Structural evolution of Co clusters in Co15Cu85 granular alloys by EXAFS spectroscopy

Abstract Metastable Co 15 Cu 85 alloys, produced by melt-spinning, have been annealed at increasing temperatures ( T A =400°C, 450°C, 500°C, 550°C, 600°C, 650°C) in order to study the evolution of the Co clusters and its relation with magnetic and transport properties. A maximum value of 10% in the magnetoresistance at room temperature is found after annealing at 500°C, when the samples are nearly superparamagnetic. A transition to a ferromagnetic behavior is detected with increasing annealing temperature. EXAFS measurements have allowed us to follow in great detail the appearance and evolution of the Co clusters inside the Co–Cu matrix as a function of the annealing temperature. This sensitive technique allows to detect that when annealing at 500°C, the Co segregation process is interrupted, presumably because the solubility of Co in Cu becomes appreciable at that temperature.