Spin gaps in the ordered states of La_2LiXO_6 (X= Ru, Os) and their relation to the distortion of the cubic double perovskite structure in 4d~3 and 5d~3 magnets

摘要

Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4J3 and double pervoskite anliferromagnets La_2LiRuO_6 and La_2LiO_sO_6. These reveal the development of an inelastic spin gap in La_2LiRuO_6 and La_2LiOsO_6 of ～1.8(8) and 6(1) meV, below their respective ordering temperatures T_N. ～23.8 and 30 K. The bandwidths of the spin excitations are shown to be ～5.7(9) and 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_2LiRuO_6 and La_2LiOsO_6 with those in cubic Ba_2YRuO_6 and Ba_2YOsO_6 as well as with those in other monoclinic La_2NaRuO_6, La_2NaOsO_6, and Sr_2ScOsO_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.