Electromagnon excitation in the field-induced noncollinear ferrimagnetic phase ofBa2Mg2Fe12O22studied by polarized inelastic neutron scattering and terahertz time-domain optical spectroscopy

We have studied magnetic excitations in a field-induced noncollinear commensurate ferrimagnetic phase of ${\mathrm{Ba}}₂{\mathrm{Mg}}₂{\mathrm{Fe}}₁₂{\mathrm{O}}₂₂$ by means of polarized inelastic neutron scattering (PINS) and terahertz (THz) time-domain optical spectroscopy under magnetic field. A previous THz spectroscopy study reported that the field-induced phase exhibits electric-dipole-active excitations with energies of around 5 meV [Kida et al., Phys. Rev. B 83, 064422 (2011)]. In the present PINS measurements, we observed inelastic scattering signals around 5 meV at the zone center in the spin-flip channel. This directly shows that the electric-dipole-active excitations are indeed of magnetic origin, that is, electromagnons. In addition, the present THz spectroscopy confirms that the excitations have oscillating electric polarization parallel to the $c$ axis. In terms of the spin-current model (Katsura-Nagaosa-Balatsky model), the noncollinear magnetic order in the field-induced phase can induce static electric polarization perpendicular to the $c$ axis, but not dynamic electric polarization along the $c$ axis. We suggest that the electromagnon excitations can be explained by applying the magnetostriction model to the out-of-phase oscillations of the magnetic moments, which is deduced from the present experimental results.