  The geometrically frustrated double perovskite Ba2YRuO6 has magnetic 4d3 Ru5+
ions decorating an undistorted face-centered cubic (FCC) lattice. This material
has been previously reported to exhibit commensurate long-range
antiferromagnetic order below T_N = 36K, a factor f = 15 times lower than its
Curie-Weiss temperature Theta_CW = -522 K, and purported short-range order to
T* = 47K. 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 wavevector below T_N = 36K, with the transition to long-range order
occurring at T* = 47K. We also report spin waves extending to 14 meV, a
surprisingly small bandwidth in light of the large Theta_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.
