Electrically driven spin excitation in a ferroelectric magnet DyMnO$_{3}$

In multiferroic manganites, there have been recent experimental and theoretical arguments on the possibility of the presence of the low-lying spin excitation, called $electromagnon$, where the spin excitation electrically becomes active [1]. Here we report on a complete set of low-energy (1--10 meV) electrodynamics of spin excitations for a multiferroic DyMnO$_3 $ with variations of the light polarization in a variety of phases tuned by both temperature (5--250 K) and magnetic field (0--70 kOe) [2]. We identify the pronounced absorption continuum (1--8 meV) with a peak feature around 2 meV, which is electric-dipole active only for the light $E$-vector along the $a$-axis. This absorption band grows in intensity with lowering temperature from the spin-collinear paraelectric phase above the ferroelectric transition, but irrespectively of the direction of the $bc$ or $ab$ spiral spin plane. The possible origin of this electric-dipole active band is argued in terms of the gigantic fluctuations of spins and spin-current. [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [2] N. Kida {\it et al.}, arXiv:0711.2733