Origin of Multiferroicity in MnWO4

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MnWO4 is regarded as a canonical example of multiferroic materials, where the multiferroic activity is caused by a spin-spiral alignment. We argue that, in reality, MnWO4 has two sources of the spin-spirality, which conflict with each other. One is the Dzyaloshinskii-Moriya (DM) interactions, reflecting the P2/c symmetry of the lattice. The P2/c structure of MnWO4 has an inversion center, that connects two Mn sublattices. Therefore, from the viewpoint of DM interactions, different Mn sublattices are expected to have opposite spin chirality. Another source is competing isotropic exchange interactions, which tend to form a spin-spiral texture with the same chirality in both magnetic sublattices. Thus, there is a conflict between DM and isotropic exchange interactions, which makes these two sublattices inequivalent and, therefore, breaks the inversion symmetry. Our theoretical analysis is based on the low-energy model, derived from the first-principles electronic structure calculations.