Magnetic transitions in CaMn7 O12 Raman observation of spin-phonon couplings

Abstract : The quadruple perovskite calcium manganite (CaMn7O12) is a multiferroic material in which a giant magnetically induced ferroelectric polarization has been reported, making it potentially very interesting for magnetoelectric applications. Here we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2≃50K. In particular, we show that the mode at 432 cm-1 associated with Mn(B)O6 (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the (a,b) plane. At low energies, two large low energy excitations are observed at 25 and 47 cm-1. The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electromagnetoactive modes. © 2019 American Physical Society.