Hybrid excitations due to crystal-field, spin-orbit coupling and spin-waves in LiFePO${}_4$

Abstract

We report on the spin waves and crystal field excitations in single crystal LiFePO${}_4$ by inelastic neutron scattering over a wide range of temperatures, below and above the antiferromagnetic transition of this system. In particular, we find extra excitations below $T_N=50$ K that are nearly dispersionless and are most intense around magnetic zone centers. We show that these excitations correspond to transitions between thermally occupied excited states of Fe${}^{2+}$ due to splitting of the $S=2$ levels that arise from crystal field and spin-orbit interaction. These excitations are further amplified by the highly distorted nature of the oxygen octahedron surrounding the iron atoms. Above $T_N$, magnetic fluctuations are observed up to at least 720~K, with additional excitation around 4 meV, likely caused by single-ion splittings through spin-orbit and crystal field interactions. The latter weakens slightly at 720~K compared to 100~K, which is consistent with calculated cross-sections using a single-ion model. Our theoretical analysis, using the MF-RPA model, provides both detailed spectra of the Fe $d-$ shell and estimates of the average ordered magnetic moment and $T_N$. By applying the MF-RPA model to a number of existing spin-wave results from other Li$M$PO${}_4$ ($M=$ Mn, Co, and Ni), we are able to obtain reasonable predictions for the moment sizes and transition temperatures.

7 pages, 4 figures

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Cited by in corpus (5)

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• Orbital magnetization in two-dimensional materials from high-throughput computational screening
• Exceptional field dependence of antiferromagnetic magnons in LiFePO${}_4$
• Linear magnetoelectricity in the Zintl phase pnictides (Ba, Ca, Sr)${\mathrm{Mn}}_2{(\mathrm{P},\mathrm{As},\mathrm{Sb})}_2$ from first principles calculations