Dynamical magnetoelectric effects induced by the Dzyaloshinskii-Moriya interaction in multiferroics

TL;DR

This paper investigates the dynamical coupling between ferroelectricity and magnetism in multiferroics with helical magnetic order, revealing how spin-phonon interactions induce hybrid modes and influence spin chirality and neutron scattering.

Contribution

It introduces a novel understanding of dynamical Dzyaloshinskii-Moriya interactions and their effects on spin-chirality and phonon modes in multiferroic materials.

Findings

Hybridization of spin waves and electric polarization fluctuations creates low-lying phonon modes.

Dynamical Dzyaloshinskii-Moriya interaction couples spin chirality to spin fluctuations.

Finite neutron scattering intensity arises from on-site inversion of spin chirality.

Abstract

We study the dynamical interplay between ferroelectricity and magnetism in a multiferroic with a helical magnetic order. We show that the dynamical exchange-striction induces a biquadratic interaction between the spins and transverse phonons resulting in quantum fluctuations of the spontaneous ferroelectric polarization $\mathbf{P}$ in the ferroelectric phase. The hybridization between the spin wave and the fluctuation of the electric polarization leads to low-lying transverse phonon modes. Those are perpendicular to $\mathbf{P}$ and to the helical spins at small wave vector but then turn parallel to $\mathbf{P}$ at a wave vector close to the magnetic modulation vector. For helical magnetic structure, the spin chirality which determines the direction of $\mathbf{P}$, also possesses a long-range order. Due to the dynamical Dzyaloshiskii-Moriya interaction, the spin-chirality is strongly coupled to the spin fluctuation which implies an on-site inversion of the spin-chirality in the ordered spin-1/2 system and results in a finite scattering intensity of polarized neutrons from a cycloidal helimagnet.