Strong coupling between 4f valence instability and 3d ferromagnetism in Yb(x)Fe4Sb12 studied by resonant x-ray emission spectroscopy.

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe(4)Sb(12) and Yb(0.88)Fe(4)Sb(12), using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x=0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe(4)Sb(12) is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe(4)Sb(12) and 17 GPa for Yb(0.88)YbFe(4)Sb(12).

Heavy-electron-like behavior in cubic PrCu(4)Au.

The magnetization M(H), magnetic susceptibility χ(T), electrical resistivity ρ(T) and specific heat C(T) properties of PrCu(4)Au have been investigated. A clear antiferromagnetic transition T(N) is observed at 2.5 K in C(T) and χ(T). The internal magnetic field at the Pr nucleus, H(HF), is obtained to be 195 T from the nuclear specific heat observed in C(T) below 0.7 K. The ground state of Pr 4f(2) in the cubic crystalline electric field is inferred to be a magnetic triplet Γ(5) from the magnitude of the magnetization at 2 K, which coincides with the value of the 4f(2) magnetic moments deduced from H(HF). The large value of the electronic specific heat coefficient, γ, remains at the zero-temperature limit even in external magnetic fields. On the other hand, the entropy up to T(N) is somewhat less than Rln3. These anomalous heavy-electron-like behaviors probably originate in the fact that both the magnetic moments and the quadrupole moments are involved in the ground state of Pr 4f(2).