  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.
