The magnetic ground state in the double perovskite system $\mathrm{S}{\mathrm{r}}_{2-x}\mathrm{C}{\mathrm{a}}_x\mathrm{CoOs}{\mathrm{O}}_6$ changes from an antiferromagnet $(x=0)$, to a spin glass $(x=1)$, to a ferrimagnet $(x=2)$ as the Ca content increases. This crossover is driven by chemical pressure effects that control the relative strength of magnetic exchange interactions. The synthesis, crystal structure, and magnetism of $\mathrm{SrCaCoOs}{\mathrm{O}}_6$ and $\mathrm{C}{\mathrm{a}}_2\mathrm{CoOs}{\mathrm{O}}_6$ are investigated and compared with $\mathrm{S}{\mathrm{r}}_2\mathrm{CoOs}{\mathrm{O}}_6$. Both compounds adopt a monoclinic crystal structure with rock-salt ordering of $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{6+}$ and $a^{-}{a}^{-}{b}^{+}$ octahedral tilting, but the average Co–O–Os bond angle evolves from $158.0{\left(3\right)}^{\circ }$ in $\mathrm{SrCaCoOs}{\mathrm{O}}_6$ to $150.54{\left(9\right)}^{\circ }$ in $\mathrm{C}{\mathrm{a}}_2\mathrm{CoOs}{\mathrm{O}}_6$ as the smaller $\mathrm{C}{\mathrm{a}}^{2+}$ ion replaces $\mathrm{S}{\mathrm{r}}^{2+}$. While this change may seem minor, it has a profound effect on the magnetism, changing the magnetic ground state from antiferromagnetic in $\mathrm{S}{\mathrm{r}}_2\mathrm{CoOs}{\mathrm{O}}_6$ $(T_{\mathrm{N}1}=108\mathrm{K}$, $T_{\mathrm{N}2}=70\mathrm{K})$, to a spin glass in $\mathrm{SrCaCoOs}{\mathrm{O}}_6$ $(T_{\mathrm{f}1}=32\mathrm{K}$, $T_{\mathrm{f}2}=13\mathrm{K})$, to ferrimagnetic in $\mathrm{C}{\mathrm{a}}_2\mathrm{CoOs}{\mathrm{O}}_6$ $(T_{\mathrm{C}}=145\mathrm{K})$. In the first two compounds the observation of two transitions is consistent with weak coupling between the Co and Os sublattices.

• Received 27 February 2015
• Revised 5 August 2015

DOI:https://doi.org/10.1103/PhysRevB.92.094435