Roles of spin fluctuations and rattling in magnetic and thermoelectric properties of AT4Sb12 (A=Ca, Sr, Ba, La; T=Fe, Ru, Os)

Magnetic and thermoelectric properties of the skutterudites AT4Sb12 filled with non-magnetic atoms, A=Ca, Sr, Ba and La, are found to depend strongly on the transition-metal elements, T=Fe, Ru and Os. The strong spin fluctuations in the Fe-based compounds manifest themselves as an enhanced specific-heat coefficient γ=100 mJ/K2 mol. The maximum of the magnetic susceptibility at 50 K observed for A=Ca, Sr and Ba is a characteristic of a nearly ferromagnetic system. The increase of the paramagnetic Curie temperature from 3 to 54 K on going from A=La, Ba, Sr to Ca indicates the enhancement of ferromagnetic interactions in the Fe 3d narrow band. Except for A=La, a marked shoulder appears in the thermopower at 50 K which is totally suppressed by the application of a magnetic field of 14 T. The Ru compounds are metallic diamagnets with γ=10 mJ/K2 mol, while the Os compounds are strongly enhanced Pauli paramagnets with γ=45 mJ/K2 mol and show behaviors of a strongly coupled electron-phonon system. The lattice thermal conductivity for either A=La or T=Os does not show a peak near 50 K and its value is much smaller than other compounds. The strong phonon scattering can be caused by rattling of the relatively small La3+ ion in the T4Sb12 cage or tunneling of Sr2+ and Ba2+ ions in the Os4Sb12 cage.