Crystal and magnetic structure of ferromagnetic superconducting RuSr2GdCu2O8

The crystal and magnetic structure of a sample of (RuSr2GdCu2O8)-Gd-160 which orders magnetically at 133 K and exhibits a superconducting transition at 35 K has been investigated by neutron powder diffraction. RuO6 octahedra, which substitute at the chain copper site in a YBa2Cu3O7-type structure, are rotated by about 14 degrees around the c axis to accommodate physically reasonable Ru-O bond lengths in the plane. This results in a root 2a(p) X root 2a(p) X c supercell. The RuO6 rotations are partially ordered to form competing domains that differ in the sense of rotation. We speculate that the degree of ordering depends on annealing conditions. Surprisingly, the only structural parameters that respond to the magnetic ordering at 133 K are the Cu-Cu distance, which defines the thickness of the CuO2 double layer, and the buckling angle of the CuO2 planes. Magnetic scattering consistent with the previously proposed ferromagnetic ordering of Ru moments perpendicular to the c axis is not observed. We do not rule out ordering of Ru moments parallel to the c axis or itinerant ferromagnetism.