Lajos Pogány

Preparation and Magnetoresistance Characteristics of Electrodeposited Ni‐Cu Alloys and Ni‐Cu/Cu Multilayers

Galvanostatic electrodeposition was used to produce Ni-Cu alloys and Ni 81 Cu 19 /Cu multilayers by direct current (dc) plating and two-pulse plating, respectively, from a sulfate/citrate electrolyte. For the dc-plated Ni-Cu alloys, the deposition rate and the alloy composition were established as a function of the deposition current density, from which the appropriate deposition parameters for the constituent sublayers of the multilayers could be established. By measuring the resistivity at room temperature in magnetic fields up to H = 7 kOe, anisotropic magnetoresistance (AMR) was found for Ni 81 Cu 19 electrodeposits, whereas both giant magnetoresistance (GMR) and AMR contributions were observed for most Ni 81 Cu 19 /Cu multilayers. Finally, Ni-Cu alloys were also prepared by conventional pulse plating, varying the length of the deposition pulse (on-time) with constant separation (off-time) between the pulses. Clear evidence of a GMR contribution also appeared in these pulse plated Ni-Cu alloys which may be explained by the formation of a Cu enriched layer between the ferromagnetic layers deposited during the cathodic pulses. A quartz crystal microbalance experiment confirmed that an exchange reaction takes place during the off-time. These findings provide useful information on the formation mechanism of multilayers by the two-pulse plating technique.

Microstructure and Giant Magnetoresistance of Electrodeposited Co-Cu/Cu Multilayers

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 (fcc) and hexagonal close packed phases, while all pulsed current methods yielded multilayers with fcc structure, Giant magnetoresistance (GMR) behavior was observed in the multilayers with a maximum magnetoresistance (MR) 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 2+ 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.

Microstructure and electrical transport properties of pulse-plated nanocrystalline nickel electrodeposits

Abstract The microstructure and the electrical transport properties (the electrical resistivity, its temperature coefficient and the thermoelectric power) were investigated for pulse-plated nanocrystalline nickel electrodeposits. Transmission and scanning electron microscopy were used to study the microstructure (grain size and lattice defects) and the surface morphology respectively. The samples were prepared from the same bath as used previously for d.c. plating and the deposition current density was constant, in most cases i dep = 20 A dm −2 . In a given series, the pulse length t on was kept constant at 0.001, 0.01, 0.1, 1 or 10 s and the separation between pulses t off was varied from 0.001 s to 10 s. Systematic variations of the electrical transport parameters with t on and t off were observed, which we attempt to explain in terms of the periodic variation due to pulse-plating of the local Ni 2+ concentration at the cathode-electrolyte interface.

Correlation between interface structure and giant magnetoresistance in electrodeposited Co–Cu/Cu multilayers

Abstract An attempt has been made to understand the correlation between the interface structure and the giant magnetoresistance (GMR) properties of electrodeposited Co–Cu/Cu multilayers by measurements performed on a series produced by galvanostatic electrodeposition under the application of different capacitances connected parallel to the electrochemical cell, and this was expected to increase the width of the chemically intermixed interface between the magnetic and non-magnetic layer. In contrast to expectation, the GMR values of multilayers electrodeposited in the presence of a capacitance remained nearly unchanged even at the highest applied capacitance value, as a consequence of immiscibility of alloying elements.

Evolution of Structure with Spacer Layer Thickness in Electrodeposited Co ∕ Cu Multilayers

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 2 nm, no hcp reflections can be detected whereas clear satellite reflections appear for 2 nm 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 4 nm. The intermediate Cu thickness range is also characterized by the strongest fcc(111) texture and by the largest structural perfectness. These structural data will be very helpful in explaining magnetoresistance results on the same multilayers.