Subpicometer-scale atomic displacements and magnetic properties in the oxygen-isotope substituted multiferroic DyMnO3

We have investigated $\mathrm{DyMn}\phantom{\rule{0.16em}{0ex}}^{16}\mathrm{O}_{3}$ and its isotopically substituted counterpart $\mathrm{DyMn}\phantom{\rule{0.16em}{0ex}}^{18}\mathrm{O}_{3}$ by neutron powder diffraction, x-ray diffraction, and heat capacity measurements to investigate the mechanism leading to its magnetically induced electric polarization. $^{18}\mathrm{O}$ isotope substitution does not influence the magnetic ordering temperature of the Mn ions ${T}_{\mathrm{N},\mathrm{Mn}}$ or the multiferroic ordering temperature ${T}_{\mathrm{l}}$ coinciding with the onset of the spin spiral phase; however, it does reduce the ordering temperature of Dy into its incommensurate magnetic state ${T}_{\mathrm{N},\mathrm{Dy}}$ from 7.0(1) K to 5.9(1) K. The temperature dependence of the magnetic propagation vector, ${\mathbf{q}}_{\mathrm{IC}}$, changes with $^{18}\mathrm{O}$ substitution, while ${T}_{\mathrm{l}}$ remains almost constant, independent of ${\mathbf{q}}_{\mathrm{IC}}$. Pronounced changes in the lattice parameters occur at the various phase transitions. Furthermore, distinct subpicometer-scale distortions of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra and displacements of the Dy ions are observed below the ferroelectric phase transition at ${T}_{\mathrm{l}}$ in both samples, pointing toward the mechanism for electric polarization and its coupling to the orbital degrees of freedom.