Structural difference between Fe/Cu and Fe/Ag granular films produced by a cluster beam method

Fe/Cu and Fe/Ag granular alloy films have been produced using a cluster-beam source: Fe clusters from the nozzle of a high temperature crucible are directly dispersed in Cu and Ag matrices obtained by simultaneous vapor deposition. They reveal a giant magnetoresistance effect without any heat treatment. High-resolution transmission electron microscope observations indicate the presence of nanometric Fe clusters together with ten-nanometer sized geometrical and chemical fluctuations in these films. Extended X-ray absorption fine structure experiments indicate that the local structure around Fe atoms is fcc for Fe concentrations less than 22 at.% and bcc for more than 35 at.% Fe in Cu matrices, while the structure of the Fe cluster is distorted bcc-like even for low Fe content down to 12 at.% in Ag matrices. These results suggest that small Fe clusters can diffuse on the substrate to form larger clusters. Small Fe clusters also intermix with Cu and Ag matrices to give rise to nanoscale heterogeneity. However, since the immiscibility of Fe with Ag is more remarkable than that with Cu, aggregation of Fe clusters is accelerated in Ag matrices.