Electromagnon in ferrimagnetic ɛ-Fe2O3 nanograin ceramics

Electromagnons are known from multiferroics as spin waves excited by the electric component of electromagnetic radiation. We report the discovery of an excitation in the far-infrared spectra of ɛ-Fe₂O₃, which we attribute to an electromagnon appearing below 110 K where the ferrimagnetic structure becomes incommensurately modulated. Inelastic neutron scattering shows that the electromagnon energy corresponds to that of a magnon from the Brillouin-zone boundary. Dielectric measurements did not reveal any sign of ferroelectricity in ɛ-Fe₂O₃ down to 10 K, despite its acentric crystal structure. This shows that the activation of an electromagnon requires, in addition to the polar ferrimagnetic structure, a modulation of the magnetic structure. We demonstrate that a combination of inelastic neutron scattering with infrared and/or terahertz (THz) spectroscopies allows detecting electromagnons in ceramics where no crystal-orientation analysis of THz and infrared spectra is possible.