The double perovskite oxide ${\mathrm{Sr}}_2\mathrm{CrOs}{\mathrm{O}}_6$ with a $3d^3\text{−}5d^3$ configuration exhibits very high-$T_C$ ferrimagnetism (∼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 $3d^3\text{−}5d^3$ configuration generates the high-$T_C$ ferrimagnetic state and to accelerate development toward applications. We have succeeded in synthesizing a solid solution of ${\mathrm{Sr}}_2{\mathrm{Cr}}_{1–x}{\mathrm{Ni}}_x\mathrm{Os}{\mathrm{O}}_6$ under high-pressure and high-temperature conditions. ${\mathrm{Sr}}_2{\mathrm{Cr}}_{0.5}{\mathrm{Ni}}_{0.5}\mathrm{Os}{\mathrm{O}}_6$ exhibits magnetization sixfold greater ($\sim 1.2{\mu }_{\mathrm{B}}$/formula unit at 5 K) than that of ${\mathrm{Sr}}_2\mathrm{CrOs}{\mathrm{O}}_6$. This enhancement is preserved even at room temperature. X-ray absorption spectroscopy revealed that the electronic configuration is ${\mathrm{Sr}}_2{\left({{\mathrm{Cr}}^{3+}}_{2/3}{{\mathrm{Cr}}^{6+}}_{1/3}\right)}_{0.5}{{\mathrm{Ni}}^{2+}}_{0.5}{\mathrm{Os}}^{5+}{\mathrm{O}}_6$, indicating that the valence state of Os does not change from the host state $\left[{\mathrm{Os}}^{5+}\left(5d^3\right)\right]$. Instead, nonmagnetic ${\mathrm{Cr}}^{6+}\left(3d^0\right)$ is partly generated among coexisting ${\mathrm{Cr}}^{3+}\left(3d^3\right)$. 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 $3d^3\text{−}5d^3$ configuration associated with the ${\mathrm{Cr}}^{3+}\text{−}\mathrm{O}\text{−}{\mathrm{Os}}^{5+}$ 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 $3d^8\text{−}5d^3$ configuration of the ${\mathrm{Ni}}^{2+}\text{−}\mathrm{O}\text{−}{\mathrm{Os}}^{5+}$ is stronger than that of the $3d^8\text{−}3d^3$ configuration of the ${\mathrm{Ni}}^{2+}\text{−}\mathrm{O}\text{−}{\mathrm{Cr}}^{3+}$, 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.

• Received 9 August 2020
• Accepted 23 October 2020

DOI:https://doi.org/10.1103/PhysRevB.102.184418