Terahertz electromagnetic excitations in the multiferroic ${\mathrm{TbMnO}}_3$ single crystals are investigated across the magnetic field induced rotation of the magnetic spin cycloid. In addition to the electromagnon along the $a$ axis, the detailed polarization analysis of the experimental spectra suggests the existence of an electroactive excitation for $ac$ electric fields of the electromagnetic wave along the crystallographic $c$ axis. This excitation is possibly the electroactive eigenmode of the spin-cycloid in ${\mathrm{TbMnO}}_3$, which has been predicted within the inverse Dzyaloshinskii-Moriya mechanism of magnetoelectric coupling.

• Received 28 October 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.097202