ISOTOPE EFFECT OF CO DIFFUSION IN AMORPHOUS CO76.7FE2NB14.3B7

Utilizing the radiotracers Co-57 and Co-60, we have investigated the isotope effect of Co diffusion in melt-spun amorphous Co76.7Fe2Nb14.3B7 between 626 and 642 K. The ion-beam sputtering technique was employed for serial sectioning. For comparison, measurements were also carried out in fcc Co at 1049 K. The resulting isotope effect E=(D(alpha)/D(beta)-1)1[(m(beta)/m(alpha))1/2-1]=0.74+/-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+/-0.01 obtained in amorphous Co76.7Fe2Nb14.3B7 allows us to rule out a defect-mediated single-atomic-jump mechanism and is consistent with a diffusion mechanism involving about ten atomic masses.