Electromagnon resonance in a collinear spin state of the polar antiferromagnet Fe2Mo3O8

Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by a magnetic field or Zn doping. We propose that the observed electromagnon mode involves the electric polarization oscillating within the c plane induced by the collective precession of the spins through the same mechanism as producing the linear magnetoelectric effect.