Antiferromagnetic order and phase transitions in GdS as studied with X-rayresonance-exchange scattering

We report on X-ray resonance exchange and neutron scattering of metallic Gd S. At the L-II and L-III absorption edges of Gd, resonance enhancements of more than two orders of magnitude over the non-resonant magnetic scattering : are observed. Polarisation analysis proves that these enhancements are du e to dipolar transitions from the 2p to the 5d states. The branching ratio between tile L-II and L-III edges of 2.5 suggests a polarisation of the 5d electrons in the ground state. The antiferromagnetic order is of type II in the fcc lattice. Single crystal diffraction of hot neutrons suggests that the spin direction lies within the (111) planes with a value for the sublat tice magnetisation of 6.51(3) mu (B). The critical exponent for the sublatt ice magnetisation has a value of beta = 0.38(2) in agrement with a pure Hei senberg model. Above TN, a sharp component persists ill the critical diffus e scattering. Lattice distortions give indications for two additional low-t emperature phase transitions at about 49 K and 32 K. We argue that these tr ansitions are not connected to spin reorientations and discuss the possible influence of fourth-order exchange interactions.