Magnetic and transport properties of the pseudobinary systems Ce„Fe1ÀxCox...2 and „Ce1ÀyScy...Fe2

We studied the dilute substitution effect on magnetic and transport properties in an unstable ferromagnet CeFe2 with a C15 cubic Laves-phase structure. In the Co substitution system Ce(Fe12xCox)2 with x<0.10, while the Curie temperature TC decreases with increasing the Co concentration, an antiferromagnetic ordering appears in the low temperature region for x^0.05, and the transition temperature T0 from ferromagnetic to antiferromagnetic states monotonously increases with increasing the Co concentration. On the other hand, in the Sc substitution system (Ce12yScy)Fe2 with y<0.10, both the Curie temperature TC and saturation magnetization M S at 4.2 K gradually increase with increasing the Sc concentration. Despite the decrease in lattice parameter upon substitution in both the systems, an antiferromagnetic ground state is stabilized in Ce(Fe12xCox)2 , whereas a ferromagnetic ground state is stabilized in (Ce12yScy)Fe2. These results indicate that the Fe 3d – Fe 3d ferromagnetic exchange interaction and the antiferromagnetic spin correlation arising from the Ce 4 f – Fe 3d hybridization compete in CeFe2, and the enhancement/depression of the 4 f -3d hybridization effect might make the ferromagnetic ground state in CeFe2 unstable/stable. Furthermore, Ce(Fe12xCox)2 exhibited a negative giant magnetoresistance reaching about Dr/r5;60– 65 % at 4.2 K, which is accompanied by a metamagnetic transition from antiferromagnetic to ferromagnetic states. The giant magnetoresistance effect is originated from the reconstruction of Fermi surface due to the collapse of the superzone gap after the metamagnetic transition.