Partial glass isosymmetry transition in multiferroic hexagonal ErMn O3

Ferroelectric transitions of a hexagonal multiferroic, ErMnO3, are studied by x-ray scattering techniques. An isosymmetry transition, similar to that previously observed for YMnO3, approximately 300 K below the well-known ferroic transition temperature, is investigated. The partially glassy behavior of the isosymmetry transition is identified by the appearance of quasielastic scattering lines in high-energy-resolution scans. The glassy behavior is further supported by the increased interlayer decorrelation of (3×3)R30 ordering below the isosymmetry transition. The transition behavior is considered for possible hidden sluggish modes and two-step phase transitions theoretically predicted for the stacked triangular antiferromagnets. The in-plane azimuthal (orientational) ordering behaviors were also compared to the theoretical predictions. Coherent x-ray speckle measurements show unambiguously that the domain sizes decrease anomalously near both the isosymmetry and ferroic transitions. However, domain boundary fluctuations increase monotonically with an Arrhenius form with an activation energy of 0.54(5) eV through both transitions.