Lorentz microscopy and giant magnetoresistance of nanostructured heterogeneous Au-Co alloys

A detailed Lorentz transmission electron microscopy study of melt-spun Au78 Co22 and Au86 Co14, and microstructural changes upon annealing, was conducted. The microstructure of the as-cast alloys consisted of small grains with fcc Co precipitates dispersed at the grain boundaries. The interior structures of these grains were AuCo lamellar eutectic or very small Co precipitates dispersed in Au matrix, depending on the composition. Up to 10±1 at. % Co were dissolved in the Au matrix. This large amount of Co in the Au matrix reduced the mean free path, probably leading to low magnetoresistance values [MR(H=1.45 T) =2.4%-2.6%]. Annealing caused precipitation of Co out of the Au matrix and growth of some Co precipitates, which transformed to multidomain particles when exceeded 160-200 nm. Annealing at 673 K for 1 h caused a reduction of MR(H=1.45 T) to 0.7%-0.8%, mainly due to growth of Co precipitates. The magnetic and micromagnetic properties were correlated with the microstructural analyses using computer simulations. The Co lamellae and precipitates were single-domain particles and single-domain vortices, respectively.