Magnetoresistance and Structural Study of Electrodeposited Ni-Cu/Cu Multilayers

Electrodeposition was used to produce Ni-Cu/Cumultilayers by two-pulse plating (galvanostatic/potentiostatic control) from a single sulfate/sulfamate electrolyte at an optimized Cu deposition potential for the first time. Magnetoresistance measurements were carried out at room temperature for the Ni-Cu/Cu multilayers as a function of the Ni-Cu and Cu layer thicknesses and the electrolyte Cu2+ ion concentration. Multilayers with Cu layer thicknesses above 2 nm exhibited a giant magnetoresistance (GMR) effect with a dominating ferromagnetic contribution and with low saturation fields (below 1 kOe). A significant contribution from superparamagnetic (SPM) regions with high saturation fields occurred only for very small nominal magnetic layer thicknesses (around 1 nm). The presence of SPM regions was concluded from the GMR data also for thick magnetic layers with high Cu contents. This hints at a significant phase-separation in Ni-Cu alloys at low-temperature processing, in agreement with previous theoretical modeling and experiments. Low-temperature measurements performed on a selected multilayer down to 18 K indicated a strong increase of the GMR as compared to the room-temperature GMR. Structural studies of some multilayer deposits exhibiting GMR were performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns of Ni-Cu/Cu multilayers exhibited in most cases clear satellite peaks, indicating a superlattice structure which was confirmed also by cross-sectional TEM. The deterioration of the multilayer structure revealed by XRD for high Cu-contents in the magnetic layer confirmed the phase-separation concluded from the GMR data. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.090203jes] All rights reserved.