Magnetism in Ca₂CoOsO₆ and Ca₂NiOsO₆: Unraveling the Mystery of Superexchange Interactions between 3d and 5d Ions

In order to rationalize and predict the behavior of compounds containing 5d transition metal ions, an understanding of the local moments and superexchange interactions from which their magnetic properties are derived is necessary. The magnetic and electrical properties of the ferrimagnetic double perovskites Ca 2 CoOsO 6 and Ca 2 NiOsO 6 studied here provide critical insight toward that goal. First-principles density functional theory (DFT) calculations indicate, and experimental measurements confirm, that the Os(VI) moments are directed antiparallel to the Co/Ni moments. X-ray magnetic circular dichroism (XMCD) measurements reveal that the orbital moment on osmium has a magnitude that is approximately 30% of the spin moment, and the two contributions oppose each other. Both the size and direction of the orbital moment are confirmed by the DFT calculations. The size of the Os(VI) total moment is predicted to be 0.6–0.7 μ B by DFT calculations. The ferrimagnetic ground state is stabilized by strong antiferromagnetic coupling between the d 2 Os(VI) ion and the d 8 /d 7 Ni(II)/Co(II) ion. Not only does the observation of antiferromagnetic coupling violate the Goodenough–Kanamori rules, but also it is unusual in that it becomes stronger as the Os–O–Co/Ni bond angle decreases. This unusual behavior is shown to arise predominantly from coupling between Os t 2g orbitals and Ni/Co e g orbitals, mediated by the intervening oxide ion. We further find that both compounds are spin–orbit assisted Mott insulators.