Structure and giant magnetoresistance behaviour of Co-Cu/Cu multilayers electrodeposited under various deposition conditions

Electrodeposited Co-Cu/Cu multilayers were prepared under a variety of deposition conditions on either a polycrystalline Ti foil or on a silicon wafer covered by a Ta buffer and a Cu seed layer. X-ray diffraction (XRD) revealed a strong (111) texture for all multilayers with clear satellite peaks for the multilayers on Si/Ta/Cu substrates, in some cases for up to three reflections. Cross-sectional (1) transmission electron microscopy investigations indicated a much more uniform multilayer structure on the Si/Ta/Cu substrates. The bilayer periods from XRD satellite reflections were in reasonable agreement with nominal values. An analysis of the overall chemical composition of the multilayers gave estimates of the sublayer thickness changes due to the Co-dissolution process during the Cu deposition pulse. The XRD lattice spacing data indicated a behaviour close to a simple "multilayer" Vegard's law which was, however, further refined by taking into account elastic strains as well. In agreement with the structural studies, magnetoresistance data also indicated the formation of more perfect multilayers on the smooth Si/Ta/Cu substrates. An analysis of the magnetoresistance behaviour revealed the presence of superparamagnetic (SPM) regions in the magnetic layers. The contribution of these SPM regions to the total observed giant magnetoresistance was found to be dominating under certain deposition conditions, e.g., for magnetic layer thicknesses less than 1 nm (about 5 monolayers).