  We have determined the magnetic structure of the low-temperature
incommensurate phase of multiferroic YMn2O5 using single-crystal neutron
diffraction. By employing corepresentation analysis, we have ensured full
compliance with both symmetry and physical constraints, so that the electrical
polarization must lie along the b axis, as observed. The evolution of the spin
components and propagation through the commensurate-incommensurate phase
boundary points unambiguously at the exchange-striction mechanism as the
primary driving force for ferroelectricity.
