Thermoelectric properties ofCe(La)Fe4Sb12skutterudites under a magnetic field

Abstract

We show that ${\text{LaFe}}_{4}{\text{Sb}}_{12}$ is a ferromagnetic quantum critical point system and that ${\text{CeFe}}_{4}{\text{Sb}}_{12}$ is a moderate heavy fermion compound. This is supported by our thermoelectric and thermodynamic experiments under magnetic field on both compounds, and a comparison with other physical properties of these compounds. For ${\text{LaFe}}_{4}{\text{Sb}}_{12}$, the quenching of the ferromagnetic spin fluctuations explains both the negative magnetothermopower and the decrease in the electronic heat capacity under magnetic field. We propose that the negative magnetothermopower is a generic property of compounds with ferromagnetic spin fluctuations when the diffusion term by spin fluctuations dominates at low temperature. On the other hand these critical ferromagnetic fluctuations are smeared out in ${\text{CeFe}}_{4}{\text{Sb}}_{12}$ due to the antiferromagnetic coupling between the $d$ electrons of Fe and the $4f$ electron of Ce. As a result, ${\text{CeFe}}_{4}{\text{Sb}}_{12}$ is a moderate heavy fermion compound with Kondo temperature ${T}_{K}$ of about 80 K, which is consistent with the fact that cerium is almost trivalent in this material, and the partially screened magnetic moment of the cerium ions at $T⪡{T}_{k}$ is $0.3{\ensuremath{\mu}}_{\text{B}}$. Finally, in both compounds, the power laws observed at low temperature in the lattice thermal conductivity ${\ensuremath{\kappa}}_{l}$ could be explained by electron-phonon scattering.