Spin gaps in the ordered states of La2LiXO6(X=Ru,Os) and their relation to the distortion of the cubic double perovskite structure in 4d3 and 5d3 magnets

Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4d3 and 5d3 double pervoskite antiferromagnets La2LiRuO6 and La2LiOsO6. These reveal the development of an inelastic spin gap in La2LiRuO6 and La2LiOsO6 of ∼1.8(8) and 6(1) meV, below their respective ordering temperatures TN,∼23.8 and 30 K. The bandwidths of the spin excitations are shown to be ∼5.7(9)and12(1)meV, respectively, at low temperatures. Spin gaps are surprising in such magnets as the t2g levels of Ru5+ or Os5+ are expected to be half-filled, resulting in an anticipated orbital singlet for both materials. We compare these results in monoclinic double perovskites La2LiRuO6 and La2LiOsO6 with those in cubic Ba2YRuO6 and Ba2YOsO6 as well as with those in other monoclinic La2NaRuO6,La2NaOsO6, and Sr2ScOsO6 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 TN scales with the top of the spin-wave band in all members of these families that display long-range order.