Hyperfine field and ordering in bcc CoFe bulk alloys studied by 59Co NMR and Monte-Carlo simulation

Bcc CoFe alloys and ordered compounds with Fe concentrations of 50 at.%, 40 at.% and 30 at.% have been studied by 59Co NMR. Spectra with regularly spaced satellites are observed and interpreted as resulting from a step increase of the resonance frequency with each additional Fe impurity in the first coordination shell. The influence of the second-neighbour shell is also demonstrated. Satellite position analysis leads to the relationship between the hyperfine field and the first- and second-neighbour shell composition, which is found valid for the three compositions both in ordered compounds and disordered alloys. This relationship is used to determine the chemical short range order ruling the compound structure. The analysis reveals that the probability for Co to be found in perfectly ordered stoichiometric regions is higher than expected from a random distribution of the excess Co in off-stoichiometric compounds. Monte-Carlo simulations indicate that such short range order would result not only from first-neighbour interactions but also from rather large second-and/or third-neighbour interactions. However, the observations made during isothermal annealing show that the phase transformation takes place through a nucleation and growth process and not by way of homogeneous ordering. Therefore, the final short range order is also discussed in terms of kinetics of the phase transformation. (orig.). With 11 figs