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arXiv · 1608.05761

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

Yuen Yiu·Manh Duc Le·Rasmus Toft-Peterson·Georg Ehlers·Robert McQueeney·David Vaknin
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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.

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Yuen Yiu, Manh Duc Le, Rasmus Toft-Peterson, Georg Ehlers, Robert McQueeney, David Vaknin. 2016-08-20. Hybrid excitations due to crystal-field, spin-orbit coupling and spin-waves in LiFePO$_4$. https://doi.org/10.1103/physrevb.95.104409

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