The frustrated fcc antiferromagnet Ba2 YOsO6: Structural characterization, magnetic properties and neutron scattering studies

Here we report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite Ba2 YOsO6. The Fm$$\bar{3}$$m space group is found both at 290 K and 3.5 K with cell constants a0=8.3541(4) A and 8.3435(4) A, 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 Ba2 YRuO6 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. We find time-of-flight inelastic neutron results reveal a large spin gap Δ~17 meV, unexpected for an orbitally quenched, d3 electronic configuration. In conclusion, 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.