Electrically driven spin excitation in the ferroelectric magnet DyMnO3

Temperature (5-250 K) and magnetic-field (0-70 kOe) variations of the low-energy (1-10 meV) electrodynamics of spin excitations have been investigated for a complete set of light-polarization configurations for a ferroelectric magnet DyMnO3 by using terahertz time-domain spectroscopy. 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 is independent of the orientation of spiral spin plane (bc or ab), as shown on the original P-s (ferroelectric polarization) parallel to c phase as well as the magnetic-field induced P-s parallel to a phase. The possible origin of this electric-dipole active band is argued in terms of the large fluctuations of spins and spin current.