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

We report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite Ba2YOsO6. The Fm (3) over barm space group is found both at 290 K and 3.5 K with cell constants a(0) = 8.3541(4) angstrom and 8.3435(4) angstrom, respectively. Os5+ (5d(3)) ions occupy a nondistorted, geometrically frustrated face-centeredcubic (fcc) lattice. A Curie-Weiss temperature theta similar to -700 K suggests the presence of a large antiferromagnetic interaction and a high degree ofmagnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below T-N similar to 69 K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) mu(B) on Os5+. The ordered moment is much reduced from either the expected spin-only value of similar to 3 mu(B) or the value appropriate to 4d(3) Ru5+ in isostructural (BaYRuO6)-Y-2 of 2.2(1) mu(B), 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 Delta similar to 17 meV, unexpected for an orbitally quenched, d(3) electronic configuration. We discuss this in the context of the similar to 5 meV spin gap observed in the related Ru5+, 4d(3) cubic double perovskite Ba2YRuO6, and attribute the similar to 3 times larger gap to stronger SOC present in this heavier, 5d, osmate system.