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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).