Structure and magnetic properties of nano-structured heterogeneous Au-Co alloys

High-resolution and analytical transmission electron microscopy, X-ray diffraction, magnetic and magnetoresistance measurements were used to investigate nanostructures of melt-spun Au₆₆Co₁₄ and Au₇₈Co₂₂ alloys. The microstructure of the Au₈₆Co₁₄ alloy was composed of very small Co precipitates inside the Au grains with some larger Co precipitates (20-35 nm) dispersed at the grain boundaries, while the microstructure of the Au₇₈Co₂₂ alloy consisted of Au/Co lamellar eutectic grains with Co precipitates (50-70 nm) dispersed at the grain boundaries. A few grains had very small (4 nm) Co precipitates. Annealing at 773 K for 10 min caused Co depletion in the Au matrix from 5.4-10 at.% to 0.9-2.0 at.%. Annealing also caused transitions from superparamagnetic to ferromagnetic and from single to multiple domain magnetic structure of some of the small and some of the larger Co precipitates, respectively. The MR ratios (Δρ/ρ, in magnetic field of 14.5 kOe) of the as-cast Au₈₆Co₁₄ and Au₇₈Co₂₂22 alloys were 2.5% and 2.6%, respectively. Annealing of the alloys at 673 K for 1 hr reduced Δρ/ρ to 0.9-1%.