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Influence of a static magnetic field on nickel electrodeposition studied using an electrochemical quartz crystal microbalance, atomic force microscopy and vibrating sample magnetometry
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 575 (2005) 221–228 www.elsevier.com/locate/jelechem
Influence of a static magnetic field on nickel electrodeposition studied using an electrochemical quartz crystal microbalance, atomic force microscopy and vibrating sample magnetometry Andreas Bund *, Adriana Ispas Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, Mommsenstr. 13, D-01062 Dresden, Germany Received 21 July 2004; received in revised form 17 September 2004; accepted 21 September 2004 Available online 18 November 2004
Abstract We investigated the influence of static magnetic fields up to 740 mT on the electrochemical nickel deposition from a sulphamate electrolyte. The magnetic field was applied parallel to the surface of the working electrode and thus gave rise to magnetohydrodynamic convection by Lorentz forces. An electrochemical quartz crystal microbalance was used to characterize the amount of hydrogen produced on the cathode during the deposition and to calculate the efficiency of the deposition process. The amount of hydrogen produced depends on the magnetic flux density, and this dependence can be explained by the complex interplay of the electrode kinetics and hydrodynamic effects, correlated with the Lorentz force. Atomic force microscopy showed an increase of the surface roughness of the nickel layers deposited in the magnetic field. The absolute values of the magnetic properties of the layers prepared in the magnetic field differed slightly from those prepared in the absence of a magnetic field. This can be explained by the structural changes. No anisotropy of the magnetic properties was observed. 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical quartz crystal microbalance; Atomic force microscopy; Vibrating sample magnetometer; Magnetic field effects; Current efficiency; Nickel electrodeposition
1. Introduction 1.1. Magnetic field effects in electrochemical reactions When a magnetic field is applied to an electrochemical cell, all the charged species that move in the electrolyte will experience a Lorentz force, which is perpendicular to the current density and the magnetic field. This force will induce convective movements in the solution that reduce the diffusion layer thickness (magnetohydrodynamic or MHD effect) [1,2]. As a consequence, the limiting current density, jL, increases. A *
Corresponding author. Tel.: +49 351 463 34351; fax +49 351 463 37164. E-mail address: [email protected] (A. Bund). 0022-0728/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.09.014
linear dependence of jL with B1/3 (B is the magnetic flux density) was reported in the literature [3,4] in the case of macroscopic electrodes, and for microelectrodes a proportionality of jL to B was found [5]. The flow profile and the Lorentz force density are related by the Navier–Stokes equation (NSE). Taking into consideration the complex geometries of cells and electrodes usually used, an analytical solution of the NSE is difficult in most cases. There are, however, some analytical and numerical approaches for simple geometries and boundary conditions [6,7]. Besides the Lorentz force, there may be forces due to inhomogeneities of the magnetic field. The inhomogeneities can be caused by the external field itself or by concentration gradients of paramagnetic ions (Cu2+, Fe2+, Co2+, Ni2+, etc.) [5,8,9]. These gradient
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forces depend on the magnitude of the magnetic field and not on its direction. Coey and Hinds argue that the magnetic forces are six orders of magnitude smaller compared to other forces occurring in an electrochemical reaction (mainly diffusion and migration) [10]. Their conclusion was that the effects of the magnetic forces are non-negligible just in the case when they are combined with the convective movements in the solution. 1.2. Electrochemical quartz crystal microbalance The electrochemical quartz crystal microbalance (EQCM) is well suited to monitor small mass changes at the working electrode in real time and in situ. Assuming that the film on the surface has a uniform thickness and that the acoustic impedance of the deposit does not differ strongly from that of the quartz, one can apply the Sauerbrey equation (1) to calculate the mass, Dm, deposited on the surface of the quartz from the frequency shift Df [11]. pffiffiffiffiffiffiffiffiffi Df ¼ 2f02 Dm= A lq qq ; ð1Þ where f0 is the frequency of the unloaded quartz crystal, A the active area of the quartz, and lq and qq are the shear modulus and the density of the quartz, respectively. Eq. (1) is the basis for the in situ determination of mass change with respect to time, pffiffiffiffiffiffiffiffiffiffi ðdm=dtÞmeas ¼ ðdf =dtÞA lq qq = 2f02 : ð2Þ Other useful information that can be taken from the quartz crystal is its damping. Near the resonance frequency, the real part of the electrical admittance (conductance) of the quartz crystal can be described by a Lorentzian function [12–14]. The full width at half maximum (FWHM) of this resonance curve is proportional to the damping of the quartz. An alternative measure of the damping is the maximum of the conductance. Its reciprocal, the motional resistance Rm of the quartz, can be measured with special oscillator circuits. In the case of metal depositions an increase of the damping of the quartz crystal, measured either as changes in w (w is the full width at half maximum (FWHM) of the resonance curve of the quartz crystal) or Rm, can be correlated with an increase of the surface roughness of the deposited layer [13,15–18]. 1.3. Nickel electrodeposition The electrodeposition of nickel from aqueous solutions is usually accompanied by the hydrogen evolution reaction (HER). Therefore, the mass deposited with respect to time will be smaller than calculated by the Faraday law (dm/dt)calc,
ðdm=dtÞcalc ¼ Mj=nF ;
ð3Þ
where M is the molar mass of the deposited species, j the current density, n the number of electrons and F is the Faraday constant. The ratio of the measured (Eq. (2)) and the calculated (Eq. (3)) deposition rates gives the current efficiency, g (Eq. (4)), and is an important parameter for technical applications. For obvious reasons, one is interested in high current efficiencies in most cases g ¼ jNi =ðjNi þ jHER Þ ¼ ðdm=dtÞmeas =ðdm=dtÞcalc ;
ð4Þ
where jNi and jHER are the partial current densities of the nickel reduction and the HER, respectively. For nickel depositions from a Watts bath, Chiba et al. [19] reported an increase of the current efficiency of 5 to 10% (50 ± 2 C) in the presence of a magnetic field up to 0.15 T (parallel to the electric field). On the other hand, Devos et al. [20] observed a decrease of some percent when a field up to 1 T was applied perpendicular to the electric field. Shannon et al. [21] made a statistical analysis of the dependence of the surface roughness on the direction of the magnetic field. They observed that, for a horizontal magnetic field, the roughness increased, while it decreased for a vertical magnetic field. Danilyuk affirmed that, in the case of copper, some values of the magnetic field (from 60 to 160 mT) accelerate the electrodeposition reaction, and others (from 160 to 173 mT) inhibit it. In the case of nickel and tin, these regions were alternating. As a consequence changes in structure and morphology may result [22]. Given the various interactions of a magnetic field in an electrochemical reaction, it seems promising to tailor the magnetic forces to optimize the mass transport or the properties of the deposited layer. However, before such tailoring can occur, the fundamentals of the interaction of magnetic fields in an electrochemical reaction must be understood better. In this paper, we applied an EQCM with damping monitoring to study the deposition rate of nickel from a sulphamate bath in the presence of static magnetic fields. The sulphamate electrolyte was chosen because of its high relevance in technical applications, which is based on the very high plating rates and the low internal stresses in the deposit [23]. Typical technical applications comprise electroforming, surface finishing and the fabrication of microelectromechanical systems (MEMS) by the LIGA (Lithographie, Galvanik, Abformung) process [23–25]. The deposition in MEMS structures is often strongly limited by mass transport. To achieve void free filling, special additives in combination with pulse plating must be applied. A carefully designed magnetic field could be used to enhance the mass transport in microstructures.
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2. Experimental The electrolyte consisted of 1.26 M Ni(SO3NH2)2 · 4H2O + 0.32 M H3BO3 + 0.04 M NiCl2 · 6H2O. For some experiments 0.52 mM sodium dodecyl sulfate (SDS) or 0.24 mM sodium 2-ethylhexyl sulphate were added as surfactants. The electrolyte was adjusted to pH 4 by adding sulphamic acid (HSO3NH2) or nickel carbonate (NiCO3). All solutions were prepared from p.a. grade chemicals and bidistilled water. All measurements were made at room temperature. For the EQCM experiments 10 MHz optically polished AT-cut quartzes provided by KVG (Neckarbischofsheim, Germany) were used. They had two gold electrodes, of 100 nm thickness, deposited on the opposite faces of the quartz crystal, on a 5–10 nm adhesion layer of chromium. The resonance frequency of the quartz was measured with a Maxtek PLO-10 phase lock oscillator, (Santa Fe Springs, USA) and a frequency counter card (GT200, Guide Technologies). The PLO circuit also provides information about the damping of the quartz as a voltage, which was acquired with a Keithley DMM and converted to the motional resistance, Rm, of the quartz. An alternative EQCM set-up consisted of an Advantest R3758BH network analyzer (Advantest, Tokyo, Japan). This device records the admittance spectra of the resonator near its resonance frequency. Each admittance spectrum was transferred to a computer via a GPIB card and then fitted with a Lorentzian function [13]. The connection between the electrochemical cell and the network analyzer was made by a pi network adapter (TeleQuarz, Neckarbischofsheim, Germany) and a LC network (capacity C = 0.1 lF, inductance, L = 1 mH) to decouple the high and low frequency signals of the quartz and the electrochemistry, respectively. Details can be found in Ref. [13]. One gold electrode was used as the working electrode (radius 3 mm). The counter electrode was a nickel circular plate, of radius, 10 mm and thickness, 2.5 mm, placed in parallel to the working electrode. The distance between the working and the counter electrodes was approximately 15 mm. Two kinds of reference electrode were used, lab-made Hg|Hg2Cl2, and a commercial ‘‘double junction’’ Ag|AgCl|3 M KCl electrode (Metrohm, Germany). The electrochemical cylindrical cell (36 mm inner diameter and 42 mm height) was made from Teflon. The quartz was placed at the bottom of the cell, and fixed between an o-ring (Viton, solution side) and a silicone gasket (air side). The electrochemical experiments were made with a potentiostat/galvanostat Model 263A (EG&G Instruments, Princeton, NJ). The whole setup was controlled by lab-made software (Microsoft Visual C++). A water-cooled electromagnet from VEB Polytechnik, Phylatex, Karl-Marx Stadt, was used to produce
Fig. 1. Schematic representation of the experimental setup for the EQCM measurements in magnetic fields. Q: quartz crystal: C: cell (PTFE); E electrolyte; WE, CE, and RE: working, counter and reference electrode; NA: network analyzer; N and S indicate the poles of the electromagnet.
the magnetic field. The maximum flux density was 740 mT for a gap of 46 mm. The magnetic field was applied parallel to the working electrode, i.e., perpendicular to the current lines (see Fig. 1 for a schematic representation). The intensity of the magnetic field was measured with a Hall probe (Lake Shore, model 450). The surface morphology of the deposited layers was investigated by contact mode AFM (atomic force microscopy). The AFM scanner was a PicoSPM, version 2.4 (Molecular Imaging, Phoenix, AZ). Pyramidal tips of silicon nitride were used. For the characterization of the magnetic properties of the layers, thin nickel layers (ca. 3 lm) were deposited at 3.5 mA cm2 onto cylindrical copper electrodes (diameter 6 mm, height 6 mm). Hysteresis curves (from 1.0 to 1.0 T) were recorded with a vibrating sample magnetometer (VSM, model 7300, Lake Shore, OH). The magnetic field H could be applied parallel (in the following called the 0 position) and normal (90) to the plane of growth. Furthermore, the sample could be rotated in the H field to check for anisotropies.
3. Results and discussion 3.1. Hydrogen evolution and current efficiency For the relatively small current density of 0.1 mA cm2, the deposition potential shifted from 0.74 V at 0 T to 0.67 V at 530 mT (Fig. 2(a)). At 50 mA cm2 a shift from 1.46 V (0 T) to 1.40 V (740 mT) was observed (Fig. 2(b)). For the setup sketched in Fig. 1, the Lorentz force points out of the paper plane. This force density leads to a gentle stirring of the electro-
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-0.3
5 jHER(B) / jHER(B=0)
E vs SCE /V
-0.4 -0.5 -0.6 -0.7 -0.8
0
10
(a)
20 30 time / min
40
50
4 3 2 1 0 0
2 00
(a)
-1.1
400
600
B / mT
-1.2
-1.4 -1.5 -1.6
(b)
0
50
100 150 200 250 300 350 time /s
Fig. 2. Influence of the magnetic flux density on the deposition potential during galvanostatic nickel depositions. (a) j = 0.1 mA cm2, (—) 0 T, (– –) 530 mTb; (b) j = 50 mA cm2, (—) 0 T, (– –) 740 mT.
lyte in front of the quartz and thus decreases the concentration polarization (see Section 1.1). The total current density is the sum of the nickel reduction current and HER. The hydrogen evolution reaction (HER) during metal depositions from aqueous solutions is of special interest in electrochemical reactions [26–28]. One of the reasons for this interest is the risk of hydrogen embrittlement that degrades the mechanical properties of metals [29]. The partial current density of the HER can be calculated by multiplying the total current density with (1 g), where g is the current efficiency for nickel deposition, Eq. (4). For a total current density of 0.1 mA cm2, the partial current of the HER increased with increasing magnetic flux density (Fig. 3(a)), whereas it remained constant for a total current density of 50 mA cm2 (Fig. 3(b)). In these measurements, a surfactant (sodium dodecyl sulphate, SDS) had been added to the electrolyte. The results were the same when SDS was replaced by sodium 2-ethylhexyl sulphate. In Fig. 3, each data point is the average of four measurements and the error bars reflect the standard error of the mean value. In the following we propose a model, which explains the influence of a magnetic field on the current efficiency at small and large current densities, respectively. It has been shown that the current efficiency of the nickel deposition increases with increasing current density [30,31]. This is because the HER is usually limited by mass
jHER(B) / jHER(B=0)
E vs SCE /V
2 -1.3
1
0
0 (b)
200
400
600
800
B / mT
Fig. 3. Dependence of the partial current density of the HER, jHER, on the magnetic flux density during nickel deposition from a sulphamate bath with surfactant. The values of jHER have been normalized with respect to the value without a magnetic field jHER (B = 0). (a) Total current density–0.1 mA cm2; (b) 50 mA cm2.
transport, whereas the nickel reduction is activation (or mixed) controlled. Therefore, making the potential more negative will result in an increase of jNi, while jHER remains constant. As a result the current efficiency of nickel reduction increases (Eq. (4)). Under the influence of a magnetic field, both limiting current densities will increase due to the magnetohydrodynamic stirring. The increase will depend on several parameters, such as the magnetic flux density and the concentration [6,32]. As the concentration of nickel ions is much larger than the proton concentration, we can assume that the nickel deposition is influenced more strongly by the magnetic field (see Fig. 4 for a schematic representation). Because of the activation control, the absolute value of the nickel current increases strongly with the overvoltage (or with the total current density in a galvanostatic experiment under steady state conditions). For a potential close to the equilibrium potential of nickel (E1 in Fig. 4), the MHD effect increases jHER more strongly than jNi as seen in Fig. 3(a). At a higher polarization (E2 in Fig. 4) jHER and jNi are both under transport control and the effect of the magnetic field is to increase both. In a galvanostatic experiment, where
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225
0
E2
E1
jHER(B) / jHER (B=0mT)
cathodic current density
12
8
4
0
polarization Fig. 4. Schematic representation of the effect of a magnetic field on the partial current densities of the Ni reduction, jNi, and the HER, jHER. (—) HER without magnetic field, (– –) HER in magnetic field, ( ) Ni reduction without magnetic field, (– Æ – Æ –) Ni reduction in magnetic field.
jNi + jHER must be constant, jHER will not change greatly with B as is seen in Fig. 3(b). As the current efficiency was determined using the EQCM, one needs to consider the systematic errors that could be introduced by this measuring technique. First, there could be mass changes which are not directly related to the electric current, such as the deposition of Ni(OH)2 due to pH changes at the electrode [20]. This precipitation occurs at high current densities and usually leads to apparent current efficiencies greater than unity [30,31]. As this was not observed in our case and because energy dispersive X-ray analysis indicated no oxygen in the layers we assume that this side reaction is negligible. As will be shown below (Section 3.2), there is an influence of the magnetic field on the surface roughness. It is known that surface roughness effects can cause frequency and damping changes [18]. In our case, the damping changes, Dw, are of the order of some kHz while the frequency shift, Df, is of the order of 100 kHz. It has been shown [15] that the mass calculated from the frequency shift is correct as long as Dw Df. To check how the surfactant affects the hydrogen evolution in a magnetic field, nickel was deposited from electrolyte free of surfactant at a relatively small current density. The partial current density of the HER increased with the magnetic flux density (Fig. 5) as in the presence of the surfactant (Fig. 3(a)). However, when one looks at the absolute values, one sees that, in the presence of the surfactant, the HER increases more strongly. This can be explained by the reduced surface tension in the presence of the surfactant. Bubbles formed at the cathode can detach more easily from the electrode and make space for new bubbles. As a result the partial current density of the HER increases more strongly with B. The layers prepared with surfactant addition had a systematically smaller surface roughness. This finding shows the role of hydrogen bubbles for the development of surface roughness (see discussion below).
0
200
400
600
B / mT Fig. 5. Hydrogen evolution during galvanostatic deposition (j = 0.5 mA cm2) from a nickel sulphamate bath with surfactant (sodium 2ethylhexyl sulphate), (s), and without (h).
3.2. Surface morphology and roughness In a previous paper, we investigated the effect of a static magnetic field up to 1 T (parallel to the electrode) on the grain size of potentiostatically deposited nickel layers [33]. It was found that a larger amount of finegrained material was formed in the presence of the magnetic field. In the present paper, we will focus on the morphology of the deposited layer and on the surface
Fig. 6. Surface topographies of nickel layers deposited at 50 mA cm2 without a magnetic field (a) and in the presence of a magnetic flux density of 740 mT (b).
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Table 1 Dependence of the surface roughness parameters Rq and Ra of galvanostatically deposited Ni layers on the magnetic flux density B of an externally applied magnetic field 0
530
740
jtotal = 0.5 mA cm2, layer thickness 112 ± 4 nm 4.91 ± 0.54 5.33 ± 0.68 Ra (nm) Rq (nm) 6.33 ± 0.73 7.22 ± 1.26
8.68 ± 1.77 11.85 ± 2.54
jtotal = 50 mA cm2, layer thickness 4.3 ± 0.2 lm Ra (nm) 5.14 ± 1.18 23.26 ± 2.44 Rq (nm) 6.55 ± 1.51 29.66 ± 2.39
26.93 ± 2.48 34.05 ± 2.29
12 10 damping ∆Rm / Ω
B (mT)
14
8 6 4 2 0 -2 0
roughness. Comparing the AFM images in Fig. 6(a) (no magnetic field) and Fig. 6(b) (B = 740 mT), one can see that the magnetic field induces an increase of the surface roughness (note the different scales of the z-axis). To characterize the surface roughness, the standard deviation of the height values, Rq, and the mean roughness, Ra, were calculated using standard software (Table 1). Each value is the average of four independent layers, and at each layer, four determinations at different locations at the surface were made. When comparing the results of Table 1 (surface roughness increases with B) with our previous results (amount of fine grained material increases with B) [33], one must take into account that the surface morphology, the grain size and the orientation of deposits depend strongly on the electrolyte composition and on the condition of deposition [34,35]. Furthermore, a general correlation of the grain size with surface roughness is difficult [36,37]. For copper electrodepositions Hu and Wu [37] showed that an increase of the mean grain size does not necessary induce an increase in the roughness of the deposit. Qualitative in situ information about the surface roughness can be taken from the EQCM data. Some typical changes of the quartz crystal are presented in Figs. 7 and 8. Fig. 7 presents the changes in the full
5 damping ∆w / kHz
4 3 2 1 0 -1 0
100
200
300
400
time /s Fig. 7. Change of the damping of the EQCM (represented by the change of the full width at half maximum of the resonance curve, Dw) during Ni deposition at 50 mA cm2 for different magnetic flux densities (n) 0 mT, (s) 206 mT, (h) 740 mT.
500
1000
1500
2000
2500
time / s Fig. 8. Change of the damping of the EQCM (represented by the change of the motional resistance of the quartz crystal, DRm) during Ni deposition at 0.1 mA cm2 for different magnetic flux densities (n) 0 mT; (s) 410 mT; (h) 530 mT.
width at half maximum (Dw) of the resonance curve of the quartz for a deposition current density of 50 mA cm2. In the early stages of the deposition process, the damping becomes negative. This finding can be explained with a smoothing of the surface. For the relatively small current density of 0.1 mA cm2, the damping increases continuously (Fig. 8). Note that Fig. 8 presents the damping increase as the change of the motional resistance of the quartz crystal. For both current densities, the damping increases with the deposition time (i.e. layer thickness). The effect of the magnetic field is to increase the damping more strongly. The damping increase indicates that the surface becomes rougher which is in perfect agreement with the ex situ AFM information. The reason for the spikes in the damping for the highest magnetic flux density (h in Fig. 7) is not completely clear at the moment. Obviously the surface roughness goes through maxima for short time periods under the influence of a magnetic field. Interestingly the plots of the electrode potential vs time do not show these spikes (Fig. 2(b)). Future work will aim at the systematic investigation of this effect. In the following, we propose a mechanism that can explain the correlation of the surface roughness and the magnetic flux density. It is based on the interplay of magnetohydrodynamic convection with the concentration profiles and the hydrogen bubbles. In general, surface roughness develops due to uneven nucleation and crystal growth [38]. Chen and Jorne applied the Mullins–Sekerka morphological stability theory to identify the major factors that influence the evolution of surface roughness [39]. According to them, one of the most important factors that favor morphological instabilities (and thus a high surface roughness) is the increase of the concentration gradient at the electrode|electrolyte interface. Because the MHD effect increases the concentration gradient, the presence of a magnetic field will increase the surface roughness. Furthermore, as outlined
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above, nickel electrodeposition is always accompanied by hydrogen evolution. As the hydrogen bubbles represent obstacles for the flow profile induced by the magnetic field, micro-turbulences will form in front of the electrode [20,35]. Thus, the current distribution becomes inhomogeneous, which results in rougher layers. Because higher current densities induce higher Lorentz forces and thus stronger convection, the effect increases with the current density (Table 1).
The nickel ions in the electrolyte are paramagnetic species. Once they are reduced to metallic nickel they begin to form a ferromagnetic layer with a certain magnetization, even in the absence of an external magnetic field. The orientation of this magnetization is the result of the energy balance between the exchange energy, the dipolar energy, the magneto-crystalline energy and the magnetoelastic energy [40,41]. One might assume that an external magnetic field, which is applied during the deposition of the layer, might induce some magnetic anisotropy. In the following we discuss briefly which terms might cause a magnetic anisotropy in the case of nickel layers. The dipolar energy is associated with the magnetization in its own field, and depends on the shape of the grain. This leads to a shape anisotropy that favours a magnetization along the larger dimension of a ferromagnetic body. In the case of a magnetic layer, the shape anisotropy favors the in plane magnetization. It has been reported that the dipolar forces can influence the process of nucleation and growth of a layer, for example in the case of iron arborescent growth [42]. The magneto-crystalline energy is caused by the spinorbit coupling and can be the source of anisotropy, too. This anisotropy favors a magnetization along a given crystalline axis, called the easy axis of magnetization, and may be responsible for a perpendicular magnetiza-
14.4
Hc/ mT
Table 2 Dependence of some important magnetic properties of nickel layers on the magnetic flux density applied during the electrodeposition Angle between sample and field H
B (mT)a
Slope at Hc (pA m2 T1)
Ms (lA m2)
Mr (lA m2)
0
0 740
22.4 16.6
46.3 34.0
22.3 16.9
90
0 740
23.0 14.9
50.7 35.6
23.2 14.9
a
3.3. Magnetic properties of the deposited layers
14.0
13.6
227
Magnetic flux density superimposed during deposition.
tion. Nickel has an fcc crystalline structure and its easy axis is [1 1 1] and the hard axis is [1 0 0]. In the case of nickel, this anisotropy can be neglected, especially if the orientation of the grains is isotropic. The magneto-elastic energy is induced by the deformation of the crystalline cell and can result in stress anisotropy. X-ray diffractograms (not shown) did not show any shift of the reflections of nickel if the layers were deposited in a magnetic field. Therefore, we can rule out this source of anisotropy. From all the terms that may be a source of anisotropic magnetic properties, the most important in our case is the dipolar one. Hysteresis curves (not shown) showed no in-plane anisotropy for nickel layers deposited in a magnetic field, and no relevant dependence of the coercive field, Hc, on the degree of rotation in the plane of growth was found (Fig. 9). The higher values of Hc seen in Fig. 9 for the sample prepared in the magnetic field are due to a higher layer thickness (see total electric charges in caption of Fig. 9). The slope of the magnetization curve in the coercive field (dM/dH)Hc (differential susceptibility) measured in the vertical plane also showed no anisotropy. In conclusion it can be stated that no anisotropy of the magnetic properties was found. However, the absolute values of (dM/dH)Hc, the magnetization of saturation, Ms and the residual magnetization, Mr, decreased by approximately 20% for the sample deposited in a field of 740 mT (Table 2). It is known that changes in morphology of a layer will induce changes in the magnetic properties [43–45]. In our case, the magnetic field changes the surface roughness (see Section 3.2) of the deposited nickel (and maybe, the layer thickness slightly, via the influence on the current efficiency, see Section 3.1), thus inducing slight changes of the coercive field, the saturation magnetization and the residual magnetization.
13.2 0
100
200
300
400
4. Conclusions
degree of rotation Fig. 9. Coercive field strength of Ni layers deposited at 3.5 mA cm2 in the absence of a magnetic field (s, total charge Q = 7.21 C cm2) and at 740 mT (h, Q = 10.40 C cm2), respectively.
The influence of a parallel (with respect to the surface of the working electrode) static magnetic field on the electrochemical reaction, the morphology and the
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magnetic properties of electrochemically deposited nickel layer has been investigated. The layers prepared in the magnetic field had a considerably higher surface roughness. Furthermore, the magnetic field increased the hydrogen evolution at small current densities, which has been explained by a complex interplay of hydrodynamics and the electrode kinetics. No in-plane anisotropy of the magnetization was observed for the deposited layer. All changes of the magnetic properties could be explained on the basis of the morphology effects.
[15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgements This work was supported financially by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 609. The authors would like to thank Dr. Stefan Roth (Leibnitz Institute, Dresden) for his assistance in the VSM measurements and Dr. Vasile Heresanu (CNRS, Laboratoire Louis Neel, Grenoble) for interesting and helpful discussions concerning the magnetic anisotropy.
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A. Bund, M. Schneider, J. Electrochem. Soc. 149 (2002) E331. A. Bund, J. Solid State Electrochem. 8 (2004) 182. L. Daikhin, M. Urbakh, Langmuir 12 (1996) 6354. L. Daikhin, E. Gileadi, G. Katz, V. Tsionsky, M. Urbakh, D. Zagidulin, Anal. Chem. 74 (2002) 554. A. Chiba, K. Kitamura, T. Ogawa, Surf. Coat. Technol. 27 (1986) 83. O. Devos, A. Olivier, J.P. Chopart, O. Aaboubi, G. Maurin, J. Electrochem. Soc. 145 (1998) 401. J.C. Shannon, Z.H. Gu, T.Z. Fahidy, J. Appl. Electrochem. 29 (1999) 577. A.L. Danilyuk, V.I. Kurmashev, A.L. Matyushkov, Thin Solid Films 189 (1990) 247. G.A. Di Bari, in: M. Schlesinger, M. Paunovic (Eds.), Modern Electroplating, Wiley, New York, 2000, pp. 139–199. T.E. Buchheit, D.A. LaVan, J.R. Michael, T.R. Christenson, S.D. Leith, Metall. Mater. Trans. A 33 (2002) 539. W. Ehrfeld, Electrochim. Acta 48 (2003) 2857. E. Wicke, G.H. Nernst, Ber. Bunsenges. 68 (1964) 224. M.H. Abd Elhamid, B.G. Ateya, H.W. Pickering, J. Electrochem. Soc. 144 (1997) L58. K.-D. Song, K.B. Kim, S.H. Han, H.K. Lee, Electrochem. Comm. 5 (2003) 460. T. Mizuno, M. Enyo, in: R.E. White, et al. (Eds.), Modern Aspects of Electrochemistry, vol. 30, Plenum Press, New York, 1996, pp. 415–503. R.K. Dorsch, J. Electroanal. Chem. 21 (1969) 495. C. Arkam, V. Bouet, C. Gabrielli, G. Maurin, H. Perrot, J. Electrochem. Soc. 141 (1994) L103. O. Aaboubi, J.P. Chopart, J. Douglade, A. Olivier, C. Gabrielli, B. Tribollet, J. Electrochem. Soc. 137 (1990) 1796. A. Bund, S. Koehler, H.H. Kuehnlein, W. Plieth, Electrochim. Acta 49 (2003) 147. E. Brillas, J. Rambla, J. Casado, J. Appl. Electrochem. 29 (1999) 1367. T.Z. Fahidy, Prog. Surf. Sci. 68 (2001) 155. Y.G. Li, A. Lasia, J. Appl. Electrochem. 27 (1997) 643. C.-C. Hu, C.-M. Wu, Surf. Coat. Technol. 176 (2003) 75. E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth, VCH, Weinheim, 1996. C.-P. Chen, J. Jorne, J. Electrochem. Soc. 138 (1991) 3305. S. Chikazumi, Physics of Ferromagnetism, Wiley, New York, 1964. E. du Tremolet de Lacheresserie, Magnetisme, EDP Science, Grenoble, 2000. S. Bodea, L. Vignon, R. Ballou, P. Molho, Phys. Rev. Lett. 83 (1999) 2612. V. Heresanu, R. Ballou, P. Molho, J. Magn. Magn. Mater. 272– 276 (2004) 2439. H. Matsushima, T. Nohira, I. Mogi, Y. Ito, Surf. Coat. Technol. 179 (2004) 245. V. Madurga, J.J. Vergara, C. Favieres, J. Magn. Magn. Mater. 272–276 (2004) 1681.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 575 (2005) 221–228 www.elsevier.com/locate/jelechem
Influence of a static magnetic field on nickel electrodeposition studied using an electrochemical quartz crystal microbalance, atomic force microscopy and vibrating sample magnetometry Andreas Bund *, Adriana Ispas Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, Mommsenstr. 13, D-01062 Dresden, Germany Received 21 July 2004; received in revised form 17 September 2004; accepted 21 September 2004 Available online 18 November 2004
Abstract We investigated the influence of static magnetic fields up to 740 mT on the electrochemical nickel deposition from a sulphamate electrolyte. The magnetic field was applied parallel to the surface of the working electrode and thus gave rise to magnetohydrodynamic convection by Lorentz forces. An electrochemical quartz crystal microbalance was used to characterize the amount of hydrogen produced on the cathode during the deposition and to calculate the efficiency of the deposition process. The amount of hydrogen produced depends on the magnetic flux density, and this dependence can be explained by the complex interplay of the electrode kinetics and hydrodynamic effects, correlated with the Lorentz force. Atomic force microscopy showed an increase of the surface roughness of the nickel layers deposited in the magnetic field. The absolute values of the magnetic properties of the layers prepared in the magnetic field differed slightly from those prepared in the absence of a magnetic field. This can be explained by the structural changes. No anisotropy of the magnetic properties was observed. 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical quartz crystal microbalance; Atomic force microscopy; Vibrating sample magnetometer; Magnetic field effects; Current efficiency; Nickel electrodeposition
1. Introduction 1.1. Magnetic field effects in electrochemical reactions When a magnetic field is applied to an electrochemical cell, all the charged species that move in the electrolyte will experience a Lorentz force, which is perpendicular to the current density and the magnetic field. This force will induce convective movements in the solution that reduce the diffusion layer thickness (magnetohydrodynamic or MHD effect) [1,2]. As a consequence, the limiting current density, jL, increases. A *
Corresponding author. Tel.: +49 351 463 34351; fax +49 351 463 37164. E-mail address: [email protected] (A. Bund). 0022-0728/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.09.014
linear dependence of jL with B1/3 (B is the magnetic flux density) was reported in the literature [3,4] in the case of macroscopic electrodes, and for microelectrodes a proportionality of jL to B was found [5]. The flow profile and the Lorentz force density are related by the Navier–Stokes equation (NSE). Taking into consideration the complex geometries of cells and electrodes usually used, an analytical solution of the NSE is difficult in most cases. There are, however, some analytical and numerical approaches for simple geometries and boundary conditions [6,7]. Besides the Lorentz force, there may be forces due to inhomogeneities of the magnetic field. The inhomogeneities can be caused by the external field itself or by concentration gradients of paramagnetic ions (Cu2+, Fe2+, Co2+, Ni2+, etc.) [5,8,9]. These gradient
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forces depend on the magnitude of the magnetic field and not on its direction. Coey and Hinds argue that the magnetic forces are six orders of magnitude smaller compared to other forces occurring in an electrochemical reaction (mainly diffusion and migration) [10]. Their conclusion was that the effects of the magnetic forces are non-negligible just in the case when they are combined with the convective movements in the solution. 1.2. Electrochemical quartz crystal microbalance The electrochemical quartz crystal microbalance (EQCM) is well suited to monitor small mass changes at the working electrode in real time and in situ. Assuming that the film on the surface has a uniform thickness and that the acoustic impedance of the deposit does not differ strongly from that of the quartz, one can apply the Sauerbrey equation (1) to calculate the mass, Dm, deposited on the surface of the quartz from the frequency shift Df [11]. pffiffiffiffiffiffiffiffiffi Df ¼ 2f02 Dm= A lq qq ; ð1Þ where f0 is the frequency of the unloaded quartz crystal, A the active area of the quartz, and lq and qq are the shear modulus and the density of the quartz, respectively. Eq. (1) is the basis for the in situ determination of mass change with respect to time, pffiffiffiffiffiffiffiffiffiffi ðdm=dtÞmeas ¼ ðdf =dtÞA lq qq = 2f02 : ð2Þ Other useful information that can be taken from the quartz crystal is its damping. Near the resonance frequency, the real part of the electrical admittance (conductance) of the quartz crystal can be described by a Lorentzian function [12–14]. The full width at half maximum (FWHM) of this resonance curve is proportional to the damping of the quartz. An alternative measure of the damping is the maximum of the conductance. Its reciprocal, the motional resistance Rm of the quartz, can be measured with special oscillator circuits. In the case of metal depositions an increase of the damping of the quartz crystal, measured either as changes in w (w is the full width at half maximum (FWHM) of the resonance curve of the quartz crystal) or Rm, can be correlated with an increase of the surface roughness of the deposited layer [13,15–18]. 1.3. Nickel electrodeposition The electrodeposition of nickel from aqueous solutions is usually accompanied by the hydrogen evolution reaction (HER). Therefore, the mass deposited with respect to time will be smaller than calculated by the Faraday law (dm/dt)calc,
ðdm=dtÞcalc ¼ Mj=nF ;
ð3Þ
where M is the molar mass of the deposited species, j the current density, n the number of electrons and F is the Faraday constant. The ratio of the measured (Eq. (2)) and the calculated (Eq. (3)) deposition rates gives the current efficiency, g (Eq. (4)), and is an important parameter for technical applications. For obvious reasons, one is interested in high current efficiencies in most cases g ¼ jNi =ðjNi þ jHER Þ ¼ ðdm=dtÞmeas =ðdm=dtÞcalc ;
ð4Þ
where jNi and jHER are the partial current densities of the nickel reduction and the HER, respectively. For nickel depositions from a Watts bath, Chiba et al. [19] reported an increase of the current efficiency of 5 to 10% (50 ± 2 C) in the presence of a magnetic field up to 0.15 T (parallel to the electric field). On the other hand, Devos et al. [20] observed a decrease of some percent when a field up to 1 T was applied perpendicular to the electric field. Shannon et al. [21] made a statistical analysis of the dependence of the surface roughness on the direction of the magnetic field. They observed that, for a horizontal magnetic field, the roughness increased, while it decreased for a vertical magnetic field. Danilyuk affirmed that, in the case of copper, some values of the magnetic field (from 60 to 160 mT) accelerate the electrodeposition reaction, and others (from 160 to 173 mT) inhibit it. In the case of nickel and tin, these regions were alternating. As a consequence changes in structure and morphology may result [22]. Given the various interactions of a magnetic field in an electrochemical reaction, it seems promising to tailor the magnetic forces to optimize the mass transport or the properties of the deposited layer. However, before such tailoring can occur, the fundamentals of the interaction of magnetic fields in an electrochemical reaction must be understood better. In this paper, we applied an EQCM with damping monitoring to study the deposition rate of nickel from a sulphamate bath in the presence of static magnetic fields. The sulphamate electrolyte was chosen because of its high relevance in technical applications, which is based on the very high plating rates and the low internal stresses in the deposit [23]. Typical technical applications comprise electroforming, surface finishing and the fabrication of microelectromechanical systems (MEMS) by the LIGA (Lithographie, Galvanik, Abformung) process [23–25]. The deposition in MEMS structures is often strongly limited by mass transport. To achieve void free filling, special additives in combination with pulse plating must be applied. A carefully designed magnetic field could be used to enhance the mass transport in microstructures.
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2. Experimental The electrolyte consisted of 1.26 M Ni(SO3NH2)2 · 4H2O + 0.32 M H3BO3 + 0.04 M NiCl2 · 6H2O. For some experiments 0.52 mM sodium dodecyl sulfate (SDS) or 0.24 mM sodium 2-ethylhexyl sulphate were added as surfactants. The electrolyte was adjusted to pH 4 by adding sulphamic acid (HSO3NH2) or nickel carbonate (NiCO3). All solutions were prepared from p.a. grade chemicals and bidistilled water. All measurements were made at room temperature. For the EQCM experiments 10 MHz optically polished AT-cut quartzes provided by KVG (Neckarbischofsheim, Germany) were used. They had two gold electrodes, of 100 nm thickness, deposited on the opposite faces of the quartz crystal, on a 5–10 nm adhesion layer of chromium. The resonance frequency of the quartz was measured with a Maxtek PLO-10 phase lock oscillator, (Santa Fe Springs, USA) and a frequency counter card (GT200, Guide Technologies). The PLO circuit also provides information about the damping of the quartz as a voltage, which was acquired with a Keithley DMM and converted to the motional resistance, Rm, of the quartz. An alternative EQCM set-up consisted of an Advantest R3758BH network analyzer (Advantest, Tokyo, Japan). This device records the admittance spectra of the resonator near its resonance frequency. Each admittance spectrum was transferred to a computer via a GPIB card and then fitted with a Lorentzian function [13]. The connection between the electrochemical cell and the network analyzer was made by a pi network adapter (TeleQuarz, Neckarbischofsheim, Germany) and a LC network (capacity C = 0.1 lF, inductance, L = 1 mH) to decouple the high and low frequency signals of the quartz and the electrochemistry, respectively. Details can be found in Ref. [13]. One gold electrode was used as the working electrode (radius 3 mm). The counter electrode was a nickel circular plate, of radius, 10 mm and thickness, 2.5 mm, placed in parallel to the working electrode. The distance between the working and the counter electrodes was approximately 15 mm. Two kinds of reference electrode were used, lab-made Hg|Hg2Cl2, and a commercial ‘‘double junction’’ Ag|AgCl|3 M KCl electrode (Metrohm, Germany). The electrochemical cylindrical cell (36 mm inner diameter and 42 mm height) was made from Teflon. The quartz was placed at the bottom of the cell, and fixed between an o-ring (Viton, solution side) and a silicone gasket (air side). The electrochemical experiments were made with a potentiostat/galvanostat Model 263A (EG&G Instruments, Princeton, NJ). The whole setup was controlled by lab-made software (Microsoft Visual C++). A water-cooled electromagnet from VEB Polytechnik, Phylatex, Karl-Marx Stadt, was used to produce
Fig. 1. Schematic representation of the experimental setup for the EQCM measurements in magnetic fields. Q: quartz crystal: C: cell (PTFE); E electrolyte; WE, CE, and RE: working, counter and reference electrode; NA: network analyzer; N and S indicate the poles of the electromagnet.
the magnetic field. The maximum flux density was 740 mT for a gap of 46 mm. The magnetic field was applied parallel to the working electrode, i.e., perpendicular to the current lines (see Fig. 1 for a schematic representation). The intensity of the magnetic field was measured with a Hall probe (Lake Shore, model 450). The surface morphology of the deposited layers was investigated by contact mode AFM (atomic force microscopy). The AFM scanner was a PicoSPM, version 2.4 (Molecular Imaging, Phoenix, AZ). Pyramidal tips of silicon nitride were used. For the characterization of the magnetic properties of the layers, thin nickel layers (ca. 3 lm) were deposited at 3.5 mA cm2 onto cylindrical copper electrodes (diameter 6 mm, height 6 mm). Hysteresis curves (from 1.0 to 1.0 T) were recorded with a vibrating sample magnetometer (VSM, model 7300, Lake Shore, OH). The magnetic field H could be applied parallel (in the following called the 0 position) and normal (90) to the plane of growth. Furthermore, the sample could be rotated in the H field to check for anisotropies.
3. Results and discussion 3.1. Hydrogen evolution and current efficiency For the relatively small current density of 0.1 mA cm2, the deposition potential shifted from 0.74 V at 0 T to 0.67 V at 530 mT (Fig. 2(a)). At 50 mA cm2 a shift from 1.46 V (0 T) to 1.40 V (740 mT) was observed (Fig. 2(b)). For the setup sketched in Fig. 1, the Lorentz force points out of the paper plane. This force density leads to a gentle stirring of the electro-
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-0.3
5 jHER(B) / jHER(B=0)
E vs SCE /V
-0.4 -0.5 -0.6 -0.7 -0.8
0
10
(a)
20 30 time / min
40
50
4 3 2 1 0 0
2 00
(a)
-1.1
400
600
B / mT
-1.2
-1.4 -1.5 -1.6
(b)
0
50
100 150 200 250 300 350 time /s
Fig. 2. Influence of the magnetic flux density on the deposition potential during galvanostatic nickel depositions. (a) j = 0.1 mA cm2, (—) 0 T, (– –) 530 mTb; (b) j = 50 mA cm2, (—) 0 T, (– –) 740 mT.
lyte in front of the quartz and thus decreases the concentration polarization (see Section 1.1). The total current density is the sum of the nickel reduction current and HER. The hydrogen evolution reaction (HER) during metal depositions from aqueous solutions is of special interest in electrochemical reactions [26–28]. One of the reasons for this interest is the risk of hydrogen embrittlement that degrades the mechanical properties of metals [29]. The partial current density of the HER can be calculated by multiplying the total current density with (1 g), where g is the current efficiency for nickel deposition, Eq. (4). For a total current density of 0.1 mA cm2, the partial current of the HER increased with increasing magnetic flux density (Fig. 3(a)), whereas it remained constant for a total current density of 50 mA cm2 (Fig. 3(b)). In these measurements, a surfactant (sodium dodecyl sulphate, SDS) had been added to the electrolyte. The results were the same when SDS was replaced by sodium 2-ethylhexyl sulphate. In Fig. 3, each data point is the average of four measurements and the error bars reflect the standard error of the mean value. In the following we propose a model, which explains the influence of a magnetic field on the current efficiency at small and large current densities, respectively. It has been shown that the current efficiency of the nickel deposition increases with increasing current density [30,31]. This is because the HER is usually limited by mass
jHER(B) / jHER(B=0)
E vs SCE /V
2 -1.3
1
0
0 (b)
200
400
600
800
B / mT
Fig. 3. Dependence of the partial current density of the HER, jHER, on the magnetic flux density during nickel deposition from a sulphamate bath with surfactant. The values of jHER have been normalized with respect to the value without a magnetic field jHER (B = 0). (a) Total current density–0.1 mA cm2; (b) 50 mA cm2.
transport, whereas the nickel reduction is activation (or mixed) controlled. Therefore, making the potential more negative will result in an increase of jNi, while jHER remains constant. As a result the current efficiency of nickel reduction increases (Eq. (4)). Under the influence of a magnetic field, both limiting current densities will increase due to the magnetohydrodynamic stirring. The increase will depend on several parameters, such as the magnetic flux density and the concentration [6,32]. As the concentration of nickel ions is much larger than the proton concentration, we can assume that the nickel deposition is influenced more strongly by the magnetic field (see Fig. 4 for a schematic representation). Because of the activation control, the absolute value of the nickel current increases strongly with the overvoltage (or with the total current density in a galvanostatic experiment under steady state conditions). For a potential close to the equilibrium potential of nickel (E1 in Fig. 4), the MHD effect increases jHER more strongly than jNi as seen in Fig. 3(a). At a higher polarization (E2 in Fig. 4) jHER and jNi are both under transport control and the effect of the magnetic field is to increase both. In a galvanostatic experiment, where
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225
0
E2
E1
jHER(B) / jHER (B=0mT)
cathodic current density
12
8
4
0
polarization Fig. 4. Schematic representation of the effect of a magnetic field on the partial current densities of the Ni reduction, jNi, and the HER, jHER. (—) HER without magnetic field, (– –) HER in magnetic field, ( ) Ni reduction without magnetic field, (– Æ – Æ –) Ni reduction in magnetic field.
jNi + jHER must be constant, jHER will not change greatly with B as is seen in Fig. 3(b). As the current efficiency was determined using the EQCM, one needs to consider the systematic errors that could be introduced by this measuring technique. First, there could be mass changes which are not directly related to the electric current, such as the deposition of Ni(OH)2 due to pH changes at the electrode [20]. This precipitation occurs at high current densities and usually leads to apparent current efficiencies greater than unity [30,31]. As this was not observed in our case and because energy dispersive X-ray analysis indicated no oxygen in the layers we assume that this side reaction is negligible. As will be shown below (Section 3.2), there is an influence of the magnetic field on the surface roughness. It is known that surface roughness effects can cause frequency and damping changes [18]. In our case, the damping changes, Dw, are of the order of some kHz while the frequency shift, Df, is of the order of 100 kHz. It has been shown [15] that the mass calculated from the frequency shift is correct as long as Dw Df. To check how the surfactant affects the hydrogen evolution in a magnetic field, nickel was deposited from electrolyte free of surfactant at a relatively small current density. The partial current density of the HER increased with the magnetic flux density (Fig. 5) as in the presence of the surfactant (Fig. 3(a)). However, when one looks at the absolute values, one sees that, in the presence of the surfactant, the HER increases more strongly. This can be explained by the reduced surface tension in the presence of the surfactant. Bubbles formed at the cathode can detach more easily from the electrode and make space for new bubbles. As a result the partial current density of the HER increases more strongly with B. The layers prepared with surfactant addition had a systematically smaller surface roughness. This finding shows the role of hydrogen bubbles for the development of surface roughness (see discussion below).
0
200
400
600
B / mT Fig. 5. Hydrogen evolution during galvanostatic deposition (j = 0.5 mA cm2) from a nickel sulphamate bath with surfactant (sodium 2ethylhexyl sulphate), (s), and without (h).
3.2. Surface morphology and roughness In a previous paper, we investigated the effect of a static magnetic field up to 1 T (parallel to the electrode) on the grain size of potentiostatically deposited nickel layers [33]. It was found that a larger amount of finegrained material was formed in the presence of the magnetic field. In the present paper, we will focus on the morphology of the deposited layer and on the surface
Fig. 6. Surface topographies of nickel layers deposited at 50 mA cm2 without a magnetic field (a) and in the presence of a magnetic flux density of 740 mT (b).
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Table 1 Dependence of the surface roughness parameters Rq and Ra of galvanostatically deposited Ni layers on the magnetic flux density B of an externally applied magnetic field 0
530
740
jtotal = 0.5 mA cm2, layer thickness 112 ± 4 nm 4.91 ± 0.54 5.33 ± 0.68 Ra (nm) Rq (nm) 6.33 ± 0.73 7.22 ± 1.26
8.68 ± 1.77 11.85 ± 2.54
jtotal = 50 mA cm2, layer thickness 4.3 ± 0.2 lm Ra (nm) 5.14 ± 1.18 23.26 ± 2.44 Rq (nm) 6.55 ± 1.51 29.66 ± 2.39
26.93 ± 2.48 34.05 ± 2.29
12 10 damping ∆Rm / Ω
B (mT)
14
8 6 4 2 0 -2 0
roughness. Comparing the AFM images in Fig. 6(a) (no magnetic field) and Fig. 6(b) (B = 740 mT), one can see that the magnetic field induces an increase of the surface roughness (note the different scales of the z-axis). To characterize the surface roughness, the standard deviation of the height values, Rq, and the mean roughness, Ra, were calculated using standard software (Table 1). Each value is the average of four independent layers, and at each layer, four determinations at different locations at the surface were made. When comparing the results of Table 1 (surface roughness increases with B) with our previous results (amount of fine grained material increases with B) [33], one must take into account that the surface morphology, the grain size and the orientation of deposits depend strongly on the electrolyte composition and on the condition of deposition [34,35]. Furthermore, a general correlation of the grain size with surface roughness is difficult [36,37]. For copper electrodepositions Hu and Wu [37] showed that an increase of the mean grain size does not necessary induce an increase in the roughness of the deposit. Qualitative in situ information about the surface roughness can be taken from the EQCM data. Some typical changes of the quartz crystal are presented in Figs. 7 and 8. Fig. 7 presents the changes in the full
5 damping ∆w / kHz
4 3 2 1 0 -1 0
100
200
300
400
time /s Fig. 7. Change of the damping of the EQCM (represented by the change of the full width at half maximum of the resonance curve, Dw) during Ni deposition at 50 mA cm2 for different magnetic flux densities (n) 0 mT, (s) 206 mT, (h) 740 mT.
500
1000
1500
2000
2500
time / s Fig. 8. Change of the damping of the EQCM (represented by the change of the motional resistance of the quartz crystal, DRm) during Ni deposition at 0.1 mA cm2 for different magnetic flux densities (n) 0 mT; (s) 410 mT; (h) 530 mT.
width at half maximum (Dw) of the resonance curve of the quartz for a deposition current density of 50 mA cm2. In the early stages of the deposition process, the damping becomes negative. This finding can be explained with a smoothing of the surface. For the relatively small current density of 0.1 mA cm2, the damping increases continuously (Fig. 8). Note that Fig. 8 presents the damping increase as the change of the motional resistance of the quartz crystal. For both current densities, the damping increases with the deposition time (i.e. layer thickness). The effect of the magnetic field is to increase the damping more strongly. The damping increase indicates that the surface becomes rougher which is in perfect agreement with the ex situ AFM information. The reason for the spikes in the damping for the highest magnetic flux density (h in Fig. 7) is not completely clear at the moment. Obviously the surface roughness goes through maxima for short time periods under the influence of a magnetic field. Interestingly the plots of the electrode potential vs time do not show these spikes (Fig. 2(b)). Future work will aim at the systematic investigation of this effect. In the following, we propose a mechanism that can explain the correlation of the surface roughness and the magnetic flux density. It is based on the interplay of magnetohydrodynamic convection with the concentration profiles and the hydrogen bubbles. In general, surface roughness develops due to uneven nucleation and crystal growth [38]. Chen and Jorne applied the Mullins–Sekerka morphological stability theory to identify the major factors that influence the evolution of surface roughness [39]. According to them, one of the most important factors that favor morphological instabilities (and thus a high surface roughness) is the increase of the concentration gradient at the electrode|electrolyte interface. Because the MHD effect increases the concentration gradient, the presence of a magnetic field will increase the surface roughness. Furthermore, as outlined
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above, nickel electrodeposition is always accompanied by hydrogen evolution. As the hydrogen bubbles represent obstacles for the flow profile induced by the magnetic field, micro-turbulences will form in front of the electrode [20,35]. Thus, the current distribution becomes inhomogeneous, which results in rougher layers. Because higher current densities induce higher Lorentz forces and thus stronger convection, the effect increases with the current density (Table 1).
The nickel ions in the electrolyte are paramagnetic species. Once they are reduced to metallic nickel they begin to form a ferromagnetic layer with a certain magnetization, even in the absence of an external magnetic field. The orientation of this magnetization is the result of the energy balance between the exchange energy, the dipolar energy, the magneto-crystalline energy and the magnetoelastic energy [40,41]. One might assume that an external magnetic field, which is applied during the deposition of the layer, might induce some magnetic anisotropy. In the following we discuss briefly which terms might cause a magnetic anisotropy in the case of nickel layers. The dipolar energy is associated with the magnetization in its own field, and depends on the shape of the grain. This leads to a shape anisotropy that favours a magnetization along the larger dimension of a ferromagnetic body. In the case of a magnetic layer, the shape anisotropy favors the in plane magnetization. It has been reported that the dipolar forces can influence the process of nucleation and growth of a layer, for example in the case of iron arborescent growth [42]. The magneto-crystalline energy is caused by the spinorbit coupling and can be the source of anisotropy, too. This anisotropy favors a magnetization along a given crystalline axis, called the easy axis of magnetization, and may be responsible for a perpendicular magnetiza-
14.4
Hc/ mT
Table 2 Dependence of some important magnetic properties of nickel layers on the magnetic flux density applied during the electrodeposition Angle between sample and field H
B (mT)a
Slope at Hc (pA m2 T1)
Ms (lA m2)
Mr (lA m2)
0
0 740
22.4 16.6
46.3 34.0
22.3 16.9
90
0 740
23.0 14.9
50.7 35.6
23.2 14.9
a
3.3. Magnetic properties of the deposited layers
14.0
13.6
227
Magnetic flux density superimposed during deposition.
tion. Nickel has an fcc crystalline structure and its easy axis is [1 1 1] and the hard axis is [1 0 0]. In the case of nickel, this anisotropy can be neglected, especially if the orientation of the grains is isotropic. The magneto-elastic energy is induced by the deformation of the crystalline cell and can result in stress anisotropy. X-ray diffractograms (not shown) did not show any shift of the reflections of nickel if the layers were deposited in a magnetic field. Therefore, we can rule out this source of anisotropy. From all the terms that may be a source of anisotropic magnetic properties, the most important in our case is the dipolar one. Hysteresis curves (not shown) showed no in-plane anisotropy for nickel layers deposited in a magnetic field, and no relevant dependence of the coercive field, Hc, on the degree of rotation in the plane of growth was found (Fig. 9). The higher values of Hc seen in Fig. 9 for the sample prepared in the magnetic field are due to a higher layer thickness (see total electric charges in caption of Fig. 9). The slope of the magnetization curve in the coercive field (dM/dH)Hc (differential susceptibility) measured in the vertical plane also showed no anisotropy. In conclusion it can be stated that no anisotropy of the magnetic properties was found. However, the absolute values of (dM/dH)Hc, the magnetization of saturation, Ms and the residual magnetization, Mr, decreased by approximately 20% for the sample deposited in a field of 740 mT (Table 2). It is known that changes in morphology of a layer will induce changes in the magnetic properties [43–45]. In our case, the magnetic field changes the surface roughness (see Section 3.2) of the deposited nickel (and maybe, the layer thickness slightly, via the influence on the current efficiency, see Section 3.1), thus inducing slight changes of the coercive field, the saturation magnetization and the residual magnetization.
13.2 0
100
200
300
400
4. Conclusions
degree of rotation Fig. 9. Coercive field strength of Ni layers deposited at 3.5 mA cm2 in the absence of a magnetic field (s, total charge Q = 7.21 C cm2) and at 740 mT (h, Q = 10.40 C cm2), respectively.
The influence of a parallel (with respect to the surface of the working electrode) static magnetic field on the electrochemical reaction, the morphology and the
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magnetic properties of electrochemically deposited nickel layer has been investigated. The layers prepared in the magnetic field had a considerably higher surface roughness. Furthermore, the magnetic field increased the hydrogen evolution at small current densities, which has been explained by a complex interplay of hydrodynamics and the electrode kinetics. No in-plane anisotropy of the magnetization was observed for the deposited layer. All changes of the magnetic properties could be explained on the basis of the morphology effects.
[15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgements This work was supported financially by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 609. The authors would like to thank Dr. Stefan Roth (Leibnitz Institute, Dresden) for his assistance in the VSM measurements and Dr. Vasile Heresanu (CNRS, Laboratoire Louis Neel, Grenoble) for interesting and helpful discussions concerning the magnetic anisotropy.
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