Spin gap in CeFe4Sb12 studied by heat capacity and inelastic neutron scattering

The magnetic properties of the skutterudite compound CeFe4Sb12 have been investigated by heat capacity and inelastic neutron scattering measurements. Heat capacity measurements reveal a broad peak centered at 125 K, whose magnitude is much larger than that expected from a Schottky anomaly due to a cubic crystal electric field. At 5 K, inelastic neutron scattering experiments clearly show the existence of a broad magnetic peak at 40(3) meV. The absence of quasielastic scattering at this temperature, together with the almost total account of the magnetic signal in the inelastic peak, shows that the excitation has a different origin than a splitting of the electronic levels due to crystal field potential. Instead, we propose a model in which the signal originates from inelastic excitations across two hybridization bands near the Fermi energy, usually referred to as a spin gap. A simple phenomenological two-level model can account for the peak in the specific heat, with a spin-gap energy of 36(2) meV, which is in very good agreement with the inelastic scattering data. Further, at 300 K, the inelastic response becomes purely quasielastic, which is in agreement with the theoretical calculations. Interestingly, the spin-gap energy in CeFe4Sb12 exhibits a universal scaling behavior with the Kondo temperature TK. The relation between the spin-gap energy and the associated anomalies in the heat capacity or thermal expansion is discussed for a series of Ce- and Yb-based compounds