  We employ neutron spherical polarimetry to determine the nature and
population of the coexisting antiferromagnetic domains in multiferroic YMn2O5.
By applying an electric field, we prove that reversing the electrical
polarization results in the population inversion of two types of in-plane
domains, related to each other by inversion. Our results are completely
consistent with the exchange striction mechanism of ferroelectricity, and
support a unified model where cycloidal ordering is induced by coupling to the
main magnetic order parameter.
