  Resonant and non-resonant X-ray scattering studies on HoFe3(BO3)O4 reveal
competing magnetic ordering of Ho and Fe moments. Temperature and X-ray
polarization dependent measurements employed at the Ho L3 edge directly reveal
a spiral spin order of the induced Ho moments in the ab-plane propagating along
the c-axis, a screw-type magnetic structure. At about 22.5 K the Fe spins are
observed to rotate within the basal plane inducing spontaneous electric
polarization, P. Components of P in the basal plane and along the c-axis can be
scaled with the separated magnetic X-ray scattering intensities of the Fe and
Ho magnetic sublattices, respectively.
