Concentration dependence of the Co moment in amorphous alloys of Co with Y, La, and Zr

The concentration dependence of the Co moment has been measured in amorphous ${\mathrm{Y}}_{1-x}{\mathrm{Co}}_x$, ${\mathrm{La}}_{1-x}{\mathrm{Co}}_x$, and ${\mathrm{Zr}}_{1-x}{\mathrm{Co}}_x$ alloys ($0.33<~x<~0.88$). For the amorphous alloy systems the concentrations of Y, La, or Zr at which the Co moment disappears is much higher than for the crystalline systems. Furthermore the critical concentration is the same for Y and La ($1-x\approx 0.5$) but lower for Zr ($1-x\approx 0.4$). The result that Y and La yield similar data indicates that the size of the non-magnetic atom does not play a significant role in the magenetic properties. This is in contrast to the results for amorphous YFe and LaFe alloys. The lower critical concentration for the Zr alloys suggests a charge-transfer mechanism for the reduction of the Co moment upon alloying. The data were found to fit a simple charge-transfer model in which Y or La contributes about 1.4 electrons to the Co $d$ states while Zr contributes 2.2. However, it was also observed that the Co moment was proportional to ${(x-x_0)}^{\frac{1}{2}}$, where $x_0$ is the critical concentration of Co, and that the Co-Co exchange constant was independent of concentration. These latter results suggest that the Co moments are quite localized in the amorphous alloys.