Pyroelectric effect and possible ferroelectric transition of helimagnetic GdMn2O5, TbMn2O5 and YMn2O5

In a series of papers, we have reported the results of dielectric, magnetic and magnetoelectric studies of RMn(2)O(5) (R = rare earth between Nd and Lu, Y or Bi), proposing the possibility of ferroelectricity in these oxides, but the direct demonstration of ferroelectricity was lacking. In this paper we present the results of pyroelectric current measurements of GdMn2O5, TbMn2O5 and YMn2O5 single crystals between 4.2 and 273 K. These results indicated the existence of spontaneous polarization at low temperatures. The spontaneous polarization was switched by an applied electric field. Its possible direction is along the b axis of an orthorhombic crystal. The supposed temperature dependence of spontaneous polarization was complicated except for the case of GdMn2O5, suggesting a ferrielectric origin of the polarization.