Spin gaps in the ordered states of La$_2$LiXO$_6$ (X = Ru, Os) and their relation to distortion of the cubic double perovskite structure in 4$d^3$ and 5$d^3$ magnets

Abstract: Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4$d^3$ and 5$d^3$ double pervoskite antiferromagnets La$2$LiRuO$_6$ and La$_2$LiOsO$_6$. These reveal the development of an inelastic spin gap in La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$ of $\sim$ 1.8(8) meV and 6(1) meV, below their respective ordering temperatures, $T_N$, $\sim$ 23.8 K and 30 K. The bandwidth of the spin excitations is shown to be $\sim$ 5.7(9) to 12(1) meV, respectively, at low temperatures. Spin gaps are surprising in such magnets as the t${2g}$ levels of Ru$^{5+}$ or Os$^{5+}$ are expected to be half-filled, resulting in an anticipated orbital singlet for both materials. We compare these results in monoclinic double perovskites La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$ with those in cubic Ba$_2$YRuO$_6$ and Ba$_2$YOsO$_6$, as well as with those in the more strongly monoclinic La$_2$NaRuO$_6$, La$_2$NaOsO$_6$, and Sr$_2$ScOsO$_6$, and model the inelastic magnetic scattering with linear spin wave theory using minimal anisotropic exchange interactions. We discuss the possible role of the distortion of the face-centered cubic, double perovskite structure on the spin gap formation and geometric frustration in these materials, and show that $T_N$ scales with the top of the spin wave band in all members of these families that display long range order.