Observation of transient lattice disorder at the onset of multiferroic ordering in Eu 1 − x Ho x MnO 3 by Raman spectroscopy

The orthorhombic rare-earth manganate ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ho}}_{x}{\mathrm{MnO}}_{3}$ ($x\ensuremath{\ge}0.15$) exhibits multiferroism below ${T}_{\text{MF}}$ $\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with a static electric polarization caused by a cycloidal spin order. We observe transient disorder effects in the crystal lattice in a very narrow temperature interval at the onset of the multiferroic ordering. This disorder manifests itself in a distinct transitory violation of the symmetry selection rules for lattice vibration modes in Raman spectroscopy: polarized Raman spectra show a clear increase of the intensity of symmetry-forbidden phonon modes. Possible scenarios of transient disorder in a narrow temperature range are discussed, with special reference to the development of 90\ifmmode^\circ\else\textdegree\fi{} domain walls in ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ho}}_{x}{\mathrm{MnO}}_{3}$, which are associated with the onset of a spontaneous reorientation of the spin cycloids with decreasing temperature.