Strong Effect of Yb Filling Fraction on the Magnetic Ground Sate of the Filled Skutterudite YbFe4Sb12

YbFe 4 Sb 12 single crystals with the Yb-site filling fraction close to 100% have been successfully synthesized by the Sb self-flux method under a pressure of 4 GPa. The grown single crystals do not exhibit ferromagnetic ordering down to the lowest temperature investigated, in contrast with the ferromagnetic transition reported for Yb-deficient single crystals grown at ambient pressure. The electronic specific heat coefficient of the present crystals is evidently smaller than those reported for Yb-deficient samples, indicating that the Fermi energy is located at the high-energy side of a peak in the electronic density of states dominated by the contribution of Fe-3d electrons. Therefore, the appearance of the ferromagnetic state in the Yb-deficient samples can be naively understood within the simple Stoner-type argument. Marked enhancement of the positive magnetoresistance was found in the temperature region where the spin-fluctuation scattering of conduction electrons dominates.