Amorphous ferromagnetic Ag-X(X=Ni,Co,Gd) alloys

The Curie points ($T_C$) of amorphous Ag-$X$ ($X=\mathrm{N}\mathrm{i},\mathrm{C}\mathrm{o},\mathrm{G}\mathrm{d}$) films prepared by getter sputtering on substrates held at 77°K, were determined without warming up the films above 77°K. As revealed by susceptibility and electrical measurements, the films remain unchanged up to 220°K; above this temperature one observes an irreversible decrease in resistivity accompanied by an irreversible increase in $T_C$ although the films are still amorphous at room temperature. The critical concentration for the appearance of ferromagnetism (41-at.% Ni) and the increase of of $T_C$ with Ni concentration (≃ 9°K/at.%) are quite close to the values reported for crystalline Ni-Cu alloys. These results suggest again the validity of the virtual-bound-state model over the rigid-band model and confirm that clustering is not a major factor in crystalline Cu-Ni alloys. An extrapolation of the present experiments to pure Ni leads to a $T_C$ of about 540°K for amorphous Ni. The validity of this extrapolation is supported by Ag-Co and Ag-Gd experiments.