Enikö Tóth-Kádár

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.

Giant magnetoresistance of electrodeposited Ni81Cu19/Cu multilayers

The room-temperature magnetoresistance (MR) characteristics were investigated for electrodeposited Ni 81 Cu 19 /Cu multilayers as a function of the constituent magnetic and nonmagnetic layer thicknesses. The maximum giant magnetoresistance (GMR) was obtained around 3 nm magnetic layer thickness and 1 nm nonmagnetic layer thickness. For multilayers with GMR behavior, especially around the optimum sublayer thickness combination, the MR curves could he considered as consisting of a low-field (H I kOe) contribution. The latter contribution can he ascribed to chemical intermixing at the interfaces between the magnetic and nonmagnetic layers due to an exchange reaction between Cu and Ni. We have identified two factors that may significantly influence the GMR of electrodeposited multilayers: (i) the position and orientation of the investigated sample section on the cathode surface during deposition and (ii) the deterioration of the particular citrate/sulfate bath used.

Giant magnetoresistance in self-supporting electrodeposited NiCu/Cu multilayers

Abstract A sulphate bath was used to produce typically 5 μm thick electrodeposited Niue5f8Cu/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 Niue5f8Cu layer thickness. Giant magnetoresistance was observed, which peaked at about 2% for a Niue5f8Cu layer thicknesses around 2 to 3 nm.

Selective catalytic hydrogenation of bifunctional compounds over amorphous nickel alloys

Abstract The hydrogenation of bifunctional carbonyl compounds (2,2,4,4-tetramethyl-1,3-cyclobutanedione (1), 5-hexen-2-one (2) and (−)−verbenone (3)) was studied over amorphous Ni-P and Ni-B alloys. The amorphous Ni alloys were Ni-P and Ni-B powders prepared by chemical reduction, and Ni-P foils prepared by electrolytic reduction or rapid quenching. A commercial Raney Ni and 3% Ni/SiO2 served as reference catalysts. The amorphous alloys exhibited lower catalytic activities, but much higher selectivities as compared to the polycrystalline nickel catalysts. Selective monohydrogenation of 1,3-diketone (1) to the corresponding hydroxy ketone took place on all amorphous catalysts, except on Ni-P foil prepared by electrolytic reduction. Here, exclusive ring-opening attributed to acidic centres occurred. The hydrogenation of unsaturated carbonyl compounds (2 and 3) took place in two consecutive steps. At 373 K the selective saturation of carbon-carbon double bond occurred, while at 398 K the carbonyl group too was reduced to a hydroxy group, yielding saturated alcohols. In contrast, the conventional polycrystalline catalysts yielded a mixture of the corresponding hydroxy ketone and diol from 1, and saturated ketones and alcohols from 2 and 3, even at low temperature.

Electronic Transport in Nanocrystalline Metals: A Study of Electrodeposited Nickel Foils

The electrical resistivity (ρ), its temperature coefficient (a) and the thermoelectric power (S) has been studied at room temperature for nanocrystalline electrodeposited Ni foils prepared under different deposition conditions. With decreasing crystallite size, ρ increased, α decreased and S became less negative in agreement with the few previously reported results on other nanocrystalline metals. The temperature dependence of ρ was measured from 4.2 K to 300 K for some samples with small (≈50 nm) crystallite size. A large residual resistivity was observed and the temperature variation of ρ was also different from that of well-annealed, defect-free Ni with large crystallite size. It was concluded that the large residual resistivity cannot be completely ascribed to crystallite boundaries only but the presence of a high density of other types of lattice imperfections due to the nanocrystalline structure must also be assumed in the electrodeposited Ni foils whereas the different temperature behavior can be explained by considering that the electronic mean free path of the matrix is higher than the average crystallite size at low temperatures and the situation is reversed at room temperature.