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

The geometrically frustrated double-perovskite Ba2YRuO6 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 TN∼ 36 K, a factor f∼15 times lower than its Curie-Weiss temperature ΘCW=−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 TN=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 ΘCW. 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.