Both electric- and magnetic-dipole active spin excitations, i.e., electromagnons, which mediate the dynamical magnetoelectric effect, have been investigated for a multiferroic perovskite of manganite by optical spectroscopy at terahertz frequencies. Upon the magnetoelectric resonance at 1 meV in the multiferroic phase with the $bc$-plane spin cycloidal order, a gigantic dynamical magnetoelectric effect has been observed as a nonreciprocal directional dichroism or birefringence. The light $k$-vector-dependent difference ($\Delta \kappa ={\kappa }_{+}-{\kappa }_{-}$) of the extinction coefficient (${\kappa }_{\pm }$) is as large as $\Delta \kappa \sim 1$ or $2\Delta \kappa /({\kappa }_{+}+{\kappa }_{-})\sim 0.7$ at the lowest-lying electromagnon energy. We clarified the mutual coupling of the $E^{\omega }\parallel a$-polarized electromagnons of the different origins, leading to the enhancement of the magnetoelectric resonance.

• Received 30 March 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.037204