Crystal and magnetic structure of ferromagnetic superconducting RuSr2GdCu2O8

The crystal and magnetic structure of a sample of (RuSr2GdCu2O8)-Gd-160 whi ch orders magnetically at 133 K and exhibits a superconducting transition a t 35 K has been investigated by neutron powder diffraction. RuO6 octahedra, which substitute at the chain copper site in a YBa2Cu3O7-type structure, a re 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 r oot 2a(p) X c supercell. The RuO6 rotations are partially ordered to form c ompeting domains that differ in the sense of rotation. We speculate that th e degree of ordering depends on annealing conditions. Surprisingly, the onl y 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, a nd the buckling angle of the CuO2 planes. Magnetic scattering consistent wi th the previously proposed ferromagnetic ordering of Ru moments perpendicul ar to the c axis is not observed. We do not rule out ordering of Ru moments parallel to the c axis or itinerant ferromagnetism.