Strong coupling of Sm and Fe magnetism in SmFeAsO as revealed by   magnetic x-ray scattering
  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. 
S. Nandi, Y. Su, Y. Xiao, S. Price, X. F. Wang, X. H. Chen, J.   Herrero-Mart\'in, C. Mazzoli, H. C. Walker, L. Paolasini, S...
cond-mat.str-el cond-mat.supr-con