LAUSR

${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ is an insulating double perovskite compound which undergoes antiferromagnetic transitions at 140 (${T}_{\text{N1}}$) and 67 K (${T}_{\text{N2}}$). To study the underlying electronic and magnetic interactions giving rise to this behavior we have performed inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) experiments on polycrystalline samples of ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$. The INS data reveal that the spectrum of spin excitations remains ungapped below ${T}_{\text{N1}}$, however below ${T}_{\text{N2}}$ a gap of 6.8 meV develops. The RIXS data reveals splitting of the ${T}_{2g}$ multiplet consistent with that seen in other $5{d}^{3}$ osmium based double perovskites. Together these results suggest that spin-orbit coupling is important for ground state selection in $3d\text{\ensuremath{-}}5{d}^{3}$ double perovskite materials.