Ágnes Cziráki

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.

Structure and properties of fine-grained electrodeposited nickel

Abstract The structure and some physical properties (electrical resistivity and its temperature coefficient, thermoelectric power, Curie temperature) were investigated for electrodeposited Ni foils. The values of these parameters deviated from those of equilibrium, coarse-grained physical properties could be ascribed to the nanocrystalline structure of the deposits which was verified by transmission electron microscopy and X-ray diffraction.

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.

Microstructure and growth of electrodeposited nanocrystalline nickel foils

In the present work, the structure of electrodeposited pure Ni foils has been investigated by X-ray diffractometry, transmission electron microscopy and by measuring their electrical transport properties. It was found that the as-deposited Ni foils have a nanocrystalline structure covered by a thin amorphous Ni layer on the substrate side: the growth of the electrodeposited foils starts in amorphous form followed by nanocrystalline layers. To explain the formation of the amorphous Ni layer, it is supposed that foreign atoms are incorporated into the nucleating Ni films.

A kinetic model of the spontaneous passivation and corrosion of zinc in near neutral Na2SO4 solutions

The corrosion kinetics of zinc in sulphate solutions is determined mainly by the mass transport and migration properties of oxide/hydroxide film formed by spontaneous passivation. We have found that the corrosion reaction in 0.5 mol/dm 3 Na 2 SO 4 pH = 5 solution is under anodic control, whereas in 0.1 mol/dm 3 Na 2 SO 4 solution it is under cathodic control. The electrochemical investigations have cleared up that the SO 4 2- -ion concentration influences the rate constant of Zn dissolution due to complex-formation and the porosity of the oxide layer which in turn influences the exchange rate of hydrogen evolution on Zn as well. Subsequent X-ray diffraction, scanning and transmission electronmicroscopy analyses of the formed films have revealed some structural and compositional differences in the corroded Zn surface and at the corrosion layer/solution boundary, respectively. In solution of lower SO 4 2- -ion concentration, a depletion space-charge layer is detectable in the ZnO film. In a narrow potential range of the corrosion potential the Mott-Schottky plot of the electrode capacitances-electrode potential data gives a donor density ∼ 7 x 10 19 cm -3 arising from the n-type non-stoichiometry of the ZnO. The less porous corrosion film behaves as a weakly degenerate ZnO semiconductor. The flat band potential in the space charge layer has been found to be ∼ - 1.08 V vs see.

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.

Giant magnetoresistance in self-supporting electrodeposited NiCu/Cu multilayers

Abstract A sulphate bath was used to produce typically 5 μm thick electrodeposited NiCu/Cu multilayer foils with up to several thousand repeats. After removing the Ti substrate, the room-temperature magnetoresistance was studied as a function of the ferromagnetic NiCu layer thickness. Giant magnetoresistance was observed, which peaked at about 2% for a NiCu layer thicknesses around 2 to 3 nm.

Structural transformations and their relation to the optoelectronic properties of chromium oxide thin films

The crystallization behavior of chemically vapor deposited chromium oxide films was characterized by X-Ray diffractometry and the vibrational properties of the films were analyzed by Fourier transform infrared (FTIR) spectroscopy as a function of the annealing temperature and the technological conditions. The effect of the oxygen content in the CVD reactor on the optical characteristics (complex refractive index, optical band gap) was considered.

Structural evolution during growth of electrodeposited Co-Cu/Cu multilayers with giant magnetoresistance

Abstract The maximum room-temperature giant magnetoresistance (GMR) of electrodeposited Co–Cu/Cu multilayers produced during this work was approximately 9% at 8 kOe, and it was found to decrease with increasing bilayer repeat number. A transmission electron microscopy study has revealed the fine details of the microstructure formed during growth. At the beginning of the deposition very small, nano-sized crystallites formed with both hexagonal close-packed (hcp) and face-centred cubic (fcc) crystal structures containing a high level of internal stress. The Cu-content of these small crystallites was found to depend strongly on their crystal structure (fcc or hcp). After this initial polycrystalline region, the size of crystallites increases, forming an fcc superlattice with increasing average Cu concentration at the first hundreds of repeat periods. This increase is not monotonous across the whole sample thickness. As another effect, the bending of layer planes becomes more remarkable as the growth progresses. The above inhomogeneities formed during the deposition of hundreds of bilayers could be responsible for the decrease in GMR with increasing total thickness of the multilayered samples.