Neutron diffraction study of magnetic ordering associated with ferroelectricity in TmMn₂O₅

We have studied magnetic orderings of a ferroelectric TmMn ₂O₅ by single-crystal neutron diffraction and dielectric measurements. Below T_N1 = 44 K, TmMn₂O₅ shows the sequence of magnetic phase transitions at T = 36.4, 35.1 and 24.9 K, associated with changes in the magnetic propagation vector q = (q_x, 0, q_z). Such magnetic phase transitions are quite similar to those of YMn₂O₅ with non-magnetic Y ion reported previously. In addition, we observed another magnetic phase transition at T = 4.8 K which implies a dramatic reorientation of Mn moments induced by the rare-earth Tm moment. The agreement between the temperatures of the magnetic and ferroelectric phase transitions indicates that the ferroelectric ordering is strongly correlated with the magnetic ordering.