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.
@article{arxiv.0710.2065,
title = {Commensurate magnetic structures ofRMn2O5 (R=Y, Ho, Bi) determined by Single-crystal Neutron Diffraction},
author = {C. Vecchini L. C. Chapon and P. J. Brown and T. Chatterji and S. Park S-W. Cheong and P. G. Radaelli},
journal= {arXiv preprint arXiv:0710.2065},
year = {2007}
}