Antiferromagnetic order and domains in Sr3Ir2O7 probed by x-ray resonant scattering

We report on a detailed x-ray resonant scattering study of the bilayer iridate compound Sr3Ir2O7 at the Ir L-2 and L-3 edges. Resonant scattering at the Ir L-3 edge has been used to determine that Sr3Ir2O7 is a long-range ordered antiferromagnet below T-N approximate to 230 K with an ordering wave vector q = (1/2, 1/2, 0). The energy resonance at the L-3 edge was found to be a factor of similar to 30 times larger than that at the L-2 edge. This remarkable effect has been seen in the single-layer compound Sr2IrO4 and has been linked to the observation of a J(eff) = 1/2 spin-orbit insulator. Our result shows that despite the modified electronic structure of the bilayer compound, caused by the larger bandwidth, the effect of strong spin-orbit coupling on the resonant magnetic scattering persists. Using the program SARAh, we have determined that the magnetic order consists of two domains with propagation vectors k(1) = (1/2, 1/2, 0) and k(2) = (1/2, -1/2, 0), respectively. A raster measurement of a focused x-ray beam across the surface of the sample yielded images of domains of the order of 100 mu m, with odd and even L components, respectively. Fully relativistic, monoelectronic calculations using the Green's function technique for a muffin-tin potential have been employed to calculate the relative intensities of the L-2,L-3 edge resonances, comparing the effects of including spin-orbit coupling and the Hubbard U term. A large L-3 to L-2 edge intensity ratio (similar to 5) was found for calculations including spin-orbit coupling. Adding the Hubbard U term had no significant effect on the calculated spectra.