Dynamical magneto-electric coupling in helical magnets

TL;DR

This paper theoretically investigates the collective mode dynamics in helical magnets coupled to electric polarization, revealing hybridized spin-electrical modes and proposing experimental detection methods for dynamical magneto-electric effects.

Contribution

It uncovers the nature of soft modes in helical magnets, showing they are hybridized spin waves and electric polarization, differing from traditional phonon-based expectations.

Findings

Soft modes are hybridized spin waves and electric polarization, not phonons.

Identification of low-lying modes: phason and helical plane rotation.

Proposes a new experiment to detect dynamical magneto-electric coupling.

Abstract

Collective mode dynamics of the helical magnets coupled to electric polarization via spin-orbit interaction is studied theoretically. The soft modes associated with the ferroelectricity are not the transverse optical phonons, as expected from the Lyddane-Sachs-Teller relation, but are the spin waves hybridized with the electric polarization. This leads to the Drude-like dielectric function $\epsilon(\omega)$ in the limit of zero magnetic anisotropy. There are two more low-lying modes; phason of the spiral and rotation of helical plane along the polarization axis. The roles of these soft modes in the neutron scattering and antiferromagnetic resonance are revealed, and a novel experiment to detect the dynamical magneto-electric coupling is proposed.