Transition from single-jump type to highly cooperative diffusion during structural relaxation of a metallic glass

The isotope effect E=(${\mathit{D}}_{\mathrm{\alpha }}$/${\mathit{D}}_{\mathrm{\beta }}$-1)/[(${\mathit{m}}_{\mathrm{\beta }}$/${\mathit{m}}_{\mathrm{\alpha }}$ ${)}^{1/2}$-1] of cobalt diffusion has been measured in melt-spun amorphous ${\mathrm{Co}}_{76.7}$ ${\mathrm{Fe}}_2$ ${\mathrm{Nb}}_{14.3}$ ${\mathrm{B}}_7$ ribbon at different stages of structural relaxation. A drastic drop of the isotope effect from E>0.5 in the as-quenched glass to E=0.1 in the relaxed state wass observed. While the latter value relflects highly cooperative diffusion, the large isotope effect in the as-quenched ribbon points to the prevalence of single-atom jumps and vacancylike holes of excess volume.