Abstract
We show that LaFe4Sb12 is a ferromagnetic quantum critical point system and that CeFe4Sb12 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 LaFe4Sb12, 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 CeFe4Sb12 due to the antiferromagnetic coupling between the d electrons of Fe and the 4f electron of Ce. As a result, CeFe4Sb12 is a moderate heavy fermion compound with Kondo temperature TK 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 TTk is 0.3B. Finally, in both compounds, the power laws observed at low temperature in the lattice thermal conductivity l could be explained by electron-phonon scattering.