Exotic magnetism on the FCC lattice of 5$d^n$ double perovskites

In the search for exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the $J_{eff} = \frac{1}{2}$ spin-orbital Mott state in the 5$d^5$ layered perovskites iridates. The double perovskite structure can accommodate numerous 5$d$ ions and therefore offers a playground for systematic studies of the exotic ground states stabilized by strong spin-orbit coupling (SOC). Here, we report time-of-flight neutron scattering measurements on the antiferromagnetic (AF), frustrated system, Ba$_2$YOsO$_6$. This 5$d^3$ system undergoes a magnetic transition to a long range ordered AF state below $T_N$ = 70K. Our results reveal a large spin gap $\Delta$ = 18(2)meV, unexpected for an orbitally quenched $d^3$ system. We compare this result to the recent observation of a $\Delta$ = 5meV spin gap in the related 4$d^3$ system, Ba$_2$YRuO$_6$, and conclude to an effect of enhanced SOC.