Microstructure formation in electrodeposited Co–Cu/Cu multilayers with GMR effect: Influence of current density during magnetic layer deposition

The influence of the current density applied during deposition of magnetic layers on the microstructure formation in electrodeposited Co–Cu/Cu multilayers and on their giant magnetoresistance was investigated using a combination of magnetoresistance measurements, wide-angle and small-angle X-ray scattering, high-resolution transmission electron microscopy, atomic force microscopy and X-ray spectroscopy. The magnetoresistance measurements revealed that a reduction in the current density stimulates a transition from the formation of the magnetic layers with predominantly ferromagnetic character to the formation of superparamagnetic regions. As based on electrochemical considerations, it was supposed that such a change in the magnetic properties can be caused by an increased amount of Cu codeposited with Co at low current densities. It turned out from the structural studies that a pronounced segregation of Co and Cu occurs at low current densities. In accordance with their very low mutual solubility at room temperature, no atomic scale intermixing of Co and Cu could be detected. The segregation of Cu and Co was related to the fragmentation of the magnetic layers, to the enhancement of the local lattice strains, to the increase in the interface corrugations, to the partial loss of the multilayer periodicity and, finally, to the formation of Co precipitates in the Cu matrix.

Research highlights
► Microstructure of electrodeposited Co/Cu multilayers depends on deposition current.
► A very important quantity is the amount of Cu that is codeposited with Co.
► The amount of Cu codeposited with Co increases with decreasing current density.
► Cu is not distributed homogeneously in Co layers but segregates at grain boundaries.
► This leads to the fragmentation of Co layers, which strongly affects the MR effect.