A comparative Raman study between PrMnO(3), NdMnO(3), TbMnO(3) and DyMnO(3)

In this paper, we present a detailed Raman study of the non-multiferroic compounds PrMnO (3) and NdMnO (3) and the multiferroic compounds TbMnO (3) and DyMnO (3) as a function of temperature and magnetic field. All studied systems show anomalous phonon shifts close to the Néel transition T (N). In PrMnO (3) and NdMnO (3), the frequency softenings are partly attributed to an orbital-spin-phonon coupling whereas in TbMnO (3) and DyMnO (3), the relatively weak frequency shifts are rather attributed to an expansion of the Mn−O bond lengths. On the other hand, the frequencies of TbMnO (3) phonons are shifted as a function of magnetic field, while those of PrMnO (3) remain unaffected. These frequency shifts are interpreted in terms of local oxygen rearrangements under magnetic field that could play an important role in the multiferroicity of TbMnO (3) and DyMnO (3).