Fig. 1: Terahertz spin excitations for multiferroic EYMO with cycloidal spin spiral.

a Spontaneous P driven by cycloidal spin spiral. Opposite helicities of spin spiral with opposite P directions are illustrated. b Optical conductivity spectra of EYMO³⁰; E||a for above 3 THz and the electromagnon (red), and E||c, H||a for the antiferromagnetic resonance (AFMR, blue) and the lower-lying electromagnon (dashed line) estimated from the former study¹⁸. (See Supplementary Fig. 2 for the derivation of terahertz spectra.) Power spectra of excitation terahertz and near-infrared pulses (right panel). c Time evolution of electric field (left axis) and magnetic field (right axis) of terahertz pulse. The power spectrum of (c) is plotted in (b). d Spin dynamics of electromagnon with \({E}_{{THz}}{||a}\). The neighboring spin oscillations are in opposite phase, and their rotation axis of spins is perpendicular to the spin spiral plane (upward arrow). e Spin dynamics of AFMR with \({H}_{{THz}}{||a}\). The rotation axis of spin is parallel to the a axis. (see Supplementary Fig. 3 for more detail of the lower-lying electromagnon).