Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the $4d^3$ and $5d^3$ double pervoskite antiferromagnets ${\mathrm{La}}_2{\mathrm{LiRuO}}_6$ and ${\mathrm{La}}_2{\mathrm{LiOsO}}_6$. These reveal the development of an inelastic spin gap in ${\mathrm{La}}_2{\mathrm{LiRuO}}_6$ and ${\mathrm{La}}_2{\mathrm{LiOsO}}_6$ of $\sim 1.8\left(8\right)$ and 6(1) meV, below their respective ordering temperatures $T_N,\sim 23.8$ and 30 K. The bandwidths of the spin excitations are shown to be $\sim 5.7\left(9\right)\text{and}12\left(1\right)\mathrm{meV}$, respectively, at low temperatures. Spin gaps are surprising in such magnets as the $t_{2g}$ levels of ${\mathrm{Ru}}^{5+}$ or ${\mathrm{Os}}^{5+}$ are expected to be half-filled, resulting in an anticipated orbital singlet for both materials. We compare these results in monoclinic double perovskites ${\mathrm{La}}_2{\mathrm{LiRuO}}_6$ and ${\mathrm{La}}_2{\mathrm{LiOsO}}_6$ with those in cubic ${\mathrm{Ba}}_2{\mathrm{YRuO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{YOsO}}_6$ as well as with those in other monoclinic ${\mathrm{La}}_2{\mathrm{NaRuO}}_6,{\mathrm{La}}_2{\mathrm{NaOsO}}_6$, and ${\mathrm{Sr}}_2{\mathrm{ScOsO}}_6$ 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.

• Received 15 May 2018
• Revised 11 August 2018

DOI:https://doi.org/10.1103/PhysRevB.98.104434