Lattice dynamics and spin-phonon coupling inCaMn2O4: A Raman study

Abstract

We present polarized Raman spectra of ${\mathrm{CaMn}}_{2}{\mathrm{O}}_{4}$ (marokite) measured in several exact scattering configurations from oriented single crystals. Most of the 11A${}_{g}$ + 12B${}_{1g}$ + 10B${}_{2g}$ + 9B${}_{3g}$ Raman-allowed modes are identified and assigned to definite atomic motions by comparison to the predictions of ab initio lattice dynamical calculations. The Raman modes at 474 cm${}^{\ensuremath{-}1}$ (${A}_{g}$), 478 cm${}^{\ensuremath{-}1}$ (${B}_{1g}$), and 614 cm${}^{\ensuremath{-}1}$ (${B}_{1g}$) exhibit anomalous softening at temperatures below ${T}_{\mathrm{N}}\ensuremath{\approx}220$ K due to a spin-phonon coupling which depends strongly on the phonon eigenvectors. There are no indications for a structural transition in the temperature range between 80 and 850 K. We argue quantitatively that the mechanism of the phonon softening below ${T}_{\mathrm{N}}$ is similar to that in the undoped manganese perovskites.