Terahertz time-domain spectroscopy of electromagnons in multiferroic perovskite manganites

TL;DR

This paper reviews recent terahertz spectroscopy studies of electromagnons in multiferroic perovskite manganites, identifying their optical features and polarization dependence across various magnetic phases, and discusses their possible origins.

Contribution

It provides a comprehensive overview of electromagnon observations in $R$MnO$_3$ compounds, including polarization dependence and phase evolution, with systematic experimental data and theoretical insights.

Findings

Electromagnons are active only for light polarized along the a-axis.

Electromagnon intensity increases with spiral spin order development.

Electromagnons are independent of the spiral spin plane orientation.

Abstract

Recent spectroscopic studies at terahertz frequencies for a variety of multiferroics endowed with both ferroelectric and magnetic orders have revealed the possible emergence of a new collective excitation, frequently referred to as electromagnon. It is magnetic origin, but becomes active in response to the electric field component of light. Here we give an overview on our recent advance in the terahertz time-domain spectroscopy of electromagnons or electric-dipole active magnetic resonances, focused on perovskite manganites--$R$MnO$_3$ ($R$ denotes rare-earth ions). The respective electric and magnetic contributions to the observed magnetic resonance are firmly identified by the measurements of the light-polarization dependence using a complete set of the crystal orientations. We extract general optical features in a variety of the spin ordered phases, including the $A$-type antiferromagnetic, collinear spin ordered, and ferroelectric $bc$ and $ab$ spiral spin ordered phases, which are realized by tuning the chemical composition of $R$, temperature, and external magnetic field. In addition to the antiferromagnetic resonances of Mn ions driven by the magnetic field component of light, we clarify that the electromagnon appears only for light polarized along the a-axis even in the collinear spin ordered phase and grows in intensity with evolution of the spiral spin order, but independent of the direction of the spiral spin plane ($bc$ or $ab$) or equivalently the direction of the ferroelectric polarization $P_{\rm s}$ ($P_{\rm s}\| c$ or $P_{\rm s}\| a$). A possible origin of the observed magnetic resonances at terahertz frequencies is discussed by comparing the systematic experimental data presented here with theoretical considerations based on Heisenberg model.