Hyperfine fields at Co clusters in bulk Ag and Cu by a first-principles study: the role of the nearest neighbourhood

The hyperfine field distribution at substitutional Co clusters in fcc Cu and Ag hosts as a function of the chemical composition of the Co nearest neighbourhoods is studied. These clusters contain up to 135 Co and the theoretical ab initio calculations are performed with the real-space-LMTO-ASA method. We find that Co hyperfine fields exhibit an almost linear dependence on the number of host atoms in the first co-ordination shell. This result supports what has been recently suggested by an empirical model based on results inferred from NMR experiments. Nevertheless, we also observe a site dependence that is not completely described by this simple empirical model and discuss the role played by further neighbourhoods.