  Orthorhombic HoMnO3 is a multiferroic in which Mn antiferromagnetic order
induces ferroelectricity. A second transition occurs within the multiferroic
phase, in which a strong enhancement of the ferroelectric polarization occurs
concomitantly to antiferromagnetic ordering of Ho 4f magnetic moments. Using
the element selectivity of resonant X-ray diffraction, we study the magnetic
order of the Mn 3d and Ho 4f moments. We explicitly show that the Mn magnetic
order is affected by the Ho 4f magnetic ordering transition. Based on the
azimuthal dependence of the (0 q 0) and (0 1-q 0) magnetic reflections, we
suggest that the Ho 4f order is similar to that previously observed for Tb 4f
in TbMnO3, which resembles an ac-cycloid. This is unlike the Mn order, which
has already been shown to be different for the two materials. Using
non-resonant diffraction, we show that the magnetically-induced ferroelectric
lattice distortion is unaffected by the Ho ordering, suggesting a mechanism
through which the Ho order affects polarization without affecting the lattice
in the same manner as the Mn order.

Strongly Correlated Electrons