Spiral spin structure in the commensurate magnetic phase of multiferroic RMn ₂O ₅

Crystal and magnetic structure analyses have been performed for single crystals of multiferroic materials RMn ₂O ₅ (R = Y, Ho, Er) using the neutron diffraction technique. For all the compounds, the magnetic structure in the commensurate magnetic phase, where spontaneous electric polarization occurs, was determined to be a transverse spiral spin structure propagating along the c-axis. The results demonstrate that the spin configuration for Mn ⁴⁺ and Mn ³⁺ ions is essentially the same in all three materials, suggesting that the ferroelectricity of the commensurate magnetic phase originates from the spin configurations of Mn ions. By contrast, the alignment of the induced 4f-moment of Ho ³⁺ ions is quite different from that of Er ³⁺ ions, which might give a rich variety of magnetic field response for magnetic and dielectric properties in the RMn ₂O ₅ system.