An Effective Model of Magnetoelectricity in Multiferroics $RMn_2O_5$

TL;DR

This paper presents an effective theoretical model explaining the magnetoelectric coupling and phase transitions in multiferroic $RMn_2O_5$, highlighting the interplay of magnetic and ferroelectric orders and predicting electromagnon spectrum features.

Contribution

It introduces a novel model describing the coupling between Ising-type orders and magnetic frustration in $RMn_2O_5$, elucidating phase transitions and excitations.

Findings

Magnetoelectric coupling is between ferroelectric and magnetic orders.

Frustrated magnetic structure causes commensurate-incommensurate transition.

Predicted distinct electromagnon spectrum features in the incommensurate phase.

Abstract

An effective model is developed to explain the phase diagram and the mechanism of magnetoelectric coupling in multiferroics, $RMn_2O_5$. We show that the nature of magnetoelectric coupling in $RMn_2O_5$ is a coupling between two Ising-type orders, namely, the ferroelectric order in the b axis, and the coupled magnetic order between two frustrated antiferromagnetic chains. The frustrated magnetic structure drives the system to a commensurate-incommensurate phase transition, which can be understood as a competition between a collinear or col-plane order stemming from the `order by disorder' mechanism and a chiral symmetry order. The low energy excitation is calculated and the effect of the external magnetic field is analyzed. Distinct features in the electromagnon spectrums in the incommensurate phase are predicted.