  Precise magnetic structures of RMn2O5, with R= Y, Ho, Bi in the
commensurate/ferroelectric regime, have been determined by single-crystal
neutron diffraction. For each system, the integrated intensities of a large
number of independent magnetic Bragg reflections have been measured, allowing
unconstrained least-squares refinement of the structures. The analysis confirms
the previously reported magnetic configuration in the ab-plane, in particular
the existence of zig-zag antiferromagnetic chains. For the Y and Ho compounds
additional weak magnetic components parallel to the c-axis were detected which
are modulated in phase quadrature with the a-b components. This component is
extremely small in the BiMn2O5 sample, therefore supporting symmetric exchange
as the principal mechanism inducing ferroelectricity. For HoMn2O5, a magnetic
ordering of the Ho moments was observed, which is consistent with a
super-exchange interaction through the oxygens. For all three compounds, the
point symmetry in the magnetically ordered state is m2m, allowing the polar
b-axis found experimentally.
