Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO2 layers is due to strong competition among intra- and inter-layer bond-directional pseudo-dipolar interactions of the spin-orbit entangled Jeff=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.
@article{arxiv.1205.4381,
title = {Dimensionality Driven Spin-Flop Transition in Layered Iridates},
author = {J. W. Kim and Y. Choi and Jungho Kim and J. F. Mitchell and G. Jackeli and M. Daghofer and J. van den Brink and G. Khaliullin and B. J. Kim},
journal= {arXiv preprint arXiv:1205.4381},
year = {2012}
}