Evolution of the Nanostructure of Giant Magnetoresistance Heterogeneous Alloys CoxAg1-x Upon Annealing

Heterogeneous alloys CoxAg1-x with atomic concentrations x=0.20 and 0.35, presenting giant magnetoresistance properties, have been studied by Total Electron Yield X-ray absorption spectroscopy at liquid nitrogen temperature at the Co K edge after annealings at various temperatures. For both sample concentrations, the Co atoms are mainly in small filament agglomerates for the as-deposited and low temperature annealed samples (T<150°C). In this temperature range, the local Co atomic environment is very stable and disordered. A structural change has been detected by XANES and EXAFS analysis between the 250 and 400°C annealings. After a 400°C annealing during 10 min, the nanostructures evolve towards larger, more compact and well ordered granules. The Co-Co distance varies from 0.246 to 0.248 nm as the annealing temperature increases indicating that the small filament-shaped agglomerates are under strain in the Ag matrix. The Debye-Waller factor of the Co0.35Ag0.65 samples decreases continously as the annealing temperature increases, as this factor drops suddenly after a 250°C annealing for the Co0.20Ag0.80 sample. The Co agglomeration is steadily larger for the Co0.35Ag0.65 sample than for the Co0.20Ag0.80 one. The Co-Ag contribution is negligible for the Co0.35Ag0.65 sample, as it represents approximately one third of the nearest neighbors for the Co0.20Ag0.80 sample from the as-deposit up to a 250°C annealing stage.