Observation of transient lattice disorder at the onset of multiferroic ordering in Eu1-xHoxMnO3 by Raman spectroscopy

The orthorhombic rare-earth manganate Eu1-xHoxMnO3 (x >= 0.15) exhibits multiferroism below T-MF similar to 25 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 degrees domain walls in Eu1-xHoxMnO3, which are associated with the onset of a spontaneous reorientation of the spin cycloids with decreasing temperature.