We have studied Ir spin and orbital magnetic moments in the double perovskites La${}_{2-x}$Sr${}_x$CoIrO${}_6$ by x-ray magnetic circular dichroism. In La${}_2$CoIrO${}_6$, Ir${}^{4+}$ couples antiferromagnetically to the weak ferromagnetic moment of the canted Co${}^{2+}$ sublattice and shows an unusually large negative total magnetic moment ($-0.38$ ${\mu }_{\mathrm{B}}$/f.u.) combined with strong spin-orbit interaction. In contrast, in Sr${}_2$CoIrO${}_6$, Ir${}^{5+}$ has a paramagnetic moment with almost no orbital contribution. A simple kinetic-energy-driven mechanism including spin-orbit coupling explains why Ir is susceptible to the induction of substantial magnetic moments in the double perovskite structure.

• Received 10 October 2011

DOI:https://doi.org/10.1103/PhysRevB.85.224422