On the magnetic structure of Sr3Ir2O7: an x-ray resonant scattering   study
  This report presents azimuthal dependent and polarisation dependent x-ray resonant magnetic scattering at the Ir L3 edge for the bilayered iridate compound, Sr3Ir2O7. Two magnetic wave vectors, k1=(1/2,1/2,0) and k2=(1/2,-1/2,0), result in domains of two symmetry-related G-type antiferromagnetic structures, noted A and B, respectively. These domains are approximately 0.02 mm^2 and are independent of the thermal history. An understanding of this key aspect of the magnetism is necessary for an overall picture of the magnetic behaviour in this compound. Azimuthal and polarisation dependence of magnetic reflections, relating to both magnetic wave vectors, show that the Ir magnetic moments in the bilayer compound are oriented along the c axis. This contrasts with single layer Sr2IrO4 where the moments are confined to the ab plane. 
S. Boseggia, R Springell, H C Walker, A T Boothroyd, D Prabhakaran, S   P Collins and D F McMorrow
cond-mat.str-el