Covalency and spin-orbit coupling driven magnetism in the double-perovskite iridates Sr2MIrO6 (M = Ca, Mg)

We have investigated the effect of spin-orbit coupling (SOC) and strong covalency on the magnetism of double-perovskite iridates (Sr2MIrO6, M= Ca, Mg) using first principles as well as model Hamiltonian calculations. Our calculation shows that strong Ir-d and O-p covalency in these double-perovskite iridates results in the deviation from the expected 6+ charge state of Ir, leading to an unquenched orbital moment in the presence of SOC. We have studied in detail the nature of magnetism and our calculations of isotropic exchange interactions for these double-perovskite iridates suggest the systems to be frustrated. We argue that the SOC driven easy plane anisotropy possibly promotes long range magnetic order suggested for these systems indicating the importance of SOC in both these systems. Further, our model calculation considering ideal d(3) electronic configuration for Ir emphasizes the role of SOC, noncubic crystal field and Ir-Ir hopping in the reduction of the spin moment.