In the search for new exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the Jeff = ½ spin orbital Mott state in the 5d5 layered perovskites iridates . The double perovskite structure, where the magnetic ions form a face-centered-cubic (fcc) sublattice, can accommodate a large variety of 5d transition metal elements, and therefore offers an ideal playground for systematic studies of the exotic magnetic and non-magnetic ground states stabilized by strong spin-orbit coupling . Here, we report time-of-flight neutron scattering measurements on the antiferromagnetic, frustrated, cubic double perovskite system Ba2YOsO6. Its non-distorted fcc lattice is decorated with magnetic Os5+ (5d3) ions which undergo a magnetic transition to a long range ordered antiferromagnetic state below TN = 70 K, as revealed by magnetic Bragg peaks occuring at the  and  positions. Our inelastic data reveals a large spin gap to the spin-wave excitations Δ = 19(2) meV, unexpected for an orbitally quenched, d3 electronic configuration. We compare this result to the recent observation of a Δ=5 meV spin gap in the related cubic double perovskite Ba2YRuO6 (Ru5+, 4d3) , and conclude to a stronger spin-orbit coupling present in the heavier, 5d, osmate system.
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