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, which display giant magnetoresistance, GMR, without any heat treatment. The experimental results of small angle X-ray scattering and high-resolution transmission electron microscopy indicate that a few nanometer Fe clusters are embeded in Ag matrices with 10 nm order geometrical and chemical fluctuations. The extended X-ray absorption fine structure measurement 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-disorder scattering even in high fields, because small Fe clusters in Ag matrices reveal a spin-glass character and Fe atoms at the interface a superparamagnetic. After annealing above 570 K, MR saturates at high fields, which is ascribed to the ferromagnetism of grown Fe clusters. The concentration dependence of electrical resistivity and MR can be interpreted by geometrical and magnetic percolation of nanometric Fe clusters.