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

Here, 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 K, which is well below the magnetic ordering temperature but coincides with a previously reported metal-insulator transition and onset of structural distortion. The observed temperature dependence of the spin gap provides strong evidence for ordering of Re orbitals at ~150 K, in accordance with an earlier proposal put forward by K. Oikawa et al. based on neutron diffraction as well as recent theoretical work by Lee and Marianetti. The presence of separate orbital and magnetic ordering in Ca2FeReO6 suggests weak coupling between spin and orbital degrees of freedom and hints at a subdominant role played by spin-orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near the magnetic zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments but suggests a strong damping of the Re magnon mode.