Electromagnons in MultiferroicYMn2O5andTbMn2O5

Based on temperature dependent far infrared transmission spectra of ${\mathrm{YMn}}_2{\mathrm{O}}_5$ and ${\mathrm{TbMn}}_2{\mathrm{O}}_5$ single crystals, we report the observation of electric dipole-active magnetic excitations, or electromagnons, in these multiferroics. Electromagnons are found to be directly responsible for the steplike anomaly of the static dielectric constant at the commensurate—incommensurate magnetic transition and are the origin of the colossal magneto-dielectric effect reported in these multiferroics.