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Abstract:
The double perovskite oxide Sr2CrOsO6 with a 3d(3)-5d(3) configuration exhibits very high-T-C ferrimagnetism (similar to 725 K) at the end point of half-metallicity. Many substitution studies have been conducted theoretically and experimentally over the last two decades to shed more light on the open issue of how the 3d(3)-5d(3) configuration generates the high-T-C ferrimagnetic state and to accelerate development toward applications. We have succeeded in synthesizing a solid solution of Sr2Cr1-xNixOsO6 under high-pressure and high-temperature conditions. Sr2Cr0.5Ni0.5OsO6 exhibits magnetization sixfold greater (similar to 1.2 mu(B)/formula unit at 5 K) than that of Sr2CrOsO6. This enhancement is preserved even at room temperature. X-ray absorption spectroscopy revealed that the electronic configuration is Sr-2(Cr2/33+Cr1/36+)(0.5)Ni0.52+Os5+O6, indicating that the valence state of Os does not change from the host state [Os5+(5d(3))]. Instead, nonmagnetic Cr6+ (3d(0)) is partly generated among coexisting Cr3+ (3d(3)). X-ray magnetic circular dichroism measurements showed that the Os ions are antiferromagnetically coupled to the Cr and ferromagnetically to the Ni. The replacement of antiferromagnetic Cr by ferromagnetic Ni explains the increase of the net magnetism in this ferrimagnetic system. We infer that the strong antiferromagnetic exchange interaction of the 3d(3)-5d(3) configuration associated with the Cr3+-O-Os5+ bond still accounts for the robust high-T-C ferrimagnetism of the Ni-substituted series. We deduce from the experiments that the ferromagnetic exchange interaction of the 3d(8)-5d(3) configuration of the Ni2+-O-Os5+ is stronger than that of the 3d(8)-3d(3) configuration of the Ni2+-O-Cr3+, suggesting that the larger 5d orbital of the Os allows for a stronger virtual hopping from the Ni than the smaller 3d orbital of the Cr. The present results can help to further develop practical materials and to resolve open issues concerning the relative strengths of the various exchange interactions.
