Complex Spin Structures and Origin of the Magneto-Electric Coupling in   YMn2O5
  By combining neutron four-circle diffraction and polarized neutron diffraction techniques we have determined the complex spin structures of a multiferroic, YMn2O5, that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magneto-electric coupling, our results are consistent with both the spin-current model that requires a magnetic transverse (cycloidal) spiral structure in order to induce a spontaneous electric polarization and the magneto-restriction model. Our results also explain why the electric polarization changes at the commensurate-to-incommensurate phase transition. 
J.-H. Kim, S.-H. Lee, S. I. Park, M. Kenzelmann, J. Schefer, J.-H.   Chung, C. F. Majkrzak, M. Takeda, S. Wakimoto, S. Y. Pa...