Magnetism and anisotropy of Ir$^{5+}$ based double perovskites Sr$_{2}$CoIrO$_{6}$ and Sr$_{2}$FeIrO$_{6}$

We report on structural, thermodynamic and transport study of single-crystal double perovskites Sr$_{2}$CoIrO$_{6}$ and Sr$_{2}$FeIrO$_{6}$. The isostructural Sr$_{2}$CoIrO$_{6}$ and Sr$_{2}$FeIrO$_{6}$ feature a cubic crystal structure with pentavalent Ir$^{5+}$(5d$^{4})$ which are anticipated to have J$=$0 singlet ground states in the strong spin-orbit coupling limit. Here we observe magnetic coupling between 5d and 3d (Co, Fe) elements, which result in antiferromagnetic order at high temperatures in both double perovskites. Of the two, Sr$_{2}$CoIrO$_{6}$ displays antiferromagnetic metallic behavior with a pronounced magnetic anisotropy; in sharp contrast, the isostructural Sr$_{2}$FeIrO$_{6}$ exhibits an antiferroamagnetic, insulating ground state without discernible magnetic anisotropy. The data will be discussed and presented with comparisons drawn with similar systems.