Neutron Scattering Investigation of Rhenium Orbital Ordering in $3d-5d$ Double Perovskite Ca${}_2$FeReO${}_6$

Abstract

We have carried out inelastic neutron scattering experiments to study magnetic excitations in ordered double perovskite Ca${}_2$FeReO${}_6$. We found a well-defined magnon mode with a bandwidth of $\sim$50meV below the ferri-magnetic ordering temperature ($T_c\sim$520K), similar to previously studied Ba${}_2$FeReO${}_6$. The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of $\sim$10meV opens up below $\sim$150K, 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 spin gap provides strong evidence for ordering of Re orbitals at $\sim$150~K, in accordance with earlier proposal put forward by 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 Ca${}_2$FeReO${}_6$ suggests weak coupling between spin and orbital degrees of freedom and hints towards a sub-dominant role played by spin orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near magnetic zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments, but suggests a strong damping of Re magnon mode.

Supplemental Material available upon request; Accepted by PRB