Infrared-active optical phonons and magnetic excitations in the hexagonal manganites RMnO3 ( R=Ho , Er, Tm, Yb, and Lu)

Abstract

Optical properties of hexagonal multiferroic oxides $R\mathrm{Mn}{\mathrm{O}}_{3}$, where $R=\mathrm{Ho}$, Er, Tm, Yb, and Lu, have been studied in the far-infrared spectral range between 100 and 2000 cm${}^{\ensuremath{-}1}$ and temperatures between 1.5 and 300 K by means of several experimental techniques: Mueller matrix spectroscopic ellipsometry, rotating analyzer ellipsometry, and optical transmission spectroscopy. Spectra of the optical phonons are described in terms of the temperature dependencies of their frequency, damping, and oscillator strength. For all studies, oxide materials' clear signatures of the spin-phonon interaction have been found below the temperature of the antiferromagnetic phase transition ${T}_{N}$ due to magnetic ordering of ${\mathrm{Mn}}^{3+}$ spins. A decrease of the ionic radius for ${R}^{3+}$ ions between ${\mathrm{Ho}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ in the corresponding $R\mathrm{Mn}{\mathrm{O}}_{3}$ compounds resulted in systematic variation of the frequency for several optical phonons. A magnetic excitation at \ensuremath{\sim}190 cm${}^{\ensuremath{-}1}$ was observed at low temperatures below ${T}_{N}$ and interpreted as resulting from two-magnon absorption.