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

TL;DR

This neutron scattering study reveals orbital ordering and magnetic excitations in Ca$_2$FeReO$_6$, showing a temperature-dependent spin gap and weak spin-orbit coupling effects, indicating complex interplay between spin and orbital degrees of freedom.

Contribution

First direct observation of Re orbital ordering in Ca$_2$FeReO$_6$ via neutron scattering, linking it to a metal-insulator transition and structural distortion.

Findings

Spin gap opens below 150K, indicating Re orbital ordering.

Presence of a single magnon band suggests strong Re magnon damping.

Weak coupling between spin and orbital degrees of freedom.

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 $\it{et.\,al}$ based on neutron diffraction [J. Phys. Soc. Jpn., $\bf{72}$, 1411 (2003)] as well as recent theoretical work by Lee and Marianetti [Phys. Rev. B, $\bf{97}$, 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.