The lattice parameter of nanocrystalline Ni as function of crystallite size

An anomalous dependence on crystallite size of the lattice parameter of nanocrystalline Ni as occurring in ultrathin Ni films and in ball-milled Ni powder was observed: lattice contraction followed by lattice expansion with decrease in crystallite size. These data were determined by application of detailed in-situ X-ray diffraction measurements during annealing treatments. To this end, the lattice parameter of Ni was accurately determined by correcting for influences of residual stress and stacking faults in the thin films and the ball-milled Ni powders, respectively. The non-monotonic variation of the lattice constant was discussed in terms of competing mechanisms: interface stress induced contraction versus magnetic multidomain to single-domain transition occurring at about the crystallite size value where a minimum value for the lattice parameter is observed.