Additive Effects in Multilayer Electrodeposition: Properties of Co−Cu

J. Phys. Chem. B 2001, 105, 10867-10873

10867

Additive Effects in Multilayer Electrodeposition: Properties of Co-Cu/Cu Multilayers Deposited with NaCl Additive L. Pe´ ter,*,† Z. Kupay,†,§ A Ä . Czira´ ki,‡ J. Pa´ da´ r,† J. To´ th,† and I. Bakonyi† Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungary, and Department of Solid State Physics, Eo¨ tVo¨ s UniVersity, H-1518 Budapest, P.O. Box 32, Hungary ReceiVed: April 12, 2001; In Final Form: June 30, 2001

Electrodeposited Co-Cu/Cu multilayers have been produced with current control from a sulfate bath with and without NaCl as an additive. The addition of NaCl decreases the current efficiency of the multilayer deposition and results in multilayers with lower GMR and higher electrical resistivity than the chloride-free bath. The grain size of the deposit and the degree of orientation also decreases if the additive is present. It has been concluded that the adsorption of chloride, the formation of copper(I) intermediate, the change in the deposition mechanism, the increase in nucleation rate, and the occurrence of a non-Faradaic current transient all contribute to the enhancement of the structural disorder that leads to the loss in GMR. Some general conclusions have also been formulated about the role of additives in electrodeposition baths, especially concerning the differences in d.c. plating and pulse-plating.

1. Introduction Investigation of metallic multilayers has attracted much interest, especially since the discovery of the giant magnetoresistance (GMR) effect in magnetic-nonmagnetic multilayers with individual layer thicknesses in the nanometer range.1,2 Such multilayers are usually produced with high-vacuum methods, although the electrochemical deposition has long been considered as a favored alternative due to its lower demand for instrumentation. Despite the large number of the works published in this field,3-5 there have been several problems related to the electrochemical deposition of multilayers that are not sufficiently understood. One of the problems to be explored is the effect of bath composition on the properties of the resulting multilayer. Throughout this paper, only the so-called single-bath method will be dealt with. In this method, the ions of both the magnetic and nonmagnetic metals are present at the same time at a concentration ratio ranging from about 10 to a few hundred, and the composition modulation in the deposit is achieved with the modulation in either current or potential. The bath compositions applied are usually very similar to those developed for d.c. deposition of the magnetic metal in that the same complexing agents and main components are used. However, it was observed in the early phase of the investigation of multilayer electrodeposition that the application of surfactants, brighteners, and leveling agents are deleterious for the magnetoresistance (see, e.g., ref 6). The classification of the additives used in the d.c. electrodeposition of metals can be found in ref 7. However, the general explanation of the difference of additive effects in d.c. plating and multilayer pulse-plating is apparently still missing. * Corrresponding author. Phone: +36-1-392-2222; fax: +36-1-3922215; e-mail: [email protected]. † Hungarian Academy of Sciences. ‡ Eo ¨ tvo¨s University. § Undergraduate student at the Eo ¨ tvo¨s University, Budapest.

The empirical description of the effect of some additives and the synergetic effect of different sorts of additives on pulsed deposition of Co-Cu alloys are given in ref 8. It has been established by these authors that the additives modify the current efficiency and the rate of the exchange reaction, but an explanation at the molecular level has not been suggested. In this paper, we report results obtained for Co-Cu/Cu multilayers from a sulfate bath with and without NaCl as an additive. Current efficiency of the deposition, deposit composition, structural parameters of the deposits, resistivity, and magnetoresistance have been measured and discussed. On the basis of our data and those found in the literature, we make an attempt to explain the mechanism of the deposition of nanoscale multilayers in the presence of additives and complexing agents. 2. Experimental Section Electrodeposition was carried out with the same method as described in a previous publication.9 The bath contained 1.00 mol dm-3 CoSO4 (form CoSO4‚7H2O; Reanal), 25.0 mmol dm-3 CuSO4 (form CuSO4‚5H2O; Reanal), and occasionally NaCl at 0.5 or 2.5 mmol dm-3 concentration. All solutions were made with double distilled water, and a new solution was used every day. The bath temperature was maintained at 30 ( 0.3 °C. The bilayer number of the samples varied between 880 and 1500, to keep a nearly constant total multilayer thickness (at about 10 µm) when varying the sublayer thicknesses. The samples were deposited with current control by using a Keithley 228 current source. The current density was -0.6 and -32.5 mA cm-2 during the deposition of the copper spacer layer and the cobalt-rich magnetic layer, respectively. A mechanically polished titanium sheet was used as a substrate, and the 2 × 2 cm deposition window was shaped by covering the inactive areas with Scotch tape. After the deposition was completed, the samples were peeled off from the substrate and the weight of each sample was measured. The current efficiency for the deposition of the cobalt-rich layers was

10.1021/jp011380t CCC: $20.00 © 2001 American Chemical Society Published on Web 10/16/2001

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TABLE 1: Bilayer Number of the Samples in Group 1a deposition time of the Co-rich layer, t(Co)/s NaCl conc mmol dm-3

0 0.5 2.5

0.32 1500 1500 1500

0.45 1325 1500 1500

0.65 1325 1325 1325

0.85 1100 1100 1100

1.00 950 950 950

1.15 880 880 880

a Deposition time of copper was t(Cu) ) 5.0 s for each sample in this group.

calculated with the assumption that the current efficiency of copper deposition can be taken as 100%. Cyclic voltammetry was carried out with an EF 453 potentiostat (Electroflex, Hungary) under identical conditions than the multilayer deposition. The electrode potentials were measured and are referenced to a saturated calomel electrode (SCE). X-ray diffraction (XRD) was used to study the structure of the samples with the help of a Philips X’pert equipment. Scanning electron microscopy (SEM) was performed with a JEOL JSM 840 equipment, and the same instrument completed with an ORTEC System 5000 unit was used for electron probe microanalysis (EPMA). The elemental analysis was carried out by examining four to six spots of the sample surface and scanning an approximately 375 × 285 µm2 surface area in each case. Transmission electron micrographs were recorded with a Philips CM 20 equipment. The resistance of the samples was measured with the “four points in line” method. Calibration of the resistance tester was performed with a Cu sheet of the same lateral dimensions as the multilayer samples. Resistivity data were calculated by using the average thickness established from the weight of the samples and the weighted average density of the constituent metals (8.92 g cm-3). The error resulting from this method is low since the difference between the densities of Cu and Co is about 0.6%. For magnetoresistance measurements, a 2-mm wide strip was cut from the central area of the square-shaped samples, which was centered at 13 mm from the top edge as defined by the position of the sample during the deposition. The magnetoresistance measurements were performed at room temperature for magnetic fields up to H ) 8 kOe in the current-in-plane/fieldin-plane (CIP/FIP) configuration. Both the longitudinal (L) and transverse (T) component of the MR was measured by applying the magnetic field parallel and perpendicular, respectively, to the strip length. The magnetoresistance ratio (MR) was calculated according to the following formula: MR ) ∆R/R0 ) [R(H) - R0]/R0, where R0 is the resistance at zero external magnetic field, and R(H) is the resistance when the external magnetic field is H. The samples prepared during the present work can be divided into two groups. In group 1, the deposition time of the cobaltrich layer was varied at three different NaCl concentrations, as shown in Table 1. This series was prepared to investigate the physical properties of the samples and to establish the appropriate deposition conditions for studying the dependence of magnetoresistance on copper layer thickness. The samples in group 2 were deposited to study the magnetoresistance only, and the parameter varied was the deposition time of the copper layer at fixed deposition time of the cobalt-rich layer. 3. Results 3.1. Electrodeposition. Cyclic voltammetric (CV) curves for the deposition of Cu metal and Co-Cu alloys are shown in Figure 1. It can be seen that a fairly good limiting current region can be observed between -0.55 and -0.75 V on the cathodicgoing scan. The height of peak A preceding the stabilization of

Figure 1. Cyclic voltammetric curves obtained for deposition and dissolution of Cu and Co. Sweep rate, 15 mV s-1; other conditions are the same as for multilayer deposition. The inset shows the entire curve for the chloride-free solution.

the mass-transport limited current increases with the addition of NaCl to the solution. The current increase is similar to that observed by Kou and Hung.10 The onset of Co deposition is at about -0.80 V. The beginning of the cobalt deposition is followed by a sharp current rise and a short plateau (see B in insert of Figure 1); then beyond -1.1 V the current keeps increasing with potential. This latter part of the CV curves is identical for all chloride concentrations investigated. After the scan direction is reversed, the current-potential curve coincides with the cathodic-going curve in the -1.3 to -1.1 V region. The anodic-going scan does not show the plateau between -0.9 and -1.1 V but rather continues to decrease exponentially (see insert in Figure 1). The cobalt deposition stops at a less negative potential (about -0.7 V) than the current onset in the first segment. The mass transport limited current recovers very accurately between -0.6 and -0.5 V, and then the current increases and the stripping peak of the metals deposited can be observed. Although the stripping peak is composed of at least three subpeaks, it is difficult to establish which process is responsible for each, especially because of the presence of electrodeposited layers with continuously varying composition. In the presence of NaCl in the solution, the current becomes substantially higher at the negative edge of the stripping peak (peak C in Figure 1). The dissolution of the deposit is essentially finished at 0.5 V, but the small residual anodic current indicates that a losely adhered deposit is still present whose dissolution takes place quite slowly. It is also possible that the dissolution in this region is determined by the dissolution of the oxidized surface and the low conductivity of the oxide-rich metal residue. It was reported also previously10 that the height of the peak preceding the diffusion-limited current of Cu (peak A in Figure 1) increases with the chloride concentration of the solution. Nevertheless, the reason for this phenomenon is still unclear. The cobalt deposition occurs below -0.8 V only which can be explained by assuming that the nuleation of Co on Cu requires a high activation energy. This is easy to understand by taking into account that copper and cobalt do not form any mixed equilibrium phase. The plateau between -0.9 and -1.0 V originates from the high Co2+ concentration close to the electrode surface at the time of the beginning of the Co deposition. When the deposition of the cobalt-rich phase takes place on the surface of a cobalt-containing alloy (i.e., when the current approaches the diffusion-limited current of copper in the anodic-going scan), there is no need for the activation energy, and hence the decay of the current-potential curve is smooth. The plateau between -1.0 and -0.9 V is also missing

Properties of Co-Cu/Cu Multilayers

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10869 TABLE 2: Composition of Some Multilayer Samplesa c(NaCl)/mmol dm-3

x(Cu) substrate side

x(Cu) solution side

0 0.5 2.5

0.348 ( 0.004 0.31 ( 0.02 0.306 ( 0.005

0.39 ( 0.02 0.36 ( 0.05 0.51 ( 0.1

a x denotes the molar fraction). t(Co) ) 1 s, t(Cu) ) 5.0 s; for other data, see the experimental section.

Figure 2. Current efficiency of the electrodeposition of the magnetic layer in the multilayers. Cu deposition time, t(Cu) ) 5.0 s; other deposition conditions are as defined in the experimental section. The arrow shows the average current efficiency of the d.c. deposition for both the absence and presence of NaCl.

since the solution is already somewhat depleted due to the continuous deposition of Co. The increase of the dissolution current as a result of the Cl- addition can be explained with the complexing capability of the chloride ion that facilitates the formation of the dissolution products. The multilayer samples deposited from the chloride-free solution exhibit a gray surface with shiny spots. Although the macroscopic surface roughness at the shiny spots looks to be considerably smaller than at the rest of the surface, local electron probe microanalysis (EPMA) cannot detect any composition difference between the areas of different appearance. The thickness of the cobalt-rich layer can be varied in a wide range, but the increase in the copper layer thickness beyond about 2.5 nm leads to such a high mechanical stress that the sample spontaneously peels off from the substrate in the early phase of the deposition. Multilayer deposition from baths with NaCl additive, however, is not always possible with the same parameters as with the chloride-free solution. The higher the chloride ion concentration, the higher the magnetic layer thickness has to be chosen to obtain a continuous self-supporting deposit. Samples with too low cobalt layer thicknesses form a loosely adherent powder. The roughness of the samples established visually is higher for the samples prepared with the solutions containing chloride ions. The current efficiency of the electrodeposition of the cobaltrich layers as a function of the thickness of the magnetic layer and the NaCl concentration is plotted in Figure 2. It can be seen that the lower the NaCl concentration and the higher the thickness of the cobalt-rich layer, the higher is the current efficiency. For the samples with the chloride-free solution, the current efficiency attained a fairly constant value at 0.65 s deposition time of the magnetic layer. This current efficiency is close to that of the d.c. samples prepared with the same current density as the magnetic layer in the multilayers, indicated by the arrow in Figure 2. In the case of the solution with 2.5 mmol dm-3 chloride ions, however, the current efficiency established for the chloride-free solution is not achieved even at 1.15 s deposition time. The deviation from the current efficiency of the d.c. deposition is especially high if NaCl is present and the deposition time of the cobalt-rich layer is small. Data presented in ref 9 serve as a guide for estimating the thicknesses of the individual sublayers. In multilayers deposited from the chloride-free bath, the Cu layer thickness is dCu ≈ 1.1 nm for t(Cu) ) 5 s and the magnetic layer thickness is dCo ≈ 1.3 nm for t(Co) ) 0.32 s. Because of the increase of the current efficiency with t(Co) (Figure 2), the actual magnetic layer thickness for multilayer samples deposited from the chloridefree bath can be at most 10% higher than scaling simply with

the Co deposition time. However, this calculation method cannot be directly transferred to the samples deposited from the chloride-containing bath, as shown by the composition data below. 3.2. Composition. The composition of both the substrate and solution sides of the samples from group 1 deposited with t(Co) ) 1 s was measured by EPMA. The results are shown in Table 2. The data concerning the substrate side show that the lateral composition variation and the change of the deposit composition with chloride concentration is quite low. The copper content of each sample is higher at the solution side than at the substrate side, and the higher the chloride concentration of the deposition bath, the more significant copper content increment can be observed. While the difference in the molar fraction of copper is only about 0.05 in the absence of NaCl, the copper content of the sample deposited from solution of c (NaCl) ) 2.5 mmol dm-3 increases from x ) 0.31 to x ) 0.51 as the deposition proceeds. The large scatter of the composition measured for the solution side indicates that the lateral fluctuation of the sample composition is a function of the chloride concentration of the bath. The higher the chloride concentration, the higher lateral composition variation is obtained. It can be concluded that the presence of chloride ions in the deposition bath results in nonuniform distribution at the submillimeter scale. 3.3. Surface Morphology. The surface morphology of the samples was studied by SEM. The substrate side of each sample investigated was very similar in that the inverse of the substrate can be seen only, and the solution composition does not influence the surface morphology. However, the solution side of the samples shows different features as the chloride ion concentration varies. Figure 3 presents the characteristic SEM image of three samples deposited from baths of different NaCl concentrations but otherwise the same conditions. When NaCl was absent (Figure 3a), the surface structure is granular, the sample is dense, and the visually established thickness fluctuation is low. If the concentration of NaCl was 0.5 mmol dm-3 (Figure 3b), the sample surface is rather uneven as compared to the chloride-free deposit, the sample appears to be less dense, and small dendrites can be seen on the top of the granules. If the NaCl concentration is as high as 2.5 mmol dm-3 (Figure 3c), the dendritic surface morphology is predominant, and the cauliflowerlike granules almost disappear. The contact surface between the dendrites is low, and that explains well why the samples deposited from the chloride-rich solutions were much more fragile than those obtained from the pure sulfate solution. The surface area of the samples deposited from chloridecontaining solutions was higher than that obtained from chloridefree solutions, and hence the deposition current density was smaller in the former case. Also, the exchange reaction could take place more effectively due to the enhanced surface area. This explains the high copper content of the deposits of the samples deposited from chloride-rich solutions. 3.4. Structure. The results of the structural study by XRD for the samples deposited from the chloride-free solution were described in detail elsewhere.9 Briefly, the multilayers deposited

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Figure 4. Transmission electron micrograph of a sample deposited from the chloride-rich solution. c(Cl-) ) 2.5 mmol dm-3, t(Co) ) 0.65 s, t(Cu) ) 5.0 s; other deposition conditions are as defined in the Experimental section.

Figure 3. Scanning electron micrograph of the solution side of three samples. Figure (a) c(Cl-) ) 0; (b) c(Cl-) ) 0.5 mmol dm-3; (c) c(Cl-) ) 2.5 mmol dm-3. t(Co) ) 1.0 s, t(Cu) ) 5.0 s; other deposition conditions are as defined in the experimental section.

from the sulfate solution contained a very small amount of hcp phase only, usually less than 2%. The samples exhibited a columnar growth and a pronounced texture, the 〈111〉 direction coinciding with the growth direction. The lateral diameter of the grains (column width) ranged up to 40 nm. The multilayer structure was obvious from the satellite peaks of two-pulse plated, pulse-plated, and reverse-pulse plated samples.9 The repeat period (Λ ) dCo + dCu) of the multilayers established

from the position of the satellite peaks in the X-ray diffractogram was in good agreement with that obtained from TEM results. For instance, when t(Co) ) 0.32 s, XRD resulted in Λ ) 2.3 nm, TEM yielded Λ ) 2.3 ( 0.1 nm, and the calculation with the help of Faraday’s law and AFM surface roughness led to Λ ) 2.4 nm (dCu ≈ 1.1 nm). The multilayers deposited with NaCl as an additive also exhibit a small amount of hcp phase whose fraction with respect to the amount of the fcc phase is similar to that of the samples prepared without the additive. However, the addition of NaCl leads to a decrease in grain size. The typical grain size of the samples prepared in the presence of NaCl was only 10 nm. No satellite peaks corresponding to the (111) reflection of the fcc phase could be seen in the X-ray diffractogram. Hence, the multilayer structure of the deposit could not be established from the X-ray data. The intensity ratio of the (111) and (200) peaks of the fcc lattice indicates that the addition of NaCl leads to a loss in the preferred orientation of the crystallites and results in a more random structure than in the case of the additive-free solution. This observation is in good agreement with the morphology of the sample as shown in Figure 3c. A TEM picture obtained for a sample deposited from the chloride-rich solution can be seen in Figure 4. The multilayer structure can be clearly observed in the large crystals of the sample, although there are domains around the small crystals where the sample structure is rather random. The repeat period of the multilayer established from the TEM picture is higher than that calculated with Faraday’s law, and it exhibits a large variation at different positions of the sample. This uncertainty comes from the fact that the dendritic growth of the sample results in a high inclination angle of the multilayer plane to the substrate. Hence, the sample thinning perpendicular to the substrate plane cannot cut through the multilayers at a right angle

Properties of Co-Cu/Cu Multilayers

Figure 5. Electrical resistivity of the electrodeposited multilayer samples in zero magnetic field. Cu deposition time, t(Cu) ) 5.0 s; other deposition conditions are as defined in the experimental section. dCu ≈ 1.1 nm and dCo varies from 1.3 nm to 6.2 for samples deposited from the chloride-free bath. Arrows show the resistivity of the d.c. samples deposited from chloride-free solution (solid line) and solution of c(Cl-) ) 2.5 mmol dm-3 (dashed line).

to the layer planes. This experimental artifact was analyzed in detail in ref 11. It is also the consequence of the dendritic growth that the multilayer planes in different crystals incline at various angles to each other as well as to the substrate. Comparing the TEM picture in Figure 4 with that published in ref 9, one can conclude that the presence of chloride ions in the bath facilitates the reduction of the stress in the deposit by inducing dendritic growth. 3.5. Resistivity. The resistivity (F) of the multilayers is given in Figure 5 where the resistivity of two d.c. plated samples with the approximate composition Co95Cu5 is also indicated by the arrows. The measured resistivities of both the multilayers and the d.c. plated deposits are much higher than the room temperature resistivities of the pure metals: F (fcc-Cu) ) 1.7 µΩcm and F (hcp-Co) ) 5.8 µΩcm.12 According to the results of a structural study described in ref 9, the d.c. plated Co-rich deposit prepared from the chloridefree bath was nanocrystalline with a grain size of 20 to 40 nm. It is well-known that the resistivity of nanocrystalline metals is usually higher than that of defect-free, microcrystalline metals due to increased grain-boundary scattering.13 It has been found in a previous study14 that the room-temperature resistivity of d.c. plated pure Co deposits prepared from a bath different from that used here was as high as 10 to 15 µΩcm, and this could also be ascribed to the nanocrystalline nature of the deposits. The amount of about 5 at. % Cu dissolved in the present d.c. plated Co-rich deposits can also account for a resistivity increase of about 5 to 10 µΩcm if we assume a similar effect as in the case of Ni-rich Ni-Cu alloys.15 Furthermore, the d.c. plated Co95Cu5 deposits contain, besides the equilibrium hcp-Co phase, a significant amount of fcc-Co phase as well.9 The resistivity of fcc-Co is not known, but even if F (fcc-Co) ≈ F (hcp-Co), the two-phase structure of the d.c. plated Co-rich nanocrystalline deposits can explain the resisitivity increase as compared to the single-phase deposit. The even higher resistivities of the multilayer samples can be explained by the contribution of spin-independent electron scattering at the interfaces between sublayers of dissimilar composition as it has been demonstrated for electrodeposited Ni-Cu/Cu multilayers.15 This is due to the nanometer scale of the multilayer structure since in such cases the electronic meanfree path both in the bulk Co95Cu5 and Cu is much larger than the sublayer thickness and, therefore, the electron scattering at the interfaces dominates over bulk scattering processes within the layers. With increasing Co-layer thickness, the relative importance of the interface scattering diminishes and the total

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10871

Figure 6. Magnetoresistance of the samples at H ) 8 kOe as a funcion of the magnetic layer deposition time, t(Co), for samples deposited from solutions of different chloride concentration. Square, c(Cl-) ) 0; triangle, c(Cl-) ) 0.5 mmol dm-3; circle, c(Cl-) ) 2.5 mmol dm-3; t(Cu) ) 5 s. Filled symbols, longitudinal MR component; open symbols, transverse MR component.

Figure 7. Magnetoresistance ratio at H ) 8 kOe as a funcion of the nonmagnetic layer deposition time, t(Cu), for samples deposited from solutions of different chloride concentration. Square, c(Cl-) ) 0; triangle, c(Cl-) ) 0.5 mmol dm-3; t(Co) ) 0.65 s. Filled symbols, longitudinal MR component; open symbols, transverse MR component.

resistivity of the multilayer structure decreases, approaching the bulk values obtained for the d.c. plated samples as shown in Figure 4. The general behavior of the resistivity of the Co95Cu/Cu multilayers is very similar to that described for Ni81Cu19/Cu multilayers,15 and the differences can be well explained in terms of the higher Cu-content of the magnetic layers in the latter case. The increase of the resistivity of the Co-Cu/Cu multilayers upon the addition of NaCl to the deposition bath (Figure 4) can be attributed to the observed structural modifications of the multilayer structure due to the additives described in section 3.4. Namely, a substantial deterioration of the multilayer structure was observed in the presence of Cl- ions in the bath as revealed by the larger surface roughess and a reduction of the grain size. Both these effects can be made responsible for an increased deposit resistivity. 3.6. Magnetoresistance. Figure 6 presents the magnetoresistance ratio data of samples of group 1. It can be seen that the GMR decreases with chloride concentration. A small increase in the GMR magnitude can be observed if the deposition time of the magnetic layer is higher than 1 s, and this trend is probably due to the slight decrease in the surface roughness of the samples. The magnetoresistance ratio as a function of the copper layer deposition time at constant Co-deposition time is plotted in Figure 7. We experienced difficulties during the deposition of samples with thick copper layers because the samples peeled off from the substrate already in the early phase of the deposition. The maximum GMR can be observed at about the same Cu layer thickness as in the case of sputtered and evaporated Co/Cu multilayers (about 1 nm16-19). However, the

10872 J. Phys. Chem. B, Vol. 105, No. 44, 2001 Cu-layer thickness range over which a significant GMR effect can be observed is much wider for the electrodeposited CoCu/Cu multilayers than for the sputtered and evaporated Co/ Cu multilayers. This discrepancy can be explained by the exchange reaction as discussed in detail in ref 9 and also by the uneven thickness of the Cu spacer layer. The magnetoresistance ratio of the samples deposited with the chloridecontaining solution is always smaller than that of the samples deposited in the chloride-free bath under the same conditions. This fact indicates that the influence of the chloride additive is not restricted to the change in deposition rate of the sublayers, but the properties of the Cu/Co-Cu interface are also modified. 4. Discussion of Additive Effects 4.1. Effect of Cl- on the Electrodeposition of Copper. The effect of the chloride ion on the electrodeposition of copper from noncomplexing electrolytes has been investigated by several authors. Nagy et al.20 revealed that the presence of Clin as low as a few tens of mmol dm-3 concentration increases the exchange current density of the Cu2+/Cu+ reaction step, probably by changing the reaction mechanism to an anionbridged, inner-sphere reaction; however, the Cu+/Cu reaction was found to be unaffected by chloride ions. This experience was the same for perchloric acid and trifluoromethanesulfonic acid supporting electrolytes. The experimental results were also confirmed by theoretical calculation. The increase in the rate constant of the Cu2+/Cu+ reaction step due to chroride ions can explain the occurrence of the current rise in the cyclic voltammetric curve prior to the achievement of the mass-transport limited region (see Figure 1). Yoon et al.21 studied the effect of Cl- on copper electrodeposition from a sulfate electrolyte by the rotating ring-disk electrode. They showed that the presence of chloride ions in a low concentration as compared to the Cu2+ concentration does not affect the limiting current of the Cu deposition. However, the presence of Cl- narrowed the limiting current region. The shift of the post-limiting current region to more positive potentials was attributed to the enhanced effective surface area of the deposit. The decrease in the overpotential of the hydrogen evolution as a result of Cl- addition was excluded, although the absence of hydrogen evolution was tested in the Cl--free electrolyte only. At low overpotential, the change in deposition current at a particular potential was explained by the strong adsorption of Cl- (0.28 mmol dm-3 Cl- concentration) and the formation of a CuCl film (2.8 mmol dm-3 Cl- concentration). It was also demonstrated21 that even though the [Cl-] , [Cu2+] relationship prevailed, the formation of an insoluble CuCl film formed whose dissolution was obvious from the ring current. The only dissolution product considered was CuCl2-. The conclusion was that the adsorbed Cl--containing species or the Cl--bridged film inhibits the surface diffusion of the Cu atoms deposited. The findings published in ref 21 have consequences on multilayer pulse-plating as well. The hindrance in surface diffusion of copper atoms leads to a microscopic roughening that destroys the multilayer structure by making the thickness of the copper layer uneven as compared to the chloride-free solution. The possibility of the formation of a Cu(I)-containing film should also be taken into account. If such a film is produced during the low-current pulse, the charge passing through the surface of the electrode at the beginning of the high-current pulse is consumed by the reduction of the surface film. Hence, the copper layer thickness can be much higher than that calculated from the low-pulse charge. Consequently, the cobalt

Pe´ter et al. layer thickness is lower than in the absence of chloride ions. The interface sharpness is reduced as a result of the byprocesses. The effect of Cl- was also demonstrated by scanning tunneling microscopic studies. Wu and Barkey22,23 have shown that the presence of chloride ions alters the growth of the Cu(111) surface by introducing monatomic terraces and screw dislocations. They also drew the attention to the faceting effect of chloride present in minor concentration. The results of Wu and Barkey are in good agreement with the roughness difference (Figure 3) of our multilayer samples deposited in the absence and presence of Cl-. The roughness increase and grain size reduction is also considered as a possible reason for the increase of the electrical resistivity of the multilayer samples with increasing Cl- ion concentration in the deposition bath. 4.2. Effect of Cl- on the Electrodeposition of Cobalt. Cobalt has no stable Co(I)-containing intermediate, and hence the influence of Cl- was studied from a completely different viewpoint than the copper deposition. Franklin24 and Franklin and Mathew25-27 published several works on the anion effect on the electrodeposition of transition metals, including Co. Their method was based on the measurement of the pressure dependence of the equilibrium exchange current density. The result clearly showed that the addition of Cl- to a bath with noncomplexing anions leads to the decrease in the activation volume of the deposition from 12.4 to 6.0 cm3 mol-1. These values correspond to the loss of 2 and 1 water molecules, respectively, in the rate-determining step of the deposition. Although the data of Franklin and Mathew do not account for the high-rate deposition of Co but only for the equilibrium, it can be assumed that the addition of chloride ions to a noncomplexing bath leads to a mechanistic change in the electrodeposition of Co in a wide potential range. It has to be emphasized that chloride ions, like all catalysts, promote a particular elementary reaction in both directions, and hence they affect the dissolution of Co, too. Therefore, the presence of Clmodifies the kinetics of the exchange reaction between Co and Cu2+, and hence the amount of Cu deposited by the exchange reaction during the low-current pulse may be much higher than in the absence of Cl-. This phenomenon also leads to an enhanced intermixing at the layer boundaries that is detrimental for magnetoresistance9,28,29 4.3. Influence of Brighteners, Leveling and Complexing Agents on Pulse-Plated Multilayer Samples. The current distribution during d.c. electrodeposition is affected by the cell geometry and the charge transfer properties of the cathode. If the polarization resistance of the cathode is small, the current distribution is determined by the position of the cathode and the anode. However, if the polarization resistance is high, the solution resistance can be neglected, and a uniform lateral distribution of the deposit is expected. This is the case when the surface sites are blocked by strongly adsorbed species.24 The occupation of the active surface sites by some components of the electrolyte may temporarily hinder the incorporation of the metal ions into the existing crystal structure. In the case of d.c. deposition, the adsorption of the leveling and/or brightening agent takes place to the highest extent at the peaks and edges; therefore, dendritic growth is supressed. The strong adsorption of an electrolyte component introduces disorder at the surface, and merging of neighboring grains into a single crystal becomes less likely than on a smooth surface not covered with strongly bound adsorbents. This effect necessarily leads to a grain refinement. A useful review of the mechanism of leveling and brightening can be found in ref 7.

Properties of Co-Cu/Cu Multilayers If the electrodeposition is carried out by pulse-plating, the occupation of the surface sites by nonmetallic components leads to the renucleation of the deposit during each pulse. This phenomenon has been shown for the deposition of copper in the presence of citric acid.30 Pulse-plating in the presence of a complexing agent is also an appropriate method to produce nanocrystalline nickel,31 iron,32 and palladium.33 It was also confirmed for the Pd pulse-plating that noncomplexing baths result in coarse-grained deposits, and the higher the chelating ability of the additive is, the more pronounced grain refinement can be observed.33 The adsorption of Cl- during the deposition of Co-Cu/Cu multilayers is thought to play the same role, thus leading to a reduced grain size. If the grain size becomes very low, the spinindependent electron scattering at the grain boundaries predominates over the spin-dependent scattering at the interfaces between the layers. Hence, the magnetic contribution to the overall resistance is low, and that leads to a small GMR. Grain refinement is necessarily accompanied by the loss of preferred crystal orientation that also must contribute to the decrease in GMR. The adsorption of additives (brighteners, complexing or leveling agents) strongly depends on the electrode potential. Therefore, the modulation in deposition potential or current may lead to a periodic adsorption-desorption cycle of the additives. This pulsating adsorption-desorption behavior gives rise to a pseudocapacitive (non-Faradaic) current that leads to a serious deviation of the deposition current waveform from that provided by the power supply. Regarding that the layer thickness in alternating magnetic/nonmagnetic multilayers with high GMR amounts to just a few times of the lattice parameter of the constituent metals, a little non-Faradaic component in the current can lead to a fairly high distortion in the sharpness of the composition profile. In other words, the periodic adsorptiondesorption of the additive results in an intermixed boundary between the magnetic and nonmagnetic layers that is deleterious for GMR. We think that the periodic adsorption-desorption of Cl- also contributes to the strong reduction of GMR in our samples. It has been shown by other authors as well6 that a bath developed for the deposition of multilayers with good GMR cannot properly work with additives that behave as brighteners in d.c. mode. 5. Conclusions We have demonstrated that the presence of a component in the bath applied for the electrodeposition of magnetic multilayers can be critical for magnetoresistance properties. NaCl at as low as 0.5 mmol dm-3 concentration (≈0.03 g/L) is capable of substantially modifying the properties of the resulting electrodeposited multilayers. We think that most of the effects discussed in the previous section are not specific to a particular additive but rather are general for many agents used for d.c. baths. Baths used for pulsed electrodeposition of nanoscale magnetic/nonmagnetic multilayers have to comply with a number of conditions quite different from those required for d.c. plating. First, the impurity level in the solution has to be well controlled. Second, the application of adsorbing, leveling, or brightening additives has to be reinvestigated. Third, the negative ions in the salt of the

J. Phys. Chem. B, Vol. 105, No. 44, 2001 10873 metal to be deposited have to be inert and nonadsorbing like SO42-. Acknowledgment. This work was supported by the Hungarian Scientific Research Fund (OTKA, Grant F 032046). The XRD work has been performed on an apparatus purchased by the Eo¨tvo¨s University under Grant CEF 1156. References and Notes (1) Baibich, M. N.; Broto, J. M.; Fert, A.; Nguyen Van Dau, F.; Petroff, F.; Etienne, P.; Creuzet, G.; Friederich, A.; Chazelas, V. Phys. ReV. Lett. 1988, 61, 2472. (2) Gru¨nberg, P. Acta Mater. 2000, 48, 239. (3) Ross, C. A. Annu. ReV. Mater. Sci. 1994, 24, 159. (4) Schwarzacher, W.; Lashmore, D. S. IEEE Trans. Magn. 1996, 32, 3133. (5) Dariel, M. P.; Lashmore, D. S.; Bennett, L. H. in Magnetic Multilayers; Bennett, L. H., Watson, R. E., Eds.; Word Scientific: Singapore, 1994; p 373. (6) Lenczowski, S. K. J.; Scho¨nenberger, C.; Gijs, M. A. M.; de Jonge, W. J. M. J. Magn. Magn. Mater. 1995, 148, 455. (7) Oniciu, L.; Muresan, L. J. Appl. Electrochem. 1991, 21, 565. (8) Kelly, J. J.; Kern, P.; Landolt, D. J. Electrochem. Soc. 2000, 147, 3725. (9) Pe´ter, L.; Czira´ki, A Ä .; Poga´ny, L.; Kupay, Z.; Bakonyi, I.; Uhlemann, M.; Herrich, M.; Arnold, V.; Bauer, H.-D.; Wetzig, K. J. Electrochem. Soc. 2001, 148, C168-C176. (10) Kou, S.-C.; Hung, A. Plat. Surf. Fin. 2000, 87, 140. (11) Czira´ki, A Ä .; Gero¨cs, I.; Fogarassy, B.; Arnold, B.; Reibold, M.; Wetzig, K.; To´th-Ka´da´r, E.; Bakonyi, I.; Z. Metallkd. 1997, 88, 10. (12) Kittel, Ch. Introduction to Solid State Physics, 6th ed.; Wiley: New York, 1986; p 144. (13) Bakonyi, I.; To´th-Ka´da´r, E.; To´th, J.; Czira´ki, A Ä .; Fogarassy, B. in Nanophase Materials; Hadjipanayis, G. C., Siegel, R. W., Eds.; NATO ASI Series E, Vol. 260, Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; p 423. (14) Bakonyi, I.; To´th-Ka´da´r, E.; To´th, J.; Tarno´czi, T.; Czira´ki, A Ä . in Processing and Properties of Nanocrystalline Materials; Suryanarayana, C., Singh, J., Froes, F. H., Eds.; The Minerals, Metals & Materials Society: Warrendale, PA, 1996; p 465. (15) Bakonyi, I.; To´th-Ka´da´r, E.; To´th, J.; Becsei, T.; Poga´ny, L.; Tarno´czi, T.; Kamasa, P. J. Phys.: Cond. Matter 1999, 11, 983. (16) Parkin, S. S. P.; Bhadra, R.; Roche, K. P. Phys. ReV. Lett. 1991, 66, 2152. (17) Parkin, S. S. P.; Li, Z. G.; Smith, D. J. Appl. Phys. Lett. 1991, 58, 2710. (18) Mosca, D. H.; Petroff, F.; Fert, A.; Schroeder, P. A.; Pratt, W. P., Jr.; Laloee, R. J. Magn. Magn. Mater. 1991, 94, L1. (19) Kubota, H.; Ishio, S.; Miyazaki, T.; Stadnik, Z. M. J. Magn. Magn. Mater. 1994, 129, 383. (20) Nagy, Z.; Blaudeau, J. P.; Hung, N. C.; Curtis, L. A.; Zurawski, D. J. J. Electrochem. Soc. 1995, 142, L87. (21) Yoon, S.; Schwartz, M.; Nobe, K. Plat. Surf. Fin. 1994, 81, 65. (22) Wu, Q.; Barkey, D. J. Electrochem. Soc. 1997, 144, L261. (23) Wu, Q.; Barkey, D. J. Electrochem. Soc. 1997, 147, 1038. (24) Franklin, T. C. Plat. Surf. Fin. 1994, 81, 62. (25) Franklin, T. C.; Mathew, S. A. J. Electrochem. Soc. 1987, 134, 760. (26) Franklin, T. C.; Mathew, S. A. J. Electrochem. Soc. 1988, 135, 2725. (27) Franklin, T. C.; Mathew, S. A. J. Electrochem. Soc. 1989, 136, 3627. (28) To´th-Ka´da´r, E.; Pe´ter, L.; Becsei, T.; To´th, J.; Poga´ny, L.; Tarno´czi, T.; Kamasa, P.; Bakonyi, I.; La´ng, G.; Czira´ki, A Ä .; Schwarzacher, W. J. Electrochem. Soc. 2000, 147, 3311. (29) To´th, J.; Kiss, L. F.; To´th-Ka´da´r, E.; Dinia, A.; Pierron-Bohnes, V.; Bakonyi, I. J. Magn. Magn. Mater. 1999, 198-199, 243. (30) Natter, H.; Schmelzer, M.; Janssen, S.; Hempelmann, R. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1706. (31) To´th-Ka´da´r, E.; Bakonyi, I.; Poga´ny, L.; Czira´ki, A Ä . Surf. Coat. Technol. 1996, 88, 57. (32) Natter, H.; Schmelzer, M.; Lo¨ffler, M.-S.; Kril, C. E.; Fitch, A.; Hempelmann, R. J. Phys. Chem. B 2000, 104, 2467. (33) Natter, H.; Krajewski, T.; Hempelmann, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 55.