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) ∥c phase as well as the magnetic-field induced P_s∥a phase. The possible origin of this electric-dipole active band is argued in terms of the large fluctuations of spins and spin current.