Neutron Scattering Study of the Double Perovskite FCC Antiferromagnet Ba$_2$YRuO$_6$

Magnetic cations in the rock-salt ordered double perovskite structure comprise a geometrically frustrated FCC network of edge-sharing tetrahedra. Previous measurements of the $4d^3$ Ru$^{5+}$ system Ba$_2$YRuO$_6$ [1] indicated the existence of long-range commensurate antiferromagnetic order below $T_N$ = 36K, a factor f $\sim$ 15 lower than the Curie-Weiss temperature $\Theta_W$ = -522K. We report time-of-flight neutron spectroscopy of Ba$_2$YRuO$_6$ confirming the existence of the long-range ordered state below $T_N$. The magnetic inelastic scattering extends over a bandwidth of $\sim$ 15 meV, and develops a $\sim$ 5 meV gap at the [100] magnetic ordering wavevector at and below $T_N$. Strong spin-orbit coupling in this $4d^3$ system is expected to result in a $j_{eff}$=3/2 magnetic moment. This is distinct from the $4d^1$, $j_{eff}$=3/2 moment which arises in its sister antiferromagnetic FCC compound, Ba$_2$YMoO$_6$, which displays an apparent singlet ground state and a $\sim$ 28 meV singlet-tripet gap at low temperatures [2], matching its |$\Theta_W$| $\sim$ 300K. [1] T. Aharen et al. Phys. Rev. B 80, 134423 (2009). [2] J. P. Carlo et al. Phys. Rev. B 84, 100404(R) (2011).