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\nbeen investigated using element specific x-ray resonant and non-resonant\nmagnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are\naligned along the c and a directions, respectively according to the same\nmagnetic representation $\\Gamma_{5}$ and the same propagation vector (1, 0,\n0.5). Below 5 K, magnetic order of both sublattices change to a different\nmagnetic structure and the Sm moments reorder in a magnetic unit cell equal to\nthe chemical unit cell. Modeling of the temperature dependence for the Sm\nsublattice as well as a change in the magnetic structure below 5 K provide a\nclear evidence of a surprisingly strong coupling between the two sublattices,\nand indicate the need to include anisotropic exchange interactions in models of\nSmFeAsO and related compounds.\n