STRUCTURAL AND MAGNETIC EVOLUTION IN GRANULAR FE-AG ALLOYS PRODUCED BY THE CLUSTER BEAM TECHNIQUE

Using cluster beam sources, we have obtained Fe-Ag granular films, whi ch display giant magnetoresistance, GMR, without any heat treatment. T he experimental results of small angle X-ray scattering and high-resol ution transmission electron microscopy indicate that a few nanometer F e clusters are embeded in Ag matrices with 10 nm order geometrical and chemical fluctuations. The extended X-ray absorption fine structure m easurement indicates that the short range structure of Fe clusters is bcc but distorted in the as-deposited state. The magnetoresistance (MR ) does not saturate at 140 kOe: conduction-electrons suffer spin-disor der scattering even in high fields, because small Fe clusters in Ag ma trices reveal a spin-glass character and Fe atoms at the interface a s uperparamagnetic. After annealing above 570 K, MR saturates at high fi elds, which is ascribed to the ferromagnetism of grown Fe clusters. Th e concentration dependence of electrical resistivity and MR can be int erpreted by geometrical and magnetic percolation of nanometric Fe clus ters.