Ferroelectricity Induced by Incommensurate Magnetism

TL;DR

This paper presents a phenomenological model linking incommensurate magnetic order to ferroelectricity in insulating materials, supported by a microscopic spin-phonon interaction explanation, with implications for first-principles calculations.

Contribution

It introduces a new phenomenological model connecting magnetic symmetry to ferroelectricity and formulates a microscopic spin-phonon interaction based on exchange tensor gradients.

Findings

Model explains ferroelectricity in incommensurate magnetic systems.

Relates magnetic symmetry to spontaneous polarization.

Provides a framework for first-principles calculations.

Abstract

Ferroelectricity has been found to occur in several insulating systems, such as TbMnO$_3$ (TMO) and Ni$_3$V$_2$O$_8$ (NVO) which have more than one phase with incommensurately modulated long-range magnetic order. Here we give a phenomenological model which relates the symmetries of the magnetic structure as obtained from neutron diffraction to the development and orientation of a spontaneous ferroelectric moment induced by the magnetic ordering. This model leads directly to the formulation of a microscopic spin-phonon interaction which gives explains the observed phenomena. The results are given in terms of gradients of the exchange tensor with respect to generalized displacements for the specific example of NVO. It is assumed that these gradients will now be the target of first-principles calculations using the LDA or related schemes.