Incommensurate magnetic structures of multiferroic MnWO4 studied within the superspace formalism

The magnetic structures of the incommensurate phases of the prototypic multiferroic MnWO4 are analyzed using superspace symmetry formalism. This approach shows in a very simple way the relation between the magnetic symmetry of the incommensurate phases and any tensorial property, such as ferroelectricity. The magnetic structures have been redetermined by single-crystal neutron diffraction experiments, which revealed fine details of the AF3 and AF2 magnetic structures. The spin modulations of the two manganese ions of the basic unit cell are symmetry-related in the paraelectric incommensurate-magnetic structure (AF3)—they are not forced to be collinear but they must have opposite chirality—while in the multiferroic phase (AF2) they become symmetry independent. It is due to this symmetry break that the electric polarization arises along the b axis.The authors acknowledge financial support from MICINN (Spanish government) under Projects No. MAT2009-09308 and No. CSD2007-00041 (NANOSELECT). We are grateful to the ILL, the French CEA-CRG D23 for the provision of beam time. We thank V. Petricek for his assistance in the use of JANA2006, and his very valuable comments. We thank A. M. Balbashov for providing the MnWO4 single crystal