Structural, magnetic, and transport properties of Co nanoparticles within a Cu matrix

We have studied the structural, magnetic, and transport properties of Co nanoparticles embedded in a Cu matrix using x-ray absorption fine structure (XAFS) spectroscopy, macroscopic magnetization, and transport measurements. We observed by XAFS, on annealed samples containing 3, 10, 12, and 25 at. % Co, that there is a systematic contraction of the average coordination distance around Co atoms with the increasing Co content and annealing. The results are consistent with the growing of small fcc nanoparticles and correlated to the evolution in the superparamagnetic behavior, observed by magnetization measurements. We present a simple model to connect the diameter of Co nanoparticles to the average coordination distance and its mean-square fluctuation. Our analysis leads to cluster sizes ranging from 1 nm to more than 8 nm. We found that for a particle diameter of about 4.4 nm the sample containing 10 at. % Co presents a maximum in the giant magnetoresistance (GMR). Moreover, we observed that the disorder is significantly reduced around that size, due to the hardening of Co bonds, further favoring the GMR. Our analysis gives direct structural parameters and emphasizes their role in the transport and magnetic properties of the Co-Cu system.