Ferromagnetism and giant magnetoresistance in the rare-earth fullerides Eu₆-_xSr_xC₆₀

We have studied crystal structure, magnetism, and electric transport properties of a europium fulleride Eu₆C₆₀ and its Sr-substituted compounds, Eu₆-_xSr_xC₆₀. They have a bcc structure, which is an isostructure of other M₆C₆₀ (M represents an alkali atom or an alkaline-earth atom). Magnetic measurements revealed that magnetic moment is ascribed to the divalent europium atom with S=7/2 spin, and a ferromagnetic transition was observed at T_C=10-14 K. In EU₆C₆₀, we also confirm the ferromagnetic transition by heat-capacity measurement. The striking feature in Eu₆-_xSr_xC₆₀ is very large negative magnetoresistance at low temperature; the resistivity ratio p(H=9 T)/p(H=0 T) reaches almost 10^-3 at Eu₆C₆₀. Such large magnetoresistance is the manifestation of a strong π-f interaction between conduction carriers on C₆₀ and 4f electrons of Eu.