Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3

Abstract

Terahertz and far-infrared electric and magnetic responses of hexagonal piezomagnetic YMnO${}_{3}$ single crystals are investigated. Antiferromagnetic resonance is observed in the spectra of magnetic permeability ${\ensuremath{\mu}}_{a}$ [H$\phantom{\rule{0.16em}{0ex}}(\ensuremath{\omega})$ oriented within the hexagonal plane] below the N\'eel temperature ${T}_{N}$. This excitation softens from 41 to 32 $\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ upon heating and finally disappears above ${T}_{N}$. An additional weak and heavily-damped excitation is seen in the spectra of complex dielectric permittivity ${\ensuremath{\varepsilon}}_{c}$ within the same frequency range. This excitation contributes to the dielectric spectra in both antiferromagnetic and paramagnetic phases. Its oscillator strength significantly increases upon heating toward room temperature, thus providing evidence of piezomagnetic or higher-order couplings to polar phonons. Other heavily-damped dielectric excitations are detected near 100 $\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ in the paramagnetic phase in both ${\ensuremath{\varepsilon}}_{c}$ and ${\ensuremath{\varepsilon}}_{a}$ spectra, and they exhibit similar temperature behavior. These excitations appearing in the frequency range of magnon branches well below polar phonons could remind electromagnons, however their temperature dependence is quite different. We have used density functional theory for calculating phonon dispersion branches in the whole Brillouin zone. A detailed analysis of these results and of previously published magnon dispersion branches brought us to the conclusion that the observed absorption bands stem from phonon-phonon and phonon-paramagnon differential absorption processes. The latter is enabled by strong short-range in-plane spin correlations in the paramagnetic phase.