Ferroelectricity Induced by Acentric Spin-Density Waves inYMn2O5

The commensurate and incommensurate magnetic structures of the magnetoelectric system ${\mathrm{YMn}}_2{\mathrm{O}}_5$, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.