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

Institut für Physik, Martin-Luther Universität Halle-Wittenberg - Heinrich-Damerow-Str.4 06120 Halle (Saale), Germany, EU

Received: 12 November 2008
Accepted: 13 February 2009

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 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 and to the helical spins at small wave vector but then turn parallel to at a wave vector close to the magnetic modulation vector. For helical magnetic structure, the spin chirality which determines the direction of , 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.