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

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• 作者：Ryan Morrow ; Kartik Samanta ; Tanusri Saha Dasgupta ; Jie Xiong ; John W. Freeland ; Daniel Haskel ; Patrick M. Woodward
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：June 14, 2016
• 年：2016
• 卷：28
• 期：11
• 页码：3666-3675
• 全文大小：579K
• 年卷期：0
• ISSN：1520-5002

文摘

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 Ca₂CoOsO₆ and Ca₂NiOsO₆ 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 d² Os(VI) ion and the d⁸/d⁷ 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.