  The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have
been investigated using element specific x-ray resonant and non-resonant
magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are
aligned along the c and a directions, respectively according to the same
magnetic representation $\Gamma_{5}$ and the same propagation vector (1, 0,
0.5). Below 5 K, magnetic order of both sublattices change to a different
magnetic structure and the Sm moments reorder in a magnetic unit cell equal to
the chemical unit cell. Modeling of the temperature dependence for the Sm
sublattice as well as a change in the magnetic structure below 5 K provide a
clear evidence of a surprisingly strong coupling between the two sublattices,
and indicate the need to include anisotropic exchange interactions in models of
SmFeAsO and related compounds.
