Frustrated fcc antiferromagnet Ba2YOsO6: Structural characterization, magnetic properties, and neutron scattering studies

We report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite Ba2YOsO6. The Fm3¯m space group is found both at 290 K and 3.5 K with cell constants a0=8.3541(4) Å and 8.3435(4) Å, respectively. Os5+ (5d3) ions occupy a nondistorted, geometrically frustrated face-centered-cubic (fcc) lattice. A Curie-Weiss temperature θ∼−700 K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below TN∼69 K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) μB on Os5+. The ordered moment is much reduced from either the expected spin-only value of ∼3 μB or the value appropriate to 4d3 Ru5+ in isostructural Ba2YRuO6 of 2.2(1) μB, suggesting a role for spin-orbit coupling (SOC). Triple-axis neutron scattering measurements of the order parameter suggest an additional first-order transition at T=67.45 K, and the existence of a second-ordered state. Time-of-flight inelastic neutron results reveal a large spin gap Δ∼17 meV, unexpected for an orbitally quenched, d3 electronic configuration. We discuss this in the context of the ∼5 meV spin gap observed in the related Ru5+,4d3 cubic double perovskite Ba2YRuO6, and attribute the ∼3 times larger gap to stronger SOC present in this heavier, 5d, osmate system.