Spin gap and the nature of the 4d3 magnetic ground state in the frustrated fcc antiferromagnet Ba₂YRuO₆

The geometrically frustrated double-perovskite Ba₂YRuO ₆ has magnetic 4d3 Ru5⁺ ions decorating an undistorted face-centered-cubic lattice. This material has been previously reported to exhibit commensurate long-range antiferromagnetic order below T_N∼ 36 K, a factor f∼15 times lower than its Curie-Weiss temperature Θ_CW=-522 K, and purported short-range order to T ^*=47 K. We report new time-of-flight neutron spectroscopy of Ba₂YRuO₆ which shows the development of a ∼5 meV spin gap in the vicinity of the [100] magnetic ordering wave vector below T _N=36 K, with the transition to long-range order occurring at T ^*=47 K. We also report spin waves extending to ∼14 meV, a surprisingly small bandwidth in light of the large Θ_CW. We compare the spin gap and bandwidth to relevant neutron studies of the isostructural 4d1 material Ba₂YMoO₆, and discuss the results in the framework of relatively strong spin-orbit coupling expected in 4d magnetic systems.