  We report neutron scattering experiments which reveal a large spin gap in the
magnetic excitation spectrum of weakly-monoclinic double perovskite Sr2ScOsO6.
The spin gap is demonstrative of appreciable spin-orbit-induced anisotropy,
despite nominally orbitally-quenched 5d3 Os5+ ions. The system is successfully
modeled including nearest neighbor interactions in a Heisenberg Hamiltonian
with exchange anisotropy. We find that the presence of the spin-orbit-induced
anisotropy is essential for the realization of the type I antiferromagnetic
ground state. This demonstrates that physics beyond the LS or JJ coupling
limits plays an active role in determining the collective properties of 4d3 and
5d3 systems, and that theoretical treatments must include spin-orbit coupling.
