A systematic far-infrared optical study has been performed on multiferroic oxides ${\text{Eu}}_{1-x}{\text{Y}}_x{\text{MnO}}_3$ ($x=0.1$, 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 $\left(x\right)$ and temperature. We have observed evolution of various absorption bands below $100{\text{cm}}^{-1}$, 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 $65{\text{cm}}^{-1}$, while the spectra for the collinear sinusoidal spin structure exhibit a plateaulike shape below $80{\text{cm}}^{-1}$. 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.

• Received 27 April 2009

DOI:https://doi.org/10.1103/PhysRevB.79.214431