STRUCTURAL, MORPHOLOGICAL, AND MAGNETIC STUDY OF NANOCRYSTALLINE COBALT-COPPER POWDERS SYNTHESIZED BY THE POLYOL PROCESS

Nanocrystalline CoxCu100-x (4 less than or equal to x less than or equ al to 49 at. %) powders were prepared by the reduction of metal acetat es in a polyol. The structure of powders was characterized by x-ray di ffraction (XRD), transmission electron microscopy (TEM), high resoluti on transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS) spectroscopy, solid-state nuclear magnetic res onance (NMR) spectroscopy, and vibrating sample magnetometry (VSM). As -synthesized powders were composites consisting of nanoscale crystalli tes of face-centered cubic (fcc) Cu and metastable face-centered cubic (fcc) Co. Complementary results of XRD, HRTEM, EXAFS, NMR, and VSM co nfirmed that there was no metastable alloying between Co and Cu. The N MR data also revealed that there was some hexagonal-closed-packed (hcp ) Co in the samples. The powders were agglomerated, and consisted of a ggregates of nanoscale crystallites of Co and Cu. Upon annealing, the powders with low Co contents showed an increase in both saturation mag netization and coercivity with increasing temperature. The results sug gested that during preparation the nucleation of Cu occurred first, an d the Cu crystallites served as nuclei for the formation of Co.