Neutron diffraction study of magnetic ordering associated with ferroelectricity in TmMn2O5

We have studied magnetic orderings of a ferroelectric TmMn2O5 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.