Weak antiferromagnetism of Jeff=1/2 band in bilayer iridate Sr3Ir2O7

The antiferromagnetic structure of Sr3Ir2O7, the bilayer analog of a spin-orbital Mott insulator Sr2IrO4, was revealed by resonant magnetic x-ray diffraction. Contrasting intensities of the magnetic diffraction at the Ir L-III and L-II edges show a J(eff) = 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 to 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 T-N, 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 in-plane canted antiferromagnetism in Sr2IrO4, originating from the strong bilayer antiferromagnetic coupling.