Weak antiferromagnetism of J _eff=12 band in bilayer iridate Sr ₃Ir ₂O ₇

The antiferromagnetic structure of Sr ₃Ir ₂O ₇, the bilayer analog of a spin-orbital Mott insulator Sr ₂IrO ₄, 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 Sr ₂IrO ₄. The magnitude of moment, however, was found to be smaller than that of Sr ₂IrO ₄ by a factor of 5 to 6, implying that Sr ₃Ir ₂O ₇ 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 Sr ₂IrO ₄, originating from the strong bilayer antiferromagnetic coupling.