Far-infrared optical study of electromagnons and their coupling to optical phonons inEu1−xYxMnO3(x=0.1, 0.2, 0.3, 0.4, and 0.45)

Abstract

A systematic far-infrared optical study has been performed on multiferroic oxides ${\text{Eu}}_{1\ensuremath{-}x}{\text{Y}}_{x}{\text{MnO}}_{3}$ ($x=0.1$, 0.2, 0.3, 0.4, and 0.45) in which versatile characteristics of magnetic orders including the $ab$- and $bc$-plane spiral, $A$-type antiferromagnetic, and collinear sinusoidal states show up, depending on Y content $(x)$ and temperature. We have observed evolution of various absorption bands below $100\text{ }{\text{cm}}^{\ensuremath{-}1}$, which critically depends on the nature of magnetic order and hence can be assigned to magnetic excitations. The spiral-spin structures yield the most intense absorption composed of two pronounced peak structures around 20 and $65\text{ }{\text{cm}}^{\ensuremath{-}1}$, while the spectra for the collinear sinusoidal spin structure exhibit a plateaulike shape below $80\text{ }{\text{cm}}^{\ensuremath{-}1}$. In contrast, the $A$-type antiferromagnetic phase shows no electrically active spin excitation. A quantitative analysis shows their strong mutual coupling in the spiral-spin phases as manifested by the transfer of the spectral weight between the electrically active spin excitations and the lowest optical-phonon mode.