József Pádár

Giant Magnetoresistance in Co-Cu/Cu Multilayers Prepared by Various Electrodeposition Control Modes

The giant magnetoresistance (GMR) effect was studied on electrodeposited Co-Cu/Cu multilayers of 300 bilayer repeats which were produced in an electrochemical cell with homogeneous current distribution from a bath with two solutes (CoSO 4 ,CuSO 4 ). The preparation employed the conventional potentiostatic/potentiostatic and galvanostatic/galvanostatic, as well as an unprecedented galvanostatic/potentiostatic (G/P) control. We find that the specific deposition parameters rather than the deposition mode itself are decisive for the magnitude of the GMR which could be as high as 10% measured at 1 kOe on substrate-free multilayers in optimized G/P conditions. For this new deposition mode, detailed studies on the dependence of GMR on Co and Cu layer thicknesses as well as the bath pH were performed. No oscillatory behavior of the GMR as a function of the Cu layer thickness could be observed. The results suggest the importance of a Co-dissolution and/or a Co vs. Cu exchange reaction after completing the deposition of each magnetic layer. These reactions lead to the formation of a Cu or Cu-rich interface layer prior to the electrochemical deposition of the actual Cu layer during the subsequent pulse in either deposition mode. It turned out that the properties of this interfacial layer (thickness, degree of chemical intermixing) strongly influence the resulting GMR behavior of the multilayer.

Nanocrystallization Studies of an Electroless Plated Ni-P Amorphous Alloy

Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements were performed on an electroless plated Ni-P amorphous alloy to study the influence of different heat-treatments (linear heating and isothermal annealing). The phases formed after crystallization and the average grain size of the crystallization products were determined from XRD line broadening, and the heat evolved during the structural transformations were established from DSC measurements. A detailed study of the transformation products obtained along different heating routes was performed. From these studies, a scheme of the structural transformations and their energetics was constructed. The grain boundary energies in the different nanocrystalline states were also estimated.

Ferromagnetic and Superparamagnetic Contributions in the Magnetoresistance of Electrodeposited Co-Cu/Cu Multilayers

Co-Cu/Cu multilayers with Cu layer thickness ranging from 0.37 to 3.45 nm were deposited by galvanostatic/potentiostatic method from electrolytes with 5-200 mM Cu 2 + concentration. The composition analysis revealed an increasing Cu content of the magnetic layers with c(Cu 2 + ). Magnetoresistance up to 8 kOe was measured for each sample. Samples with a thin Cu layer (i.e., 0.37 nm) exhibited anisotropic magnetoresistance. At higher Cu layer thicknesses, giant magnetoresistance was observed. The higher the Cu 2 + concentration, the smaller the slope of the magnetoresistance curves at low magnetic field and the higher the saturation field. Magnetoresistance curves were quantitatively separated into ferromagnetic and superparamagnetic contributions. While the ferromagnetic portion of the magnetoresistance varied with the Cu layer thickness in the same way, the superparamagnetic contribution was higher, the larger the Cu content of the magnetic layer. The size of the superparamagnetic regions decreased with increasing Cu content of the magnetic layers. The copper layer thickness dependence of the magnetoresistance properties could be elucidated by accounting for both the asymmetric nucleation of Co on Cu and vice versa and the variation of the Cu content of the magnetic layers. General conclusions on the electrodeposition of magnetic/nonmagnetic multilayers have also been drawn.

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.

Structure and thermal stability of a melt-quenched single-phase nanocrystalline Hf61Fe39 alloy

Abstract The formation of a single-phase nanocrystalline Hf61Fe39 alloy with the fcc-Hf2Fe structure was achieved in a single step by melt-quenching, similarly to the previously reported nanocrystalline HfNi5 phase in a melt-quenched Hf11Ni89 alloy. Structural studies by X-ray diffraction and transmission electron microscopy have been performed for the nanocrystalline Hf61Fe39 alloy in both the as-quenched state and after various heat treatments. It was found that the as-quenched state exhibits an average grain size of about 50 nm and a lattice parameter remarkably lower than the value known for the stoichiometric fcc-Hf2Fe phase. Differential scanning calorimetry has been used to reveal phase transformations and to establish the heat evolution and activation energy of transformation. The nanocrystalline Hf61Fe39 alloy shows high thermal stability with an activation energy of 1.29 eV corresponding to a grain growth above 800 K.