Composition depth profile analysis of electrodeposited alloys and metal multilayers: the reverse approach

The reverse depth profile analysis is a recently developed method for the study of a deposit composition profile in the near-substrate zone. The sample preparation technique enables one to separate the deposit and a thin cover layer from its substrate, and the initial roughness of the sample is much smaller than in the conventional sputtering direction. This technique is particularly suitable to study the zones being formed in the early phase of the electrodeposition of alloys. It has been demonstrated with the reverse depth profile analysis that in many cases when one component of an alloy is preferentially deposited, an initial zone is formed that is rich in the preferentially deposited component. This phenomenon is demonstrated for Ni Cd, Ni Sn, Fe Co Ni, Co Ni and Co Ni Cu alloys. The composition change is confined to the initial 150 nm thick deposit, and it is the result of the interplay of the deposition preference and the depletion of the electrolyte near the cathode with respect to the ion reduced preferentially. The reverse depth profile analysis made it possible to compare the measured and the calculated composition depth profile of electrodeposited multilayers. It has been shown that the decay in the composition oscillation intensity in Co/Cu multilayers with the increase of the sputtering depth can be derived from the roughness measured as a function of the deposit thickness

Electrodeposition of Ni from various non-aqueous media: the case of alcoholic solutions

Although electrodeposition from aqueous media has been widely used to obtain metallic deposits, there are cases where the application of non-aqueous solutions offers advantages over the traditional baths or even represents the only way to electrodeposit some metals. For this reason, a study of the electrodeposition of Ni from various alcoholic solutions was performed. Apart from a detailed cyclic voltammetry study of these solutions, the surface morphology, crystal structure and texture as well magnetic properties of the deposits have also been investigated. The best results were obtained with methanol as solvent, so results on Ni deposits prepared from this solution will be presented in more detail whereas the case of the other alcoholic solutions investigated will be summarized only briefly. Structural and magnetic properties of the deposits obtained have been compared to literature data on Ni samples obtained from various non-aqueous solvents

Composition, morphology and electrical transport properties of Co-Pb electrodeposits

Bath compositions were elaborated for the codeposition of Co and Pb by taking into account the chemical compatibility of Co2+ and Pb2+ with the appropriate anions. Electrolytes containing either acetate, chloride or nitrate anions were tested, but only the acetate bath proved to be appropriate for the preparation of compact Co-Pb films. Deposits were obtained with constant current, with constant potential or by using various current and potential pulses in order to investigate the possibility of multilayer formation. The variation in deposit composition as a function of the deposition parameters was elucidated by using cyclic voltammetry, current transient measurements and gravimetry. X-ray diffraction (XRD) patterns recorded for two-pulse plated deposits revealed a nanocrystalline structure with grain sizes in the range 5 to 20 nm. The XRD peaks could be well indexed to pure face-centered cubic Co and Pb, indicating that the Pb codeposited with Co is not dissolved in Co but is segregated. Both the d.c.-plated and the two-pulse plated deposits exhibited anisotropic magnetoresistance without an indication for a noticeable giant magnetoresistance contribution. This means that the observed magnetoresistance arises from spin-dependent electron scattering events dominantly within the sufficiently large Co regions and not along electron paths between two Co regions via the Pb regions. Low-temperature resistivity measurements revealed a superconducting transition slightly below that of pure Pb. This may be ascribed to a proximity effect: the ferromagnetic Co grains suppress somewhat the superconductivity of the Pb phase due to the nanoscale phase mixture of the two constituents

Near-substrate composition depth profile of direct current-plated and pulse-plated Fe-Ni alloys

Composition depth profiles of d.c.-plated and pulseplated Fe-Ni alloys have been investigated with the reverse depth profile analysis method. When d.c. plating is applied, the mole fraction of iron near the substrate is higher than during steadystate deposition since iron is preferentially deposited beside nickel and the achievement of the steadystate deposition condition takes time. The steadystate composition was achieved typically after depositing a 90-nm-thick alloy layer. In the pulseplating mode, samples with nearly uniform composition could be obtained at a duty cycle of 0.2 or smaller, and a continuous change in the composition profile could be seen as a function of the duty cycle above this value. A constant sample composition was achieved with pulseplating in a wide peak current density interval. The composition depth profile was also measured for a wide range of Fe2+ concentration. The different characteristics of the composition depth profile as a function of the deposition mode can be explained mostly in terms of mass transport effects. The elucidation of the results is fully in accord with the kinetic models of anomalous codeposition and with the assumption of the superposition of a stationary and a pulsating diffusion layer. The results achieved help to identify the conditions for the deposition of ultrathin magnetic samples with uniform composition along the growth direction

Magnetoresistance and structural study of electrodeposited Ni-Cu/Cu multilayers

Electrodeposition was used to produce Ni Cu/Cu multilayers 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

Influence of Ag additive to the spacer layer on the structure and giant magnetoresistance of electrodeposited Co/Cu multilayers

In order to explore the possible surfactant effect of Ag on the formation of electrodeposited multilayers, Co/Cu(Ag) multilayers were prepared by this technique and their structure and giant magnetoresistance (GMR) were investigated. The multilayers were deposited from a perchlorate bath with various amounts of Ag+ ions in the solution for incorporating Ag atoms into the multilayer stack. Without Ag addition, secondary neutral mass spectroscopy (SNMS) indicated a well-defined composition modulation of the undermost Co/Cu bilayers. However, already at an Ag content as low as 1.8 at.% incorporated, SNMS showed a deterioration of the periodic multilayer structure. In agreement with the SNMS results, superlattice satellites were visible in the X-ray diffraction (XRD) patterns of the multilayers with up to 0.3 at.% Ag. The satellites were, however, very faint even for multilayers without Ag addition, indicating that the multilayers have high interface roughness and/or poor periodicity. In the absence of Ag and at the smallest Ag content investigated by XRD, a strong central multilayer peak and the weak superlattice satellites were complemented by weak diffraction maxima from non-periodic Co and Cu domains. In the Co/Cu(Ag) multilayer containing about 25 at.% Ag, i.e., nearly as much as Cu, XRD found a separate Ag(Cu) phase. In spite of the imperfect layered structure, a multilayer-type GMR behavior was observed in all samples up to about 10 at.% Ag incorporated in the multilayer stack. The GMR magnitude increased for Ag contents up to about 1 at.%, which implies that a small amount of Ag may have a beneficial effect through a slight modification of the layer growth and/or interface formation. However, for higher Ag contents beyond this level, the GMR was reduced in line with the structural degradation revealed by XRD and SNMS. For the highest Ag contents (above about 10 at.%), the GMR exhibited a behavior characteristic of a granular magnetic alloy, in agreement with the results of the structural study

Óriás mágneses ellenállás (GMR) elektrolitikus multirétegekben = Giant magnetoresistance (GMR) in electrodeposited multilayers

Teljesültek a projekt fő célkitűzései az elektrokémiailag előállított (ED) és óriás mágneses ellenállást (GMR) mutató mágneses/nemmágneses multirétegek gócképződési és rétegnövekedési folyamatainak tanulmányozására vonatkozólag. Eddig kilenc folyóirat közlemény jelent meg, köztük egy összefoglaló cikk a Progress in Materials Science-ben, egy beküldött cikk elbírálás alatt van, további 5 cikk pedig előkészületben van. Az ED Co/Cu multirétegek nemmágneses rétegéhez adalékolt Pb, Bi és Ag elemek közül a Pb esetén találtunk javulást a GMR-ben. A fürdő hőmérsékletének nem volt jelentős hatása a GMR-re ED Co(-Ni)/Cu multirétegek esetén. Atomerő mikroszkópiával (AFM) vizsgáltuk a felületi durvaság és a GMR korrelációját ED Co/Cu mulitrétegeknél, különösen a rétegnövekedés kezdeti szakaszában, valamint különböző körülmények között készült ED Ni-Co/Cu multirétegekben. A korábban alig vizsgált ED Fe-Co/Cu multiréteg rendszerre részletesen meghatároztuk a GMR jelenség megfigyeléséhez szükséges előállítási körülményeket. Megmutattuk, hogy ED Co/Cu, Ni-Co/Cu and Fe-Co/Cu multirétegek esetén nincs oszcilláló viselkedése a GMR-nek a nemmágneses réteg függvényében. Mélységprofil analízissel megállapítottuk, hogy ED Fe-Co-Ni and Ni-Fe rétegekben spontán összetételfluktuáció alakul ki leválásnál a hordozó közelében. Modelleztük a GMR változását a mágneses tér függvényében, hogy jobban megérthessük a kísérletileg megfigyelt GMR görbéket. | The major planned tasks of the project aimed mainly at studying the nucleation and growth processes of electrodeposited (ED) magnetic/non-magnetic multilayers with giant magnetoresistance (GMR) behavior were accomplished. A total of 9 papers including a review in Progress in Materials Science have already been published, 1 paper is under review and 5 more papers are under preparation. As to the influence non-magnetic elements (Pb, Ag and Bi) in the spacer layer on GMR in ED Co/Cu multilayers, a beneficial effect was found in the case of Ag. No significant influence of bath temperature on GMR could be demonstrated for ED Co(-Ni)/Cu multilayers. Atomic force microscopy (AFM) was used to investigate the surface roughness and its correlation with GMR in ED Co/Cu multilayers especially in the early stages of multilayer formation and in ED Ni-Co/Cu multilayers prepared under various conditions. The electrochemical preparation conditions for observing GMR were elaborated in detail for ED Fe-Co/Cu multilayers rarely investigated before. It could be shown that the GMR does not exhibit an oscillatory behavior as a function of the spacer layer thickness for ED Co/Cu, Ni-Co/Cu and Fe-Co/Cu multilayers. Depth profiling studies revealed a spontaneous composition vairation in the near substrate zone of ED Fe-Co-Ni and Ni-Fe layers. The magnetic field dependence of the GMR was modelled in order to better understand the experimentally observed GMR curves in multilayers

In-depth component distribution in electrodeposited alloys and multilayers

It is shown in this overview that modern composition depth profiling methods like secondary neutral mass spectroscopy (SNMS) and glow-discharge – time-of-flight mass spectrometry (GD-ToFMS) can be used to gain highly specific composition depth profile information on electrodeposited alloys. In some cases, cross-sectional transmission electron microscopy was also used for gaining complementary information; nevertheless, the basic component distribution derived with each method exhibited the same basic features. When applying the reverse sputtering direction to SNMS analysis, the near-substrate composition evolution can be revealed with unprecedented precision. Results are presented for several specific cases of electrodeposited alloys and mulitlayers. It is shown that upon d.c. plating from an unstirred solution, the preferentially deposited metal accumulates in the near-substrate zone, and the steady-state alloy composition sets in at about 150-200 nm deposit thickness only. If there is more than one preferentially deposited metal in the alloy, the accumulation zones of these metals occur in the order of the deposition preference. This accumulation zone can be eliminated by well-controlled hydrodynamic conditions (like the application of rotating disc electrodes) or by pulse plating where the systematic decrease in the duty cycle provides a gradual transition from a graded to a uniform composition depth profile. The application of composition depth profile measurements enabled detecting the coincidence in the occurrence of some components in the deposits down to the impurity level. This was exemplified by the GD-ToFMS measurements of Ni-Cu/Cu multilayers where all detected impurities accumulated in the Cu layer. The wealth of information obtained by these methods provides a much more detailed picture than the results normally obtained with bulk analysis through conventional integral depth profiling and help in the elucidation of the side reactions taking place during the plating processes

Influence of Pb additive to the spacer layer on the structure and giant magnetoresistance of electrodeposited Co/Cu multilayers

In an effort to see the possible surfactant effect of Pb on the formation of electrodeposited multilayers, Co/Cu(Pb) multilayers were prepared by this technique and their structure and giant magnetoresistance (GMR) were investigated. The multilayers were deposited from a perchlorate bath with various amounts of Pb2+ ions in the solution. The composition analysis by energy dispersive X-ray spectroscopy revealed that the Pb mole fraction in the deposit varies in a non-monotonous manner with Pb2+ ion concentration. By fitting the measured X-ray diffraction patterns, superlattice satellites could be identified in some of these multilayers. A ferromagnetic-type GMR behavior was observed for Co/Cu(Pb) deposits prepared from baths with small Pb2+ ion concentration, corresponding to the formation of a layered structure. The GMR magnitude decreased from 8 to 10 % with increasing Pb concentration and, also, changed to a superparamagnetic-type GMR; finally, for high Pb2+ ion concentrations, the magnetoresistance behavior turned over to anisotropic magnetoresistance characteristic of bulk materials