We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn2O5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and phys
ical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary points unambiguously at the exchange-striction mechanism as the primary driving force for ferroelectricity.
The commensurate phase of multiferroic HoMn2O5 was studied by X-ray magnetic scattering, both off resonance and in resonant conditions at the Ho-L3 edge. Below 40 K, magnetic ordering at the Ho sites is induced by the main Mn magnetic order parameter
, and its temperature dependence is well accounted for by a simple Curie-Weiss susceptibility model. A lattice distortion of periodicity twice that of the magnetic order is also evidenced. Azimuthal scans confirm the model of the magnetic structure recently refined from neutron diffraction data for both Mn and Ho sites, indicating that the two sublattices interact via magnetic superexchange.
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.
