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arXiv · 1809.03647

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

Bo Yuan·J. P. Clancy·J. A. Sears·A. I. Kolesnikov·M. B. Stone·Z. Yamani·Choongjae Won·Namjung Hur·B. C. Jeon·T. W. Noh·Arun Paramekanti·Young-June Kim
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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.

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Bo Yuan, J. P. Clancy, J. A. Sears, A. I. Kolesnikov, M. B. Stone, Z. Yamani, Choongjae Won, Namjung Hur, B. C. Jeon, T. W. Noh, Arun Paramekanti, Young-June Kim. 2018-12-05. Neutron Scattering Investigation of Rhenium Orbital Ordering in $3d-5d$ Double Perovskite Ca$_2$FeReO$_6$. https://doi.org/10.1103/physrevb.98.214433

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