Origin of magnetic excitation gap in double perovskite Sr2FeOsO6

Sr2FeOsO6 is an insulating double perovskite compound which undergoes antiferromagnetic transitions at 140 (TN1) and 67 K (TN2). 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 Sr2FeOsO6. The INS data reveal that the spectrum of spin excitations remains ungapped below TN1, however below TN2 a gap of 6.8 meV develops. The RIXS data reveals splitting of the T2g multiplet consistent with that seen in other 5d3 osmium based double perovskites. Altogether these results suggest that spin-orbit coupling is important for ground state selection in 3d–5d3 double perovskite materials.