  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
magnetic field or Zn-doping. We propose that the observed electromagnon mode
involves the collective precession of the spins with oscillating in-plane
electric polarization through the mechanism of the linear magnetoelectric
effect.
