Neutron scattering investigation of rhenium orbital ordering in the 3d-5d double perovskite Ca2FeReO6

We have carried out inelastic neutron scattering experiments to study magnetic excitations in the ordered double perovskite Ca2FeReO6. We have found a well-defined magnon mode with a bandwidth of ∼50 meV below the ferrimagnetic ordering temperature (Tc∼520 K), similar to the previously studied Ba2FeReO6. The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of ∼10 meV opens up below ∼150 …