Nature of the magnetism of iridium in the double perovskite Sr 2 CoIrO 6

We report on our investigation on the magnetism of the iridate double perovskite ${\mathrm{Sr}}_{2}{\mathrm{CoIrO}}_{6}$, a nominally ${\mathrm{Ir}}^{5+}$ Van Vleck ${J}_{\mathrm{eff}}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-${L}_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the ${J}_{\mathrm{eff}}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-${L}_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% ${\mathrm{Ir}}^{5+}$ (${J}_{\mathrm{eff}}=0$) and ${\mathrm{Co}}^{3+}$ ($S=2$) and 10% ${\mathrm{Ir}}^{6+}$ ($S=3/2$) and ${\mathrm{Co}}^{2+}$ ($S=3/2$). The magnetic signal of this minority ${\mathrm{Ir}}^{6+}$ component is almost equally as strong as that of the main ${\mathrm{Ir}}^{5+}$ component. We infer that there is a competition between the ${\mathrm{Ir}}^{5+}\ensuremath{-}{\mathrm{Co}}^{3+}$ and the ${\mathrm{Ir}}^{6+}\ensuremath{-}{\mathrm{Co}}^{2+}$ configurations in this stoichiometric compound.