Independent ferroelectric contributions and rare-earth-induced polarization reversal in multiferroic TbMn2O5

Three independent contributions to the magnetically induced spontaneous polarization of multiferroic TbMn2O5 are uniquely separated by optical second harmonic generation and an analysis in terms of Landau theory. Two of the contributions are related to the Mn3+ and Mn4+ magnetic order and are independent of applied fields mu H-0(x) of up to +/- 7 T. The third contribution is related to the long-range antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application of a magnetic field and mediates the change of sign of the spontaneous electric polarization in TbMn2O5. The close relationship between the rare-earth long-range order and the nonlinear optical properties points to isotropic Tb3+-Tb3+ exchange and O2- spin polarization as mechanisms for this rare-earth-induced ferroelectric contribution.