  By extending our general spin-current model to non-centrosymmetric spin
dimers and performing density functional calculations, we investigate the
causes for the helical magnetic order and the origin of the giant ferroelectric
polarization of CaMn7O12. The giant ferroelectric polarization is proposed to
be caused by the symmetric exchange striction due to the canting of the Mn4+
spin arising from its strong Dzyaloshinskii-Moriya (DM) interaction. Our study
suggests that CaMn7O12 may exhibit a novel magnetoelectric coupling mechanism
in which the magnitude of the polarization is governed by the exchange
striction, but the direction of the polarization by the chirality of the
helical magnetic order.
