Collinear to Spiral Spin Transformation without Changing the Modulation Wavelength upon Ferroelectric Transition in Tb_1-xDy_xMnO₃

Lattice modulation and magnetic structures in magnetoelectric compounds Tb_1-xDy_xMnO₃ have been studied around the ferroelectric (FE) Curie temperature T_C by x-ray and neutron diffraction. Temperature-independent modulation vectors through T_C are observed for the compounds with 0.50≤x≤0.68. This indicates that ferroelectricity with a polarization (P) along the c axis in the RMnO₃ series cannot be ascribed to such an incommensurate-commensurate transition of an antiferromagnetic order as was previously anticipated. A neutron diffraction study with x=0.59 shows that the FE transition is accompanied by the transformation of the Mn-spin alignment from sinusoidal (collinear) antiferromagnetism into a transverse-spiral structure. The observed spiral structure below T_C is expected to produce P along the c axis with the “inverse” Dzialoshinski-Moriya interaction, which is consistent with observation.