TRANSITION FROM SINGLE-JUMP TYPE TO HIGHLY COOPERATIVE DIFFUSION DURING STRUCTURAL RELAXATION OF A METALLIC-GLASS

The isotope effect E = (D(alpha)/D(beta) - 1)/[(m(beta)/m(alpha))1/2 - 1] of cobalt diffusion has been measured in melt-spun amorphous Co76.7Fe2Nb14.3B7 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 was observed. While the latter value reflects 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.