Antiferromagnetic coupling between Fe and Cu moments in superconductor YBa 2(Cu 1-xFe x) 3O y

Superconductivity and magnetic properties are investigated in YBa ₂(Cu _1-xFe _x) ₃O _y with x=0-0.19 and y=6-7 through DC magnetic susceptibility and electrical resistivity. With increase of Fe contents and oxygen deficiency, T_c decreases and, furthermore, their normal-state conductivity also decreases markedly, of which temperature dependence is approximated by In T, suggesting a weakly localized state in two dimensional systems. The susceptibility of all samples obeys the modified Curie-Weiss law. The effective Bohr magneton number P_eff per Fe calculated from Curie constants decreases from 5.5 to 3.1 with an increase of Fe concentration up to x=0.19. Oxygen deficiency depresses these P_eff to a minimum value of about 1.5 but more deficiency such as y=6 raises them up to about 3.0. These results indicate that Cu spins are weakly frozen around Fe atoms through weak localization due to Fe impurity and oxygen deficiency, and that their moments antiferromagnetically couple with the Fe moment locally without long range magnetic order.