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

A detailed Lorentz transmission electron microscopy study of melt-spun Au 78 Co 22 and Au 86 Co 14 , 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 Au ∕ Co 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.