Neutron scattering investigation of rhenium orbital ordering in the 3d − 5d double perovskite Ca^sub 2^FeReO^sub 6

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 [J. Phys. Soc. Jpn. 72, 1411 (2003)] as well as recent theoretical work by Lee and Marianetti [Phys. Rev. B 97, 045102 (2018)]. 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.