Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism in Li$R$F$_4$ ($R$ = Gd, Ho, Er, Tm, and Yb)

TL;DR

This study uses inelastic neutron scattering to analyze crystal field effects in Li$R$F$_4$ magnets, revealing how crystal field interactions influence magnetic properties and phase transitions, with evidence of phonon coupling and magnetoelastic effects.

Contribution

It provides detailed quantification of crystal field excitations in Li$R$F$_4$ and links these to magnetic behavior, including phonon coupling and magnetoelastic effects, advancing understanding of rare-earth magnetism.

Findings

Crystal field transition energies and wavefunctions quantified for all studied compounds.

Coupling between crystal field and phonons causes lineshape broadening and level splitting.

Magnetoelastic coupling observed in LiHoF$_4$ affects excitation broadening.

Abstract

We present a systematic study of the crystal field interactions in the Li$R$F$_4$, $R$ = Gd, Ho, Er, Tm and Yb, family of rare-earth magnets. Using detailed inelastic neutron scattering measurements we have been able to quantify the transition energies and wavefunctions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to lineshape broadening in LiTmF$_4$ and level splitting in LiYbF$_4$. Furthermore, using high resolution neutron scattering from LiHoF$_4$, we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.