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The purpose of the present study was to provide a reliable value for the specific grain-boundary resistivity ρSGBR of Ni metal. New results are presented on the room-temperature electrical resistiv...

The room-temperature magnetoresistance (MR) characteristics of nanocrystalline (nc) Ni metal with various grain sizes (between 30 and 100 nm) are investigated in this work for the first time. The nc-Ni foils were produced by electrodeposition and the results are compared with data measured on coarse-grained (bulk) pure Ni metal samples prepared by cold-rolling and annealing. The MR(H) curves measured in magnetic fields up to H = 9 kOe are analyzed in detail to determine the anisotropic magnetoresistance (AMR) ratio. The magnitude of the AMR ratio was found to be around 2.5% for bulk Ni and in the range from about 2 to 2.5% for the nc-Ni samples, the latter data not exhibiting a systematic dependence on the grain size. On the other hand, the field-induced resistivity anisotropy splitting ∆ρAMR in the magnetically saturated state of the nc-Ni series was found to be proportional to the zero-field resistivity of the same samples with different grain sizes. The slope of this proportionality relation provided an AMR ratio of 2.4% for all nc-Ni samples, matching well the value for the bulk Ni samples. Thus, the AMR ratio for polycrystalline Ni metal seems to be fairly independent of the microstructural features. This also means that the AMR ratio is an inherent characteristic of the Ni metal matrix and it remains the same even if the matrix resistivity changes (e.g., by introducing grain boundaries) without noticeably modifying the electronic density of states at least in the vicinity of the Fermi level.

Depth profile of electrodeposited Fe–Co–Ni alloys was measured using secondary neutral mass spectrometry with the reverse sputtering direction. A spontaneous near-substrate composition modulation was found for each sample studied. The initial deposit is very rich in Fe, which decays due to the depletion of the electrolyte in the vicinity of the cathode surface. The near-substrate modulation pattern can be elucidated by taking into account the mutual deposition preference of the iron-group metals and the mass transport of the ions in the solution. The composition fluctuations of Fe and Co at distances larger than 100 nm from the substrate are strictly correlated. Composition fluctuations beyond the near-substrate zone are attributed to the instability of the depleted electrolyte layer in the vicinity of the cathode. A critical analysis of earlier depth profile data for electrodeposited Fe–Co–Ni samples is also given.

It has long been known that a concentration gradient can develop in electrodeposits along the growth direction as a result of the combined impact of anomalous codeposition and mass trasnport limitation of the reactants. It is shown in the present paper that this composition change in electrodeposited Co–Ni–Cu/Cu multilayers can be verified by secondary neutral mass spectrometry (SNMS). Various electrodeposited samples have been analyzed and the resulting composition profiles have been compared to the calculation based on the non-destructive analysis of the overall composition. The change of the Co:Ni ratio in the magnetic layer along the growth direction has been verified. A calculation method has been proposed by which the local composition of the magnetic layer can be estimated from the thickness dependence of the overall multilayer composition. It has also been shown that the deviation of the experimental depth profile data from the estimation based on the variation of the total composition with the Co–Ni–Cu layer thickness can be ascribed to the roughness increase during the sputtering in the SNMS instrument.

Co–Ni–Cu/Cu multilayers have been deposited from a sulfate/sulfamate bath by controlling the current and the potential for the deposition of the magnetic and the copper layer, respectively. The magnetic layer thickness was varied between 1.2 nm and 12 nm, while the Cu layer thickness was in the range of 1.1–2.3 nm. Alloys with direct current have also been produced for comparison. All multilayer deposits exhibited a face centered cubic structure, while a small amount of hexagonal close packed phase was also present in the d.c. deposited alloys. The composition analysis showed that the codeposition of Co and Ni can be classified as anomalous codeposition for both d.c. and compositionally modulated deposits, regardless of the simultaneous deposition of Cu. The composition of the magnetic layer was affected by its layer thickness. This finding led to the conclusion that there must be a composition gradient in the magnetic layer in the growth direction. The magnetoresistance of the multilayers increased with increasing copper layer thickness, while the superparamagnetic contribution in the magnetoresistance increased with decreasing deposition current density of the magnetic layers.

The magnetoresistance (MR) and magnetization were studied at room temperature and down to 4.2 K for Ni 81 Cu 19 (3 nm)/Cu( d Cu ) multilayers with 0.5 nm ⩽d Cu ⩽3 nm and for a bulk Ni 81 Cu 19 alloy, all produced by electrodeposition. For most of the multilayers, giant magnetoresistance (GMR) was observed with a maximum around 1–1.5 nm Cu-layer thicknesses. The room-temperature MR curves consisted of a rapidly varying low-field component up to about 1 kOe and a slowly varying high-field component persisting beyond 18 kOe. For Cu-layer thicknesses around the maximum GMR, the low-field component dominated whereas for d Cu >1.5 nm, it became comparable to or smaller than the high-field component. For the bulk alloy, anisotropic magnetoresistance was observed. According to the room-temperature magnetization curves, both the bulk alloy and the multilayers exhibited ferromagnetic (FM) behaviour with technical saturation around 1–3 kOe. For the multilayers, the low-field MR curves with a small peak splitting were characteristic of uncoupled or weakly coupled magnetic layers. Magnetic measurements from 5 to 300 K indicated that besides an FM contribution, these multilayers also exhibit a superparamagnetic (SPM) component with blocking temperature around 220 K. From an analysis of the present results, the low-field MR component could be attributed to the FM domains with their magnetizations exhibiting a random in-plane distribution in the zero-field (demagnetized) state and the high-field MR component to a contribution due to SPM regions. All these considerations are supported by the complete saturation of the MR in a magnetic field of 4 kOe at T =4.2 K.

It turned out from room temperature electrical resistivity and magnetoresistance studies of electrodeposited Co metal and Co(Ru) dilute alloys that the incorporation of a small amount of Ru into Co significantly reduces the anisotropic magnetoresistance (AMR). The influence of Ru on the AMR of Co is explained by a drastic change of the asymmetry of d-band spin-up and spin-down electronic states at the Fermi level as revealed also by recent electronic band-structure calculations on Co-rich Co-Ru alloys.

Conventional and reverse depth profile analysis of electrodeposited Fe–Co–Ni alloys was performed by secondary neutral mass spectrometry (SNMS). It was found that the reverse sputtering method gave a much better depth resolution at the vicinity of the substrate. The reverse SNMS spectra showed that the deposition of Fe–Co–Ni alloys starts with the formation of an Fe-rich zone followed by an increase in Co concentration, then the nickel content increases and a steady-state alloy composition is achieved. At high current density, the initial depth pattern reproduces itself twice before the composition becomes stable. It was concluded that the varying depth profile is a consequence of the anomalous nature of the codeposition of the alloy components, the depletion of the electrolyte with respect to the metal salts, and the dependence of the intensity of the hydrogen evolution on the deposit surface composition. Keywords: Electrodeposition, Fe–Co–Ni alloys, Anomalous codeposition, Depth profile analysis, Secondary neutral mass spectrometry (SNMS)

Electrodeposited Ni-Cu alloys and nanoscale Ni-Cu/Cu multilayers were produced by direct-current plating and pulse-plating, respectively. The room-temperature electrical resistivity and thermopower as well as the Curie temperature for the Ni-Cu electrodeposits were in good agreement with relevant data reported for metallurgically processed Ni-Cu alloys. The same parameters were investigated also for the multilayers as a function of the constituent magnetic and non-magnetic layer thicknesses. The electrical resistivity of the multilayers was much larger than calculated for a parallel resistance model and their thermopower was more negative than expected on the basis of a volume average model, by using bulk values of both parameters for the sublayer materials. These differences were ascribed to surface scattering processes which can be significant in nanoscale multilayer structures.

This work is an extension of our previous structural study performed by conventional transmission electron microscopy (TEM) on electrodeposited Ni–Cu/Cu multilayers exhibiting giant magnetoresistance (GMR). It could be established from the present atomic-resolution structural studies that (i) the layered structure found by conventional TEM to be flat and smooth shows an interface roughness spread over a few atomic monolayers; (ii) the atomic planes cross, in most cases, continuously the Ni–Cu/Cu interface, i.e., forming a coherent superlattice; (iii) the atomic planes exhibit a periodic distortion due to the unrelaxed strain induced by the lattice mismatch between the magnetic and non-magnetic constituent layers.

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.

The microstructure and electrical transport properties were investigated for d.c.-plated Ni electrodeposits prepared from different types of electrolysing bath with deposition current densities ( i dep ) ranging from 2.5 to 50 A dm −2 onto either Cu or Ti substrates, which were subsequently removed from the deposits. The contamination of the Ni foils was investigated by several analytical techniques, and the impurity content (mainly Co and Fe) was found to be of the order of a few tenths of an atomic per cent. With decreasing deposition current density, the deposit grain size was found to decrease markedly, e.g. deposits with an average grain size of about 30 nm could be produced with i dep =5 A dm −2 from a bath containing one nickel salt (NiSO 4 ) only. The surface morphology on the bath side of the electrodeposited Ni foils was characteristic for the type of bath, and its roughness correlated well with the observed grain sizes. The room-temperature resistivity increased and the temperature coefficient of resistivity decreased markedly with decreasing grain size as expected, whereas the shift of the room-temperature thermoelectric power towards less negative values is not yet completely understood. In discussing the factors controlling the deposit grain size, it is concluded that the Ni 2+ ion content at the cathode-electrolyte interface and the amount of adsorbed hydrogen at the cathode surface may have a decisive role.

It has been shown recently that techniques such as severe plastic deformation and electrodeposition are able to yield porosity free massive nanocrystalline metal samples in a single-step process. In the present work, a comparison of the microstructures is made for nc nanocrystalline Ni and Cu samples prepared in the form of disk-shaped foils by these two techniques.

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.

The grain-growth process of electrodeposited nanocrystalline Ni foils upon heating has been studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The direct structural observations have shown that the exothermic calorimetric peak detected at about 600 K corresponds to a grain growth process. Furthermore, the TEM pictures indicate that dislocation loops are formed during grain growth. An almost complete recovery of the Curie point has also been observed, although its correlation with the structural changes is still to be revealed.

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.

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.

Magnetization studies were performed on amorphous Ni100−xPx alloys with 15 ≤ x ≤ 23 in the temperature range from 4.2 to 300 K and in fields up to 9 kOe. Samples with x

Mechanical, chemical and electrochemical polishing of the sensor core material was investigated on the operation of the Fluxset magnetic field sensor. A significant improvement of the sensitivity was found after proper polishing. The phenomenon was interpreted, taking into account chemical changes in the surface region, the effect of surface roughness on domain wall motion, volume changes in the core material, and the skin effect.

The overall quality of multilayer thin films prepared by electrodeposition could strongly be influenced by the surface and interface roughness. The roughness, however, may increase with the number of layers. For that very reason the reliable analysis of the first few layers is essential. However, in depth profiling methods based on sputtering techniques the first layer is always found at the bottom of the sputter crater. Since the depth resolution might decrease during sputtering, the analysis of the first few layers is often very difficult. In order to circumvent this problem, we performed SNMS depth profiling in the direction from the substrate. We prepared thin film samples in two ways. First, Co/Cu multilayer stacks were electrodeposited on Si/Cr/Cu substrates and SNMS depth profiling was carried out from the direction of the topmost layer. Secondly, electrodeposited Co/Cu multilayer stacks were coated with a few microns thick Ni layer and detached from the Si substrate in order to study the film structure from the side of the substrate. Using this latter method, which we denote as “reverse depth profiling”, we were able to analyze the first and, probably, the most even layers of the thin film structure with high resolution.

The time evolution of the hydrogen absorption of melt-quenched Zr50Ni50 amorphous alloy ribbons was investigated for the as-quenched state and after a relaxation heat treatment. Hydriding was performed from the gas phase at 200 °C and 35 atm H2 gas pressure for charging times up to t = 48 h. The absorption process was monitored by measuring the mass increase, the thermoelectric power and the magnetization of the hydrided ribbons. For both the as-quenched and relaxed state, homogeneously hydrided ribbons could only be obtained at small ( H M ) and high ( H M ≈ 1 ) hydrogen contents. The maximum amount of absorbed hydrogen was found to be the same for both states; however, the hydrogen absorption process in the relaxed ribbons required considerably longer times than for the as-quenched state. As in our previous study of a Zr47.5Ni52.5 metallic glass, the magnetization measurements indicated also for the present alloy the occurrence of a phase separation process leading to the development of strongly magnetic nickel-rich clusters for long charging times.

The influence of hydrogen on some physical properties and on the micro-structure of a nearly equiatomic, melt-quenched NiZr amorphous alloy was investigated. Hydrogen absorption from the gas phase at 190 °C and 35–40 atm pressure was measured by weight as a function of the charging time. Considerable hydrogen absorption occurred only for charging times around 10 h and was completed within a few hours. The hydrogen content remained constant for a hydrogen-to-metal ratio of about one for prolonged charging. The room-temperature thermopower changed considerably upon hydriding and a local thermopower probe revealed that the activation time varies considerably along the ribbon length. It was found that, during further heat treatment in a hydrogen gas atmosphere after a hydrogen-to-metal ratio of approximately one was reached, the magnetic state of the amorphous NiZr samples changed progressively from paramagnetic to superparamagnetic. The density and size of the magnetic clusters, which were nickel-rich segregations induced by the hydriding process, were determined by magnetic and scanning transmission electron microscopic measurements. After desorbing hydrogen at elevated temperatures, magnetic measurements indicated the disappearance of the superparamagnetic clusters.

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.

It is found that the magnetic state of the amorphous Ni 60 Zr 40 samples changes from paramagnetic to superparamagnetic upon hydriding. The density and the size of the magnetic clusters induced by the hydriding process was determined by magnetic and scanning transmission electron microscopic (STEM) measurements.

A comprehensive study of the magnetic properties of ${\mathrm{Ni}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ alloys prepared by electrodeposition with 11.5\ensuremath{\le}x\ensuremath{\le}23.2 and by melt quenching with 16.3\ensuremath{\le}x\ensuremath{\le}21.0 was performed for temperatures 4.2\ensuremath{\lesssim}T\ensuremath{\le}300 K in magnetic fields up to H=9 kOe and, for most of the melt-quenched alloys, for T\ensuremath{\ge}300 K including the molten state as well. The individual contributions to the magnetization were identified and determined separately. The matrix of Ni-P alloys was found to exhibit Pauli paramagnetism for x\ensuremath{\gtrsim}17 and very weak itinerant ferromagnetism for x\ensuremath{\lesssim}14. However, magnetic inhomogeneities in the form of ferromagnetic precipitates, giant-moment paramagnetic clusters and/or superparamagnetic particles could be identified throughout the whole concentration range studied and their amount and character varied significantly with alloy composition and preparation technique. In the paramagnetic phase, the temperature-independent Pauli susceptibility was not sensitive to the way of preparation, it agreed well with extrapolated room-temperature liquid-state data, decreased approximately linearly with increasing P content and extrapolated to the corresponding value of the crystalline stoichiometric compound ${\mathrm{Ni}}_{3}$P. In the ferromagnetic phase, the magnetization data of the matrix could be reasonably well accounted for in terms of the Stoner-Edwards-Wohlfarth model and the theory of Mathon, yielding 85.7 at. % Ni as the critical concentration for the onset of spontaneous magnetic order. For alloys in the intermediate composition range (14\ensuremath{\lesssim}x\ensuremath{\lesssim}17), the observed magnetization was dominated by the contribution of superparamagnetic particles.

A secondary neutral mass spectrometric (SNMS) depth profile study of electrodeposited Co/Cu multilayers was performed. Depth profile measurements were performed both in the conventional way (i.e., starting the sputtering from the final deposit surface) and in the reverse manner (i.e., detaching the multilayers from the substrate and starting the analysis from the substrate side, which was very smooth as compared to the final deposit surface). The latter method could yield significantly larger intensity fluctuations in the SNMS spectra. Surface roughness data were measured with atomic force microscopy (AFM) for multilayers with different bilayer numbers but otherwise exhibiting the same layer structure as those used for the depth profiling. The experimental AFM surface roughness evolution was used to calculate the result of the depth profile measurements quantitatively. An excellent agreement was obtained between this calculation and the SNMS measurements. It was shown that the decrease in the intensity fluctuations during the depth profile analysis stems mainly from the increase in surface roughness of the samples studied, especially in the conventional sputtering mode. It was also concluded that the thickness fluctuation of the entire multilayer deposit and that of each layer are strongly correlated.

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.

It is demonstrated that Co(Ru)/Ru multilayers with a hexagonal close-packed structure can be produced by electrodeposition. For appropriate layer thicknesses, clear giant magnetoresistance (GMR) behavior has been found. The observed GMR is very small (less than 0.1% for 8 kOe at room temperature) but not smaller than the GMR of Co/Ru multilayers prepared previously by high-vacuum methods. It is argued that neither structural imperfection of the multilayers nor chemical intermixing at the interfaces can be the main reason for the small GMR observed in the Co-Ru system for any preparation technique. It is suggested that the strongly reduced spin-dependent scattering characteristics of the interfacial Co-Ru alloys, which was described in a previous paper, might explain the low GMR of Co/Ru 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.

Electrodeposited amorphous Ni-P alloys were produced in an electrolyzing tank by using plane-parallel electrodes which did not fill the whole tank wall area. The resulting non-uniform current density distribution over the cathode area caused a lateral variation in the thickness and the phosphorus content and both parameters were measured locally on the deposits. The conditions for obtaining uniform electrodeposited amorphous Ni-P alloys and the dependence of deposit composition and cathode current efficiency on the local current density were established in detail. It was also found that during the deposition of usual foil thickness (10 - 100 μm) no bath deterioration occured.

Growth stages of amorphous Ni−P films deposited both by electrolytic and by chemical means were studied by electron microscopy. Nucleation and the growth of separated three dimensional grains have been proved.

The temperature- and field-dependence of magnetization, and the 31 P NMR linewidth and Knight shift of some amorphous Ni 100− x P x alloys with 18⩽ x ⩽ 22 prepared by melt-quenching and electrodeposition were studied below room temperature. Although all the alloys investigated contain magnetic inhomogeneities, the amount and nature of which depend on impurities and on the details of sample preparation, it could be established that the amorphous matrix of Ni 100− x P x alloys with x ⩾ 18 exhibit Pauli paramagnetism and the Pauli susceptibility was found to decrease rapidly with P content. From the results, a similarity of the electronic structure and the short-range order in amorphous Ni-P alloys and in the crystalline Ni 3 P compound is deduced.

Magnetization reversal characteristics of exchange-coupled magnetic films obtained by alternately electrodepositing soft Ni-Fe and hard Ni-Co layers on a wire substrate were studied with the help of a critical switching curve tracer. It is considered that the variety of magnetic nehaviour observed in such systems may serve, in some cases, as a simple model for understanding magnetization processes in soft magnetic materials containing magnetically hard regions larger than the domain wall width.

Room temperature measurements are presented of the 31P NMR Knight shift and the T1 and T2 relaxation times for several frequencies in Ni100−xPx (17.1 ≦ x ≦ 26.8) and Cu60Ni22P18 amorphous alloys. The experimental results are analysed in terms or the Korringa relation and of the possible electronic structure of these metallic glasses.