Electric Polarization Induced by a Proper Helical Magnetic Ordering in a Delafossite Multiferroic CuFe1-xAlxO2

TL;DR

This study demonstrates that proper helical magnetic ordering in CuFe1-xAlxO2 induces electric polarization, revealing a new class of magnetic ferroelectrics with a direct link between spin helicity and ferroelectricity.

Contribution

It shows that in CuFe1-xAlxO2, electric polarization is directly caused by proper helical magnetic order, differing from existing models like Katsura-Nagaosa-Balatsky.

Findings

Spin helicity correlates with electric polarization.

Proper helical magnetic order induces ferroelectricity.

CuFe1-xAlxO2 is a new class of magnetic ferroelectric materials.

Abstract

Multiferroic CuFe1-xAlxO2 (x=0.02) exhibits a ferroelectric ordering accompanied by a proper helical magnetic ordering below T=7K under zero magnetic field. By polarized neutron diffraction and pyroelectric measurements, we have revealed a one-to-one correspondence between the spin helicity and the direction of the spontaneous electric polarization. This result indicates that the spin helicity of the proper helical magnetic ordering is essential for the ferroelectricity in CuFe1-xAlxO2. The induction of the electric polarization by the proper helical magnetic ordering is, however, cannot be explained by the Katsura-Nagaosa-Balatsky model, which successfully explains the ferroelectricity in the recently explored ferroelectric helimagnets, such as TbMnO3. We thus conclude that CuFe1-xAlxO2 is a new class of magnetic ferroelectrics.