Spin–phonon coupling in ordered double perovskites A2CoMnO6 (A=La, Pr, Nd) probed by micro-Raman spectroscopy

Detailed Raman scattering investigations are carried out on ordered double perovskites A2CoMnO6 (A=La, Pr, Nd). Polarized Raman spectroscopy studies showed number of Raman modes concomitant with the monoclinic space group realized in ordered structure. The temperature dependent Raman data showed anomalous softening below the magnetic ordering temperature that is attributed to the spin–phonon coupling. The spin–phonon coupling strength is observed to decrease with decreasing ionic radii of the rare-earth ion. It is also observed that the antistreching Bg mode appearing at ~500 cm−1 is stronger in comparison to the stretching Ag mode appearing at ~650 cm−1. The valance state of the Mn and the Co ions below and above the magnetic ordering temperature is studied by X-ray absorption spectroscopy at the Mn and the Co K edges and it is found to be invariant due to the magnetic order.