We find evidence for long-range and short-range (ζ= 70 \AA~at 4 K)
incommensurate magnetic order on the quasi-face-centered-cubic (FCC) lattices
of the monoclinic double perovskites La2​NaRuO6​ and La2​NaOsO6​
respectively. Incommensurate magnetic order on the FCC lattice has not been
predicted by mean field theory, but may arise via a delicate balance of
inequivalent nearest neighbour and next nearest neighbour exchange
interactions. In the Ru system with long-range order, inelastic neutron
scattering also reveals a spin gap Δ∼ 2.75 meV. Magnetic
anisotropy is generally minimized in the more familiar octahedrally-coordinated
3d3 systems, so the large gap observed for La2​NaRuO6​ may result from
the significantly enhanced value of spin-orbit coupling in this 4d3
material.Comment: 5 pages, 4 figure