  We have studied the crystal and magnetic structures of the magnetoelectric
materials RMn2O5 (R = Tb, Ho, Dy) using neutron diffraction as a function of
temperature. All three materials display incommensurate antiferromagnetic
ordering below 40 K, becoming commensurate on further cooling. For R = Tb, Ho,
a commensurate-incommensurate transition takes place at low temperatures. The
commensurate magnetic structures have been solved and are discussed in terms of
competing exchange interactions. The spin configuration within the ab plane is
essentially the same for each system, and the radius of R determines the sign
of the magnetic exchange between adjacent planes. The inherent magnetic
frustration in these materials is lifted by a small lattice distortion,
primarily involving shifts of the Mn3+ cations and giving rise to a canted
antiferroelectric phase.
