Phenomenological treatment of the giant magnetoelectric effect in some ferroelectromagnets

It is shown that the colossal changes of the dielectric constant and electric polarization observed in DyMn2O5 and TbMnO3 in magnetic fields of the order of a few tesla can occur in ferroelectromagnets in which the temperatures of the ferroelectric and magnetic transitions are close and the values of the electric polarization are small. The appearance of electric polarization in the commensurate magnetic phase of TbMnO3 and the change of its orientation in a magnetic field are interpreted as being the result of a renormalization of the ferroelectric energy by the magnetoelectric interaction. The existence of a weak ferromagnetic moment along the c axis in the commensurate phase of TbMnO3 is predicted. (c) 2005 American Institute of Physics.