Spin-phonon and magnetoelectric coupling in oxygen-isotope substituted TbMnO3 investigated by Raman scattering

In order to investigate the effects leading to the strong magnetoelectric coupling in the type II multiferroic $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_{3}$ we have studied the thermal properties and temperature dependence of the lattice vibrations of ${\mathrm{TbMn}}^{16}{\mathrm{O}}_{3}$ and its isotopically substituted counterpart ${\mathrm{TbMn}}^{18}{\mathrm{O}}_{3}$. Heat capacity measurements on powder samples revealed no significant change in the ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ magnetic phase transition temperatures, as well as the multiferroic ordering temperature upon isotope substitution, indicating that a change in the dynamical modulation of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedral distortions and rotations altering the Mn-O-Mn bond angles has no influence on the magnetic properties of $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_{3}$. Raman light scattering experiments have been performed on isotopically substituted single crystals to determine the temperature induced changes in phonon energies and linewidths at the sinusoidal and multiferroic phase transitions. A detailed modeling indicates that the spin-phonon coupling can be accounted for the pronounced anomalies in the temperature dependence of the phonon behaviors at the transition to the sinusoidal spin phase at ${T}_{\mathrm{N}}^{\mathrm{Mn}}=41$ K and to the multiferroic spin-spiral phase at ${T}_{\mathrm{FE}}=28$ K. No further effects such as the appearance of the electric polarization or the electromagnon were required to explain the data, especially below the multiferroic phase transition.