Electromagnon excitations in modulated multiferroics

TL;DR

This paper extends the phenomenological theory of ferroelectricity in spiral magnets to include modulated states, highlighting the role of symmetric magnetoelectric coupling in non-uniform polarizations and their experimental implications.

Contribution

It generalizes existing models to describe both uniform and modulated ferroelectric states, emphasizing the significance of symmetric magnetoelectric coupling in complex magnetic systems.

Findings

Polarization correlation functions follow soft magnetic behavior.

Symmetric magnetoelectric coupling is crucial for non-uniform polarization.

Results applicable to various magnetic ordering systems.

Abstract

The phenomenological theory of ferroelectricity in spiral magnets presented in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe consistently states with both uniform and modulated-in-space ferroelectric polarizations. A key point in this description is the symmetric part of the magnetoelectric coupling since, although being irrelevant for the uniform component, it plays an essential role for the non-uniform part of the polarization. We illustrate this importance in generic examples of modulated magnetic systems: longitudinal and transverse spin-density wave states and planar cycloidal phase. We show that even in the cases with no uniform ferroelectricity induced, polarization correlation functions follow to the soft magnetic behavior of the system due to the magnetoelectric effect. Our results can be easily generalized for more complicated types of magnetic ordering, and the applications may concern various natural and artificial systems in condensed matter physics (e.g., magnon properties could be extracted from dynamic dielectric response measurements).