Lattice dynamics and spin-phonon coupling in CaMn2O4 : a raman study

Abstract

We present polarized Raman spectra of CaMn2O4 (marokite) measured in several exact scattering configurations from oriented single crystals. Most of the 11Ag + 12B1g + 10B2g + 9B3g 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-ˡ (Ag), 478 cm-ˡ (B1g), and 614 cm-ˡ (B1g) exhibit anomalous softening at temperatures below TN ≈ 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 TN is similar to that in the undoped manganese perovskites.