Abstract-Terahertz magnetoelectric resonance enhanced by mutual coupling of electromagnons

Y. Takahashi, Y. Yamasaki, and Y. Tokura
http://prl.aps.org/accepted/a307cY05P3d16856702e05a55632c8825b5eb5713
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 the 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 non-reciprocal directional dichroism/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 $E^{\omega}\| a$-polarized electromagnons of the different origins, leading to the enhancement of the magnetoelectric resonance.