Neutron scattering investigation of rhenium orbital ordering in the $3d\ensuremath{-}5d$ double perovskite ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$

Neutron scattering investigation of rhenium orbital ordering in the $3 d - 5 d$ 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, and Young-June Kim

Phys. Rev. B 98, 214433 – Published 19 December 2018

Abstract

We have carried out inelastic neutron scattering experiments to study magnetic excitations in the ordered double perovskite ${Ca}_{2} {FeReO}_{6}$ . We have found a well-defined magnon mode with a bandwidth of $\sim 50$ meV below the ferrimagnetic ordering temperature ( $T_{c} \sim 520$ K), similar to the previously studied ${Ba}_{2} {FeReO}_{6}$ . The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of $\sim 10$ meV opens up below $\sim 150$ 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 $\sim 150$ 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 ${Ca}_{2} {FeReO}_{6}$ 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.

Received 10 September 2018
Revised 4 December 2018

DOI:https://doi.org/10.1103/PhysRevB.98.214433

Physics Subject Headings (PhySH)

Magnetic order Magnons Spin-orbit coupling

Oxides Perovskites

Inelastic neutron scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Bo Yuan¹, J. P. Clancy¹, J. A. Sears¹, A. I. Kolesnikov², M. B. Stone², Z. Yamani³, Choongjae Won⁴, Namjung Hur⁴, B. C. Jeon^5,6, T. W. Noh^5,6, Arun Paramekanti¹, and Young-June Kim^1,*

¹Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7
²Neutron Scattering Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
³Chalk River Laboratories, National Research Council, Chalk River, Ontario, Canada K0J 1J0
⁴Department of Physics, Inha University, Incheon 402-751, Korea
⁵Center for Correlated Electron Systems, Institute for Basic Science, Seoul 08826, Republic of Korea
⁶Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea

^*yjkim@physics.utoronto.ca

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Vol. 98, Iss. 21 — 1 December 2018

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Figure 1
Powder averaged neutron intensity plots measured with incident energy $E_{i} = 120 meV$ at various temperatures: (a) 5, (b) 200, (c) 300, and (d) 450 K. The horizontal and vertical axes denote momentum $| Q |$ (Å $^{- 1}$ ) and energy transfers $ℏ ω$ (meV). Two optical phonon modes discussed in the text are indicated by black arrows in (b). The aluminum sample container background has been subtracted from each scan, and an arbitrary intensity scale has been used where red (blue) denotes higher (lower) intensity.
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Figure 2
Momentum-integrated local susceptibility $χ^{''}$ (a) at 5, 200, 300, and 450 K with incident energy $E_{i} = 120 meV$ and (b) at 20 K obtained with incident energy $E_{i} = 30 meV$ [Fig. 3]. The solid line is a fit to the phenomenological form described in the text. The Al sample container background has been subtracted, and the same arbitrary intensity scales as in Figs. 1 and 3 have been used for both (a) and (b).
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Figure 3
Powder averaged neutron intensity plots with incident energy $E_{i} = 30$ meV at (a) 20, (b) 45, (c) 85, (d) 120, (e) 160, and (f) 200 K. This data are similar to those of Fig. 1 but were obtained with a higher-resolution setup. Two modes at $\sim 10$ and $\sim 20$ meV indicated by black arrows in (b) correspond to the optical phonon modes seen in high- $E_{i}$ data in Fig. 1. Arrows in (e) denote positions for $Q = (\frac{1}{2}, \frac{1}{2}, \frac{1}{2})$ and $Q = (0, 0, 1)$ in pseudocubic notation. The Al sample container background has been subtracted from all plots.
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Figure 4
(a) Constant-energy cuts below the gap obtained by integrating inelastic neutron intensity in Fig. 3 over energy transfers $5 meV < ℏ ω < 7 meV$ . The 20 K data have been subtracted from all plots. Inset: Intensity at $| Q_{1} |$ and $| Q_{2} |$ as a function of temperature, obtained by integrating constant-energy cuts in the main plot within $1.3 Å^{- 1} < | Q | < 1.5 Å^{- 1}$ and $1.5 Å^{- 1} < | Q | < 1.7 Å^{- 1}$ . Blue solid line shows $n (T)$ for $ℏ ω = 6 meV$ . It has been scaled to match the data at $| Q_{2} |$ . (b) Temperature dependence of magnetic inelastic intensity $I_{magn}$ at $| Q_{1} |$ (black square) and $| Q_{2} |$ (blue circle). $I_{magn}$ is obtained by subtracting the phonon intensity given by $n (T)$ from integrated intensities in the inset of (a). (c) Temperature dependence of the magnetic Bragg peak intensity at $| Q_{1} |$ and zero-field cooled (ZFC) magnetization of a ${Ca}_{2} {FeReO}_{6}$ pellet. Magnetization was obtained in the presence of an applied field of 0.2 T after cooling down to 2 K in zero field. Magnetic Bragg peak intensity was obtained at the C5 triple-axis spectrometer at NRU at fixed $| Q | = 1.42 Å^{- 1}$ and $ℏ ω = 0 meV$ on the same powder sample used for time-of-flight measurements.
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Figure 5
Simulated powder averaged spin wave spectra using $F = 2.1$ and $R = 1.3$ for (a) $J_{0} = 3.1$ meV, $J_{1} = J_{2} = B_{a} = 0$ , (b) $J_{0} = 2.9$ meV, $J_{1} = - 0.7$ meV, $J_{2} = - 0.8$ meV, $B_{a} = 0$ , and (c) $J_{0} = 2.9$ meV, $J_{1} = - 0.7$ meV, $J_{2} = - 0.8$ meV, $B_{a} = 18.6$ meV. Calculation of powder averaged spectra is done using the spinw package [50]. All spectra have been corrected for magnetic form factors and instrumental resolutions. (d) Left axis: ${J_{0}, J_{1}, J_{2}}$ parameters constrained by setting zone boundary energies of both modes in the local moment model to 50 meV. The horizontal axis denotes the absolute magnitude of the nearest Fe-Re interaction $J_{0}$ . The vertical axis denotes the magnitude of next-nearest-neighbor Re-Re and Fe-Fe interactions relative to the nearest-neighbor interaction, $J_{1} / J_{0}$ and $J_{2} / J_{0}$ . Right axis: The effective anisotropy field $B_{a}$ required to produce a gap of 10 meV for each set of ${J_{0}, J_{1}, J_{2}}$ .
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Physical Review B

covering condensed matter and materials physics

Neutron scattering investigation of rhenium orbital ordering in the $3 d - 5 d$ 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, and Young-June Kim

Phys. Rev. B 98, 214433 – Published 19 December 2018

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Physical Review B

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

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, and Young-June Kim

Phys. Rev. B 98, 214433 – Published 19 December 2018

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Neutron scattering investigation of rhenium orbital ordering in the $3 d - 5 d$ double perovskite ${Ca}_{2} {FeReO}_{6}$