Direct observation of cobalt precipitates in nanocrystalline Cu-Co powder synthesised by laser ablation

Nanocrystalline Cu-Co (Cu-25Co by atomic fraction) powder for giant magnetoresistance (GMR) application has been synthesised by laser ablation method. The effect of Co precipitating evolution on the GMR properties during annealing has been investigated using energy filtered transmission electron microscopy (EFTEM) and X-ray diffraction. The as synthesised Cu-Co powder was 10 nm in diameter and formed a solid solution with an fcc structure. On the thermal stability during annealing the Cu-Co powder, the grain growth commenced before the phase separation of Cu and Co. As the result of the phase separation, nanocrystalline fcc Co particles with an average diameter of 5 nm were uniformly dispersed in Cu matrix. The relationship between the GMR property and the microstructural evolution of Co precipitate could be understood by the compositional profile showing a sinusoidal shape from EFTEM images.