Electric-Dipole Active Two-Magnon Excitation in ab Spiral Spin Phase of a Ferroelectric Magnet Gd0.7Tb0.3MnO3

Abstract

A broad continuum-like spin excitation (1–10 meV) with a peak structure around 2.4 meV has been observed in the ferroelectric a b spiral spin phase of Gd 0.7 Tb 0.3 MnO 3 by using terahertz (THz) time-domain spectroscopy. Based on a complete set of light-polarization measurements, we identify the spin excitation active for the light E vector only along the a -axis, which grows in intensity with lowering temperature even from above the magnetic ordering temperature but disappears upon the transition to the A -type antiferromagnetic phase. Such an electric-dipole active spin excitation as observed at THz frequencies can be ascribed to the two-magnon excitation in terms of the unique polarization selection rule in a variety of the magnetically ordered phases.