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

TL;DR

This study investigates the complex hybrid excitations in LiFePO$_4$ caused by crystal-field, spin-orbit coupling, and spin-waves, revealing new nearly dispersionless excitations and analyzing their temperature dependence through neutron scattering and theoretical modeling.

Contribution

The paper provides a detailed experimental and theoretical analysis of hybrid excitations in LiFePO$_4$, highlighting the role of crystal field and spin-orbit interactions in these phenomena.

Findings

Identification of nearly dispersionless excitations below $T_N$

Observation of magnetic fluctuations up to 720 K

Theoretical modeling with MF-RPA matches experimental spectra

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.