Direct current plating, pulse plating, two-pulse plating, and reverse pulse plating were used to produce electrodeposited Co-Cu alloys and Co-Cu/Cu multilayers under galvanostatic control from an electrolyte containing CoSO 4 and CuSO 4 . Atomic force microscopy, X-ray diffraction, and transmission electron microscopy were used to study the sample structure and morphology. Direct current plating resulted in a Co 95 Cu 5 alloy with nearly equal amounts of face-centered cubic ͑fcc͒ and hexagonal close packed phases, while all pulsed current methods yielded multilayers with fcc structure. Giant magnetoresistance ͑GMR͒ behavior was observed in the multilayers with a maximum magnetoresistance ͑MR͒ ratio of about 9% as measured at 8 kOe. The shape of the MR curves and the magnitude of the GMR were very similar, regardless of the sign of the current between the Co deposition pulses. The results of structural studies also confirmed the formation of a multilayer structure for each pulsed electrodeposition mode. The conclusion was that the spontaneous exchange reaction between Co and Cu 2ϩ is responsible for the formation of a pure Cu layer even under reverse pulse plating conditions. The GMR of the multilayer deposits decreased with increasing bilayer number, due to the deterioration of the microstructure as the deposit grew.

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 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).

An X-ray diffraction study of electrodeposited Co/Cu multilayers with Cu layer thicknesses ͑d Cu ͒ from 0.5 to 4.5 nm revealed that, from structural point of view, three thickness ranges can be distinguished. For d Cu Ͻ 2 nm, a few percent of hexagonal close-packed ͑hcp͒-Co fraction is present and no superlattice satellite peaks can be observed. For d Cu Ͼ 2 nm, no hcp reflections can be detected whereas clear satellite reflections appear for 2 nm Ͻ d Cu Ͻ 4 nm around the main face-centered cubic ͑fcc͒͑111͒ reflection. For d Cu Ͼ 4 nm, these satellite peaks can hardly be seen again. These findings can be explained by the presence of pinholes in the Cu layers for d Cu Ͻ 2 nm, the formation of a coherent superlattice structure for 2 nm Ͻ d Cu Ͻ 4 nm and the degradation of the multilayer structural quality for d Cu Ͼ 4 nm. The intermediate Cu thickness range is also characterized by the strongest fcc͑111͒ texture and by the largest structural perfectness. These structural data will be very helpful in explaining magnetoresistance results on the same multilayers.

No abstract