Isotope Effect of Cobalt Diffusion in Amorphous Co sub 76.7 Fe sub 2 Nb sub 14.3 B sub 7

Utilizing the radiotracers exp 57 Co and exp 60 Co, the isotope effect of cobalt diffusion in melt-spun amorphous Co sub 76.7 Fe sub 2 Nb sub 14 B sub 7 between 626-642K has been investigated. The ion-beam sputtering technique was employed for serial sectioning. For comparison, measurements were also carried out in fcc Co at 1049K. The resulting isotope effect E = (D sub alpha /D sub beta --1)/\(m sub beta /m sub alpha ) exp 1/2 --1\ = 0.74 plus /minus 0.05 in polycrystalline Co is typical of diffusion via monovacancies in densely packed structures. In contrast, the extremely small value of E = 0.1 plus/minus 0.01 obtained in amorphous Co sub 76.7 Fe sub 2 Nb sub 14.3 B sub 7 allows one to rule out a defect-mediated single-atomic-jump mechanism and is consistent with a diffusion mechanism involving about ten atomic masses.