Muon Spin Relaxation Studies of the Magnetically Frustrated Double Perovskite Ba2CaOsO6

The double perovskite structure A$_2$BB'O$_6$, in which antiferromagnetically-correlated magnetic B$'$ cations form an edge-sharing tetrahedral network, is an ideal laboratory for geometric magnetic frustration. The versatility of the perovskite structure enables systematic studies as a function of lattice distortion and moment size, and with 4d and 5d cations, spin-orbit coupling (SOC). Systems with large moments (d$^3$) tend toward antiferromagnetic order, albeit at T $<$ $|\Theta_{CW}|$. Systems with small moments (d$^1$) tend toward disorder, including glassy and singlet ground states. d$^2$ systems form a ``middle ground'' in which a variety of ground states are observed, and theory indicates a wealth of accessible behavior in systems with sizable SOC. Here we report on muon spin relaxation experiments of the 5d$^2$ system Ba$_2$CaOsO$_6$, which exhibits an undistorted cubic structure down to low temperatures, and in which long-lived muon spin precession is observed below 50K. These results are compared to related compounds, including the isostructural Ba$_2$YReO$_6$, an isoelectronic 5d$^2$ system exhibiting glassy behavior, and Ba$_2$YRuO$_6$, a 4d$^3$ system exhibiting commensurate antiferromagnetic order.