Weak antiferromagnetism of J_eff=1/2 band in bilayer iridate Sr3Ir2O7
  The antiferromagnetic structure of Sr3Ir2O7, the bilayer analogue of a spin-orbital Mott insulator Sr2IrO4, was revealed by resonant magnetic x-ray diffraction. Contrasting intensities of the magnetic diffraction at the Ir LIII and LII edges show a Jeff=1/2 character of the magnetic moment as is argued in Sr2IrO4. The magnitude of moment, however, was found to be smaller than that of Sr2IrO4 by a factor of 5-6, implying that Sr3Ir2O7 is no longer a Mott insulator but a weak antiferromagnet. An evident change of the temperature dependence of the resistivity at TN, from almost temperature-independent resistivity to insulating, strongly suggests that the emergent weak magnetism controls the charge gap. The magnetic structure was found to be an out-of-plane collinear antiferromagnetic ordering in contrast to the inplane canted antiferromagnetism in Sr2IrO4, originating from the strong bilayer antiferromagnetic coupling. 
Shigeki Fujiyama, K. Ohashi, H. Ohsumi, K. Sugimoto, T. Takayama, T.   Komesu, M. Takata, T. Arima, and H. Takagi
cond-mat.str-el