Effect of inserted Cu on current-perpendicular-to-plane-giant magnetoresistance of Fe50Co50 spin valves

We have reported that body-centered-cubic (bcc) Fe50Co50 with ultrathin Cu layers has large spin-dependent bulk scattering and enhances ADeltaR of spin valves. In order to clarify the role of Cu, two series of samples with (Fe50Co50 1 nm/Cu)x5 were prepared by two different sputtering machines. Machine A has a high rate, and machine B has a comparatively low rate. Both of ADeltaR in series A and B are increased by inserting thin Cu of about 0.1 nm, but they have a significant difference in Cu thickness dependence. While series A keeps high ADeltaR through the Cu thickness, ADeltaR in series B decreases when Cu is thickened. In order to find the reason for the difference, we took the extended x-ray absorption structure (EXAFS) spectra. By comparing ADeltaR and EXAFS, it was found that the very thin Cu of about 0.1 nm in Fe50Co50 takes a body-centered-cubic bcc structure, namely a solid solution state. Cu of 1 nm in spin valves with high ADeltaR takes a bcc and face-centered-cubic (fcc) evenly, and that with a low ADeltaR is a fcc structure. These results indicate that the existence of the solid solution Cu in Fe50Co50 is attributable to the enhancement of ADeltaR and are consistent with the report that Cu impurity in Fe and Co enhances the difference between the subband resistivity of up and down spins. (C) 2003 American Institute of Physics.