Incommensurate magnetism in PrBa₂Cu₃O_6.92

We report resonant x-ray magnetic scattering and high-resolution neutron-diffraction studies of the Pr site magnetism in high quality single crystals of PrBa2Cu3O6.92. These studies reveal that the Pr sublattice orders at 19 K in a well correlated, long period incommensurate structure with probable wave vector (0.5+/-delta,0.5,0) or (0.5,0.5+/-delta,0) with delta=0.006 r.l.u. The observed x-ray scattering results from dipole transitions, demonstrating the existence of an ordered 5d Pr moment and implying a large 4f moment at the Pr site. A spin reorientation transition to a commensurate antiferromagnetic structure of wave vector (0.5,0.5,0.5) is observed at lower temperatures.