Strong Spin-Phonon Coupling Mediated by Single Ion Anisotropy in the All-In–All-Out Pyrochlore Magnet Cd2Os2O

Abstract

Spin-phonon coupling mediated by single ion anisotropy was investigated using optical spectroscopy and first-principles calculations in the all-in-all-out pyrochlore magnet Cd_{2}Os_{2}O_{7}. Clear anomalies were observed in both the phonon frequencies and linewidths at the magnetic ordering temperature. The renormalization of the phonon modes was exceptionally large, signifying the presence of an unconventional magnetoelastic term from large spin-orbit coupling. In addition, the relative phonon frequency shifts show a strong correlation with the modulation of noncubic crystal field by the corresponding lattice distortion. Our observation establishes a new type of spin-phonon coupling through single ion anisotropy, a second-order spin-orbit coupling term, in Cd_{2}Os_{2}O_{7}.

Spin and orbital excitations through the metal-to-insulator transition in Cd2Os2O7 probed with high-resolution resonant inelastic x-ray scattering

“…An alternative proposition is that the damping is driven by spin-phonon coupling; as already observed by Raman scattering 23 and optical spectroscopy measurements. 25 From fitting the data above 100 K to an Arrhenius function: Γ = A exp (−∆/k B T ), we find that the critical energy scale ∆ = 25(3) meV. This is in good agreement with the lowest energy Os phonon modes, both infrared-active [Os-Os stretch, 24.6(5) meV] and Raman-active [Os-O(1) stretch, 28 meV].…”

“…Such a model successfully described the effect of spinphonon coupling observed in Raman scattering. 23,25 The asymmetry is a direct manifestation of coupling between a resonant mode (with lifetime ∝ 1/Γ) and a continuum of states, the magnitude of which is parametrized by the coefficient |1/q|. Note that in the subsequent analysis, it has been assumed that q ≈ 7 -indicative of moderate coupling between the resonance and continuum -is globally fixed for all temperatures and momentum transfers.…”

Unconventional spin-phonon coupling via the Dzyaloshinskii–Moriya interaction

“…The relevant physical parameters are summarized in Table 2. 11,50,60,61,64,65 Recently, Calder et al reported that the breathing phonon mode of NaOsO 3 has Δω 0~5 .0 meV, the largest observed in solids. 50 This large renormalization was attributed to the strong modulation of the IE interaction by the Os-O-Os bond stretching motion.…”

“…In general, we can describe the physics of most spin-related phenomena in solids using the spin Hamiltonian (H spin ). This can be written as a sum of the isotropic exchange (IE) interaction, the Dzyaloshinskii-Moriya (DM) interaction, the anisotropic exchange (AE) interaction, and the single-ion anisotropy (SIA): [11][12][13]…”

“…The phonon renormalization due to SPC can be directly measured by infrared (IR) spectroscopy. 11 In general, SPC phenomena have been widely investigated by measuring anomalies in spin wave dispersion with either neutron or X-ray scattering experiments. 37,38 However, in Y 2 Ir 2 O 7 the constituent Ir ions strongly absorb neutrons, making neutron scattering experiments difficult.…”