Resonant X-ray diffraction study of the strongly spin-orbit-coupled mott insulator CaIrO₃

We performed resonant x-ray diffraction experiments at the L absorption edges for the post-perovskite-type compound CaIrO₃ with a (t _2g)5 electronic configuration. By observing the magnetic signals, we could clearly see that the magnetic structure was a striped ordering with an antiferromagnetic moment along the c axis and that the wave function of a t _2g hole is strongly spin-orbit entangled, the J_eff=1/2 state. The observed spin arrangement is consistent with theoretical work predicting a unique superexchange interaction in the J_eff=1/2 state and points to the universal importance of the spin-orbit coupling in Ir oxides, independent of the octahedral connectivity and lattice topology. We also propose that nonmagnetic resonant scattering is a powerful tool for unraveling an orbital state even in a metallic iridate.