Magnetic-field-controlled multitude of spin excitation modes in magnetoelectric Co4Nb2O9 as investigated by magnetoterahertz spectroscopy

Abstract

Gapped spin orders induce electric polarization in multiferroics with strong magnetoelectric (ME) coupling. The sensitivity of terahertz (THz) radiation to the spin gaps and the dielectric medium can uniquely access this technologically relevant ME effect. Here, we implement magneto-THz time-domain spectroscopy to demonstrate the manifestation of spin-wave excitations responsible for the ME coupling in multiferroic compound ${\mathrm{Co}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$. We observed two sharp and a weak spin-gap resonance mode at 0.77, 1.58, and 0.91 THz at 5 K below the antiferromagnetic ordering in the absence of magnetic field. In an applied magnetic field, the strength of the 0.77 THz mode decreases while two field-induced modes, namely, the Goldstone mode and magnon excitation, appear at 0.27 and 1.50 THz, respectively. This THz optical evidence of a rich manifestation of field-controlled magnetic resonances, a mutual transfer of optical spectral weight between the zero-field and field-induced excitations, and associated change of complex refractive index in magnetic fields unravel a unique ME coupling in ${\mathrm{Co}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$.