metastable magnetic state. A relatively small electric polarization induced by the field parallel to the tetragonal axis may indicate a more complex magnetic ordering in Bi$_2$CuO$_4$.