We report on the spin waves and crystal field excitations in single crystal LiFePO4 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 TN=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 Fe2+ 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 TN, 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 TN. By applying the MF-RPA model to a number of existing spin-wave results from other LiMPO4 (M= Mn, Co, and Ni), we are able to obtain reasonable predictions for the moment sizes and transition temperatures.
@article{arxiv.1608.05761,
title = {Hybrid excitations due to crystal-field, spin-orbit coupling and spin-waves in LiFePO$_4$},
author = {Yuen Yiu and Manh Duc Le and Rasmus Toft-Peterson and Georg Ehlers and Robert McQueeney and David Vaknin},
journal= {arXiv preprint arXiv:1608.05761},
year = {2017}
}