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

Abstract

The Curie points (${T}_{C}$) of amorphous Ag-$X$ ($X=\mathrm{N}\mathrm{i},\phantom{\rule{0ex}{0ex}}\mathrm{C}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{d}$) films prepared by getter sputtering on substrates held at 77\ifmmode^\circ\else\textdegree\fi{}K, were determined without warming up the films above 77\ifmmode^\circ\else\textdegree\fi{}K. As revealed by susceptibility and electrical measurements, the films remain unchanged up to 220\ifmmode^\circ\else\textdegree\fi{}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 (\ensuremath{\simeq} 9\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}K for amorphous Ni. The validity of this extrapolation is supported by Ag-Co and Ag-Gd experiments.