Utilizing the radiotracers ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ and ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$, we have investigated the isotope effect of Co diffusion in melt-spun amorphous ${\mathrm{Co}}_{76.7}$${\mathrm{Fe}}_2$${\mathrm{Nb}}_{14.3}$${\mathrm{B}}_7$ between 626 and 642 K. The ion-beam sputtering technique was employed for serial sectioning. For comparison, measurements were also carried out in fcc Co at 1049 K. The resulting isotope effect E≡(${\mathit{D}}_{\mathrm{\alpha }}$/${\mathit{D}}_{\mathrm{\beta }}$-1)/[(${\mathit{m}}_{\mathrm{\beta }}$/${\mathit{m}}_{\mathrm{\alpha }}$${)}^{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 ${\mathrm{Co}}_{76.7}$${\mathrm{Fe}}_2$${\mathrm{Nb}}_{14.3}$${\mathrm{B}}_7$ allows us to rule out a defect-mediated single-atomic-jump mechanism and is consistent with a diffusion mechanism involving about ten atomic masses.

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