Coupled Spin-Phonon Excitations in Helical Multiferroics

TL;DR

This paper investigates how spin and phonon interactions influence magnetoelectric excitations in multiferroic RMnO3, revealing novel low-energy phonon modes and polarization effects due to exchange-striction and spin fluctuations.

Contribution

It demonstrates the impact of exchange-striction and Dzyaloshinskii-Moriya interactions on spin-phonon dynamics and polarization in helical multiferroics, highlighting new quantum fluctuation phenomena.

Findings

Low-lying phonon modes perpendicular and parallel to polarization P.

Reversal of local polarization and spin chirality by spin fluctuations.

Finite differential neutron scattering intensity from cycloidal magnets.

Abstract

Both the Dzyaloshiskii-Moriya interaction and the exchange-striction are shown to affect dynamically the magnetoelectric excitations in the perovskite multiferroic RMnO3. The exchange-striction results in a biquadratic interaction between the spins and the transverse phonons, giving rise to quantum fluctuations of the ferroelectric polarization P. This leads to low-lying phonon modes that are perpendicular to P and to the helical spins at small wave vector but are parallel to P at a wave vector close to the magnetic modulation vector. For spin-1/2 helimagnet, the local polarization can be completely reversed by the spin fluctuation, and so does the direction of the on-site spin chirality, which allows for a finite differential scattering intensity of polarized neutrons from a cycloidal magnet.