  The non-centrosymmetric magnetic structure of ErMn$_2$O$_5$ has been shown to
be very similar to that of HoMn$_2$O$_5$ (Vecchini {\it et al.}, 2008 Phys.
Rev. B {\bf 77} 134434). The magnetic modulation at 25 K has propagation vector
$\vec k$=(1/2,0,1/4) and the symmetry imposes very few constraints on the
magnetic configurations allowed. Only by combining the results of bulk
magnetisation measurements, powder and single crystal neutron diffraction and
spherical neutron polarization analysis was it possible to distinguish clearly
between different models. The susceptibility measurements show that the erbium
magnetic moments are aligned parallel to the c-axis indicating strong
single-ion anisotropy. Spherical neutron polarimetry demonstrates the presence
of two unequally populated chirality domains in ErMn$_2$O$_5$ single crystals.
X-ray diffraction measurements on an ErMn$_2$O$_5$ powder using synchrotron
radiation show that the buckling angles of the Mn-O-Mn bond change below the
transition to the ferroelectric phase.
