Electrodeposition of Co-Ni and Co-Ni-Cu systems in sulphate-citrate medium

Article abstract of DOI:10.1016/j.electacta.2005.04.010

Electrodeposition of Co-Ni and Co-Ni-Cu alloys was performed in a sulphate-citrate medium. Experimental electrodeposition parameters (pH, cobalt(II), nickel(II) and citrate concentrations) were varied in order to analyse their influence on the deposition. Anomalous Co-Ni codeposition occured in the citrate medium. High [Ni(II)]/[Co(II)] ratios (above 5) were suitable for the preparation of homogeneous magnetic Co-rich Co-Ni deposits of hexagonal close-packed (hcp) structure or face centred cubic (fcc) structure as a function of the deposition potential. The presence of very low copper(II) concentrations (<10^-2 mol dm^-3) in the nickel-cobalt bath makes it possible to incorporate copper in the deposits in amounts ranging from 5 to 60% Cu, although uniform deposits are obtained only for low copper percentages. These ternary deposits are solid solutions with fcc structure and magnetic behaviour both dependent on the deposition potential.

Full text of DOI:10.1016/j.electacta.2005.04.010

Electrochimica Acta 51 (2005) 146–153
Electrodeposition of Co–Ni and Co–Ni–Cu systems
in sulphate–citrate medium
∗,1
1
´
´
´
Elvira Gomez , Salvador Pane, Elisa Valles
Laboratori d’Electrodeposicio´ i Corrosio´ (Electrodep), Departament de Qu´ımica F´ısica,
Facultat de Qu´ımica, Universitat de Barcelona, Mart´ı i Franq ue`s, 1, 08028 Barcelona, Spain
Received 8 February 2005; received in revised form 13 April 2005; accepted 15 April 2005
Available online 10 May 2005
Abstract
Electrodeposition of Co–Ni and Co–Ni–Cu alloys was performed in a sulphate–citrate medium. Experimental electrodeposition parameters
(pH, cobalt(II), nickel(II) and citrate concentrations) were varied in order to analyse their influence on the deposition. Anomalous Co–Ni
codeposition occured in the citrate medium. High [Ni(II)]/[Co(II)] ratios (above 5) were suitable for the preparation of homogeneous magnetic
Co-rich Co–Ni deposits of hexagonal close-packed (hcp) structure or face centred cubic (fcc) structure as a function of the deposition potential.
The presence of very low copper(II) concentrations (<10−2 mol dm−3) in the nickel–cobalt bath makes it possible to incorporate copper in
the deposits in amounts ranging from 5 to 60% Cu, although uniform deposits are obtained only for low copper percentages. These ternary
deposits are solid solutions with fcc structure and magnetic behaviour both dependent on the deposition potential.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Cobalt–nickel; Cobalt–nickel–copper; Magnetic alloy; Electrodeposition; Ternary alloy
1. Introduction
position in this medium, where the less noble constituent is
deposited preferentially.
Electrodeposition has been shown to be a useful tool
for preparation of alloys or heterogeneous films of different
metals for several magnetic applications. Co-based binary
alloys with magnetic or magnetoresistive properties were
studied and prepared by electrodeposition in our laboratory:
Co–Ni alloy was deposited from chloride and implemented in
MEMS technology [1–4] and Co–Cu heterogeneous deposits
[5–8] were obtained from a sulphate–citrate medium.
The functional properties of electrodeposited Co–Ni alloy
depend greatly on their composition, which should be turn
strongly affected by deposition parameters. A characteristic
feature is that in the chloride medium studied, the Co/Co + Ni
ratio in the alloy is considerably higher than in the bath, in-
dicating the anomalous character of Co–Ni alloy electrode-
The aim of the present study is to test the possibilities
of the electrodeposition to introduce copper on the Co–Ni
system and to prepare ternary Co–Ni–Cu alloys from a
sulphate–citrate bath without additives. Co–Ni–Cu alloys
would be used as either magnetic or magnetoresistive ma-
terials with better corrosion resistance than Co–Cu system.
For this ternary system, research works related to the code-
position from pyrophosphate or ammonium citrate media
[9,10] are in the bibliography and recently, to the prepara-
tion of Co–Ni (Cu)/Cu multilayers [11–15]. A previous study
of the behaviour of Co–Ni electrodeposition in the selected
medium is necessary due to the controversy given in the liter-
ature about that Co–Ni anomalous codeposition occurs only
in baths that contain simple cobalt and nickel salts [16].
The present paper shows the results of laboratory research
centred both in the deposition of Co–Ni and Co–Ni–Cu al-
loys using sulphate–citrate solutions. The objective was to
determine the way in which the composition of alloys, mor-
phology and structure are dependent upon several variations
∗
Corresponding author. Tel.: +34 93 402 12 34; fax: +34 93 402 12 31.
E-mail addresses: e.gomez@ub.edu (E. Go´mez), e.valles@ub.edu
(E. Valle´s).
1
ISE members.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.04.010

E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
147
of the electrolyte composition and electrolysis parameters as
the concentration of cobalt and nickel ions, citrate concen-
tration and applied potential.
were separated from the electrode and mounted on sili-
con. Cu K␣ radiation (λ = 1.5418 A) was selected by means
of a diffracted-beam flat graphite monochromator crystal.
Diffractograms were obtained with 2θ ranging from 10◦ to
100◦ with a step size of 0.05 and a measuring time of 5 s per
step.
˚
2. Experimental
The magnetic measurements were taken at room tempera-
ture with a Manics DSM8 pendulum-type magnetometer and
a SQUID magnetometer.
The electrochemical measurements were performed
in a conventional three-electrode cell using a potentio-
stat/galvanostat Autolab with PGSTAT equipment and GPES
software. The temperature was maintained at 25 ◦C. Chem-
icals used were CoSO4·7H2O, NiSO4·6H2O, CuSO4·5H2O
and Na3C6H5O7·2H2O, all of analytical grade. All solutions
were freshly prepared with water which was doubly distilled
and then treated with a Millipore Milli Q system. Before each
experiment the solutions were de-aerated by argon bubbling
and they were maintained under argon atmosphere during the
experiment.
3. Results
3.1. Co–Ni electrodeposition in sulphate–citrate bath
The preliminary study of the cobalt–nickel electrodeposi-
tion process in sulphate–citrate medium was performed using
a total Ni(II) + Co(II) concentration of 0.2 mol dm−3, varying
the [Ni(II)]/[Co(II)] ratio between 10 and 0.33, at different
sodium citrate concentrations (0.2–0.5 mol dm−3) and solu-
tion pH (3–5).
A working electrode of vitreous carbon (Metrohm) of
0.0314 cm2 was used. It was polished to a mirror finish using
alumina of different grades “3.75 and 1.85 ␮m” and cleaned
ultrasonically for 2 min in water before each experiment. The
reference electrode was Ag/AgCl/1 mol dm−3 NaCl mounted
in a Luggin capillary containing 0.5 mol dm−3 Na2SO4 solu-
tion. All potentials are referred to this electrode. The counter
electrode was a 3 cm diameter platinum spiral located par-
allel to the working electrode in order to optimize current
distribution.
Fig. 1A shows the voltammetric response of
a
0.175 mol dm−3 NiSO4 + 0.025 mol dm−3 CoSO4 + 0.2 mol
dm−3 C6H5Na3O7, pH 4, solution, at stationary condi-
tions, next to the voltammograms of 0.2 mol dm−3 Ni(II)
and 0.2 mol dm−3 Co(II) solutions in the same complexing
medium. At these conditions, the onset of the voltammet-
ric reduction current appeared at intermediate potential value
between those corresponding to nickel and cobalt. Complex
oxidation response was recorded with no clear features. The
stirring of solution during the deposition/oxidation process
favoured the evolution of the hydrogen formed during the
cathodic scan, minimizing its partial oxidation [17–19] and
leading to more defined peaks for cobalt and alloy oxidation
(Fig. 1B). AgradualdecreaseoftheQox/Qred ratiointheorder
Co(II)/Co > Ni(II) + Co(II)/alloy > Ni(II)/Ni was observed.
IncreasingsodiumcitrateconcentrationorsolutionpH, the
deposition of the alloy was shifted to negative potentials and,
simultaneously, the Qox/Qred ratio was drastically decreased.
To analyse the initial stages of the Co–Ni deposition in
citrate medium, potentiodynamic stripping of deposits was
made under stirring. For deposits obtained at low deposition
Voltammetric experiments were carried out at 50 mV s−1
,
scanning initially towards negative potentials. Only one cy-
cle was run in each voltammetric experiment. Chronoamper-
ometric experiments were done from an initial potential at
which no process occurred to a potential value at which re-
duction process occurred. Stripping experiments of deposits
oxidation were recorded at 10 mV s−1, from a potential value
at which no current was detected.
Deposits were prepared with stirring (ω = 100 rpm, mag-
netic stirrer), since more uniform and reproducible deposits
can be obtained under hydrodynamically controlled condi-
tions specially at low [Cu(II)]/([Co(II)] + [Ni(II)]) ratios. The
morphology of the deposits was examined with a Hitachi S-
2300 Scanning Electron Microscope.
Film composition was determined by electron probe mi-
croanalysis (EPMA), performed on a Cameca SX-50 electron
microprobe and/or inductively coupled plasma mass spec-
trometry (ICP-MS). For chemical analysis, the deposits were
dissolved in 5 ml of 1% nitric acid solution and measurements
were taken on a Perkin-Elmer spectrometer Elan 6000. Certi-
fied standard solutions were used to calibrate the instrument.
The results of ICP-MS and EPMA analysis were in agree-
ment in all cases. The efficiency of the deposition process
was calculated by comparing the reduction charge deduced
from chemical analysis and that involved in the preparation
of the film.
potentials and very low charges (around 60–300 mC cm−2
)
(Fig. 2A, points a and b) a small oxidation peak at negative
potentials was detected (Fig. 2B, curves a and b) although
no displacement in the oxidation peak was observed. Upon
increasing the deposition time (Fig. 2A, points c and d), a sec-
ond peak centred at more positive potentials was developed
(Fig. 2B, curves c and d). This second oxidation peak gradu-
ally shifted to positive potentials as the deposition potential
was made more negative (Fig. 3).
In order to elucidate the stripping response, a set of de-
posits prepared at low charges were chemically analysed us-
ing ICP-MS. The nickel percentage of the deposits, which
oxidized only under the first peak, varied between 38 and
57% Ni, increasing Ni percentage as the deposition poten-
XRD analysis was performed on a Philips MRD diffrac-
tometer. To avoid the vitreous carbon response, the films

148
E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
Fig. 2. Solution(c)ofFig. 1. (A)Potentiostaticcurvefrom−500to+980 mV.
ω = 100 rpm. (B)Strippingcurvesofdepositsobtainedatdifferenttimesfrom
the j–t transient of (A). (a) 30 s, (b) 50 s, (c) 100 s and (d) 150 s; 10 mV s−1
.
Fig. 1. Cyclic voltammograms at 50 mV s−1 of (a) 0.2 mol dm−3 NiSO4 +
0.2 mol dm−3 Na3C6H5O7, (b) 0.2 mol dm−3 CoSO4 + 0.2 mol dm−3
Na3C6H5O7
and (c) 0.175 mol dm−3
NiSO4 + 0.025 mol dm−3
CoSO4 + 0.2 mol dm−3 Na3C6H5O7, pH
4 solutions. (A) Quiescent
conditions and (B) ω = 100 rpm.
tial decreased. However, the analysis of deposits obtained
increasing deposition time showed a nickel percentage in the
range 17–23%. These deposits oxidize mainly under the sec-
ond peak. A second cobalt-rich deposit was formed on the
first nickel-richer deposit, which oxidized at more positive
potentials as the nickel percentage increased.
Fig. 3. Solution (c) of Fig. 1. Stripping curves of deposits obtained poten-
tiostatically at ω = 100 rpm, at (a) −980 mV, 150 s; (b) −1010 mV, 70 s and
(c) −1050 mV, 30 s.
Table 1
Influence of deposition potential on the composition of the Co–Ni
deposits obtained from
CoSO4 + 0.2 mol dm−3 Na3C6H5O7, pH 4 solution
a
0.175 mol dm−3 NiSO4 + 0.025 mol dm−3
Deposits involving charges between 600 mC cm−2 and
380 C cm−2 were prepared. In order to assure the repro-
ducibility of the results, three replicas were analysed at each
condition and no differences higher than 1% were found.
Table 1 shows the average percentages and confirms the pres-
ence of cobalt rich deposits whose specific composition was
slightly dependent on the deposition potential. Results cor-
roborate constant composition with increasing charge at the
stirred conditions selected. Then, high [Ni(II)]/[Co(II)] ra-
tios led to cobalt-rich deposits, showing that over an initial
deposit with higher nickel percentage, anomalous codeposi-
tion takes place also in the citrate medium used, as occurs in
other baths [1,20–24].
−E (mV)
t (min)
Q (C cm−2
)
Ni (%)
Co (%)
980
2.5
0.6
17
17
83
83
320
4.2
83
267
4.2
330
1.9
76
390
3.8
1010
1050
1070
17
18
18
83
82
82
20
19
21
80
81
79
12
19
14
440
5.5
8.3
6.5
16
23
23
77
77

E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
149
Fig. 5. X-ray diffractograms of Co–Ni deposits of Q = 318 C cm−2
of charge obtained from
a
0.175 mol dm−3 NiSO4 + 0.025 mol dm−3
CoSO4 + 0.2 mol dm−3 Na3C6H5O7, pH 4 solution, ω = 100 rpm. (a)
−980 mV, 17% Ni and (b) −1050 mV, 21% Ni.
morphological changes: clear acicular morphology was ob-
served, similar to that of pure hcp cobalt, when main Co–Ni
hcp structure was detected (Fig. 6A). On the other hand, de-
posits of fcc structure corresponded to nodular morphology
(Fig. 6B). The structural–morphological changes observed
Fig. 4. X-ray diffractograms of (A) nickel electrodeposited from a
0.2 mol dm−3 NiSO4 + 0.2 mol dm−3 Na3C6H5O7, pH
solution at
4
−1000 mV, Q = 318 C cm−2, ω = 100 rpm and (B) cobalt electrodeposited
from a 0.2 mol dm−3 CoSO4 + 0.2 mol dm−3 Na3C6H5O7, pH 4 solution at
−1025 mV, Q = 318 C cm−2, ω = 100 rpm.
The structure of high-charge deposits prepared at dif-
ferent potentials was analysed from XRD. Pure cobalt and
nickel electrodeposits obtained at different deposition poten-
tials from the same medium were also analysed.
Diffractograms of nickel deposits (Fig. 4A) show the
peaks of the fcc structure, with a cell parameter around
˚
a = 3.524 A. Electrodeposited cobalt was hcp structure, with
˚
˚
cell parameters a = 2.504 A, c = 4.098 A and 1 0 0 + 1 1 0 pre-
ferred orientation (Fig. 4B). All Co–Ni deposits showed
several peaks that revealed their crystalline nature, but the
diffraction peaks obtained varied as a function of the deposi-
tion potentials (Fig. 5). Deposits of 17% Ni obtained at low
potentials (Fig. 5, curve a) showed the peaks of an hcp struc-
ture, with a small peak centred at 51◦ 2θ assigned to a sec-
ondary fcc structure. Diffraction peaks were shifted to greater
2θ values with respect to those of hcp deposited cobalt as a
consequence of nickel incorporation in the deposit. Cell vol-
ume of Co–Ni hcp deposits was lower than that of hcp cobalt.
Deposits obtained at more negative potentials, evolved to fcc
structure. Thus, for deposits of 23% Ni (Fig. 5, curve b),
a clear fcc structure with 110 preferred orientation was de-
tected. These structural modifications were accompanied by
Fig. 6. SEM micrographs of the Co–Ni deposits of Fig. 5. (A) −980 mV,
17% Ni and (B) −1050 mV, 21% Ni.

150
E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
Fig. 7. Parallel (a) and perpendicular (b) hysteresis loops for a Co–Ni de-
posit obtained from the solution of Fig. 5 at −1050 mV, Q = 318 C cm−2
,
ω = 100 rpm.
seem more related to the different deposits growth rate than
the involved compositional changes.
Magnetic properties of Co–Ni alloys were analysed by
hysteresis loops. The magnetic response of Co–Ni deposits
obtained at different potentials was similar because no great
variation of composition was found as a function of the
potential. Saturation magnetisation was in the range of
120–140 emu g−1 (150 for electrodeposited cobalt and 52
for electrodeposited nickel), and coercivity values ranged be-
tween 70–100 Oe (Fig. 7). When parallel and perpendicular
magnetic fields were applied, slower response was obtained
in the second case. The easy axis is the in-plane direction of
the film parallel. Saturation field under the perpendicular field
was clearly higher than the saturation field under the parallel
one. These features indicated strong uniaxial anisotropy, in
addition to the usual shape anisotropy [25]. Alloy films were
spontaneously orientated in the direction parallel to the field.
Fig.
8. (A)
Cyclic
voltammograms
at
50 mV s−1
of
0.175 mol dm−3
NiSO4 + 0.025 mol dm−3
CoSO4 + 0.2 mol dm−3
Na3C6H5O7 + x mol dm−3 CuSO4, pH 4 solutions. (a) x = 7.4 × 10−4
,
(b) x = 1.5 × 10−3 and (c) 3.0 × 10−3
.
Cathodic limit: −1300 mV.
Quiescent conditions. (B) Cyclic voltammogram at 50 mV s−1 of
0.175 mol dm−3
NiSO4 + 0.025 mol dm−3
CoSO4 + 0.2 mol dm−3
Na3C6H5O7 + 3.0 × 10−3 mol dm−3 CuSO4, pH 4 solution. Cathodic limit:
−890 mV. Quiescent conditions.
ditions, although for a fixed cathodic limit the voltammetric
oxidation peak appeared at more positive potential than under
quiescent conditions. Stirring favours Cu(II) species arrival
at the electrode making easy their incorporation in the film.
3.2. Influence of copper on Co–Ni electrodeposition
The influence of low copper(II) concentrations
(in the range 7 × 10−4 to 3 × 10−3 mol dm−3
) on
the Co–Ni electrodeposition from
a
0.175 mol dm−3
NiSO4 + 0.025 mol dm−3 CoSO4 + 0.2 mol dm−3 C6H5Na3
O7, pH 4, solution was analysed.
Voltammograms recorded at different copper concentra-
tions showed a main reduction peak that gradually advanced
as copper(II) concentration increased (Fig. 8A). Zooming the
initial reduction zone, low current, assigned to copper depo-
sition, was detected. Low Qox/Qred ratios were obtained in
all cases. A main oxidation peak was detected that shifted to
positive potentials as the copper(II) concentration increased,
shifting to the position of the pure-copper oxidation in this
medium (Fig. 8B).
For a fixed copper(II) concentration, the influence of ca-
thodic limit was analysed. The oxidation peak was detected at
more negative potentialsdecreasingthe cathodic limit (Fig. 9)
probably because of a decrease of copper percentage in the
deposits. Similar behaviour was observed under stirring con-
Fig. 9. Cyclic voltammograms at 50 mV s−1
. Same solution as in
Fig. 8B.Quiescent conditions. Cathodic limit: (a) −1135 mV, (b) −1300 mV
and (c) −1400 mV.

E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
151
Fig. 10. Same solution as in Fig. 8B. Stripping curves of deposits obtained
potentiostatically at ω = 100 rpm, at (a) −950 mV, 30 s, (b) −975 mV, 20 s
and (c) −1025 mV, 10 s.
Voltammetric stripping was recorded for low-charge de-
posits prepared potentiostatically at different deposition po-
tentials (Fig. 10). In all scans, a single oxidation peak was
detected that shifted to negative potentials as the deposition
potentials was made more negative.
Deposits of 9.5 C cm−2 were prepared at different po-
tentials and analysed by means of ICP-MS. The analysis
showed the incorporation of copper in cobalt–nickel de-
posits. Specific deposit composition was dependent on de-
position potential: at low deposition potentials copper de-
position was favoured and copper-rich deposits were found.
Copper percentage clearly decreased for more negative de-
position potentials (Table 2). Ternary alloys maintain the
anomalous codeposition for cobalt–nickel, deposits present
greater proportion of cobalt than nickel, as in the binary
alloy.
These results indicate that the position of both voltammet-
ric and stripping oxidation peaks of the ternary alloy could
be related to the alloy composition. Copper-rich deposits ox-
idized at positive potentials, whereas cobalt-rich deposits ox-
idized at more negative potentials.
Increasing deposition charge, deposits obtained at low de-
position potentials showed high roughness whereas more uni-
form deposits were observed for more negative deposition
potentials (Fig. 11). The favoured incorporation of ferromag-
netic metals and the increase of nucleation involves the de-
crease of crystal size leading uniformity to the deposits. Ho-
Table 2
Composition of CoNiCu deposits of Q = 223 C cm−2 obtained at differ-
ent deposition potentials from a 0.175 mol dm−3 NiSO4 + 0.025 mol dm−3
CoSO4 + 3.0 × 10−3 mol dm−3 CuSO4 + 0.2 mol dm−3 Na3C6H5O7, pH 4
solution
Fig. 11. SEM micrographs of the Co–Ni–Cu deposits obtained at
ω = 100 rpm from the solution of Fig. 8B. Q = 223 C cm−2. (A) −950 mV,
(B) −1000 mV and (C) −1025 mV.
−E (mV)
Co (%)
Ni (%)
Cu (%)
850
875
900
925
950
975
1000
1050
1
4
7
22
42
50
58
57
2
8
8
97
88
86
71
47
35
26
12
mogeneous deposits, both in morphology and composition
throughout deposit thickness, were obtained from potentials
≤−1025 mV.
7
Structure and magnetic properties of ternary Co–Ni–Cu
deposits of low copper percentages, obtained from poten-
tials more negative than −1000 mV, were analysed. These
deposits presented a similar cobalt percentage and a gradual
11
15
16
31

152
E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
Fig. 13. (A) Magnetisation versus magnetic field and magnified detail of
Co–Ni–Cu deposits obtained from the solution cited in Fig. 8B. (a) −900,
(b) −950, (c) −1000 and (d) −1050 mV, Q = 223 C cm−2. (B) Parallel (a)
and perpendicular (b) hysteresis loops for a Co–Ni–Cu deposit obtained at
−1050 mV, Q = 318 C cm−2, ω = 100 rpm.
Fig. 12. (A) X-ray diffractograms of a Co–Ni–Cu deposit obtained at
−1050 mV using ω = 100 rpm, Q = 318 C cm−2, from solution of Fig. 8B.
(B) Detail of the fcc 1 1 1 peak in diffractograms corresponding to Co–Ni–Cu
deposits of Q = 318 C cm−2obtained at ω = 100 rpm from the same solution,
at (a) −1050 mV, 57% Co, 31% Ni, 12% Cu, (b) −1075 mV, 57% Co, 32%
Ni, 11% Cu and (c) −1100 mV, 55% Co, 35% Ni, 10% Cu.
Similar magnetic response was obtained for these de-
posits. The incorporation of copper in cobalt–nickel films
provoked a decrease of the saturation magnetisation, but a
clear decrease of the coercivity of the material (Fig. 13A). As
was observed for binary alloy slower response was obtained
when perpendicular magnetic fields were applied (Fig. 13B)
being the easy axis the in-plane direction of the film parallel.
decrease of copper content that favoured the nickel incorpo-
ration when potential values decreased.
X-ray diffractograms of the films exhibited a low signal
noise ratio; however show a clear collection of narrow peaks
that reveal their crystalline nature (Fig. 12A). The adjustment
of these peaks leads to an fcc structure. Each peak appeared
at position near that of cobalt fcc and midway between those
of copper and nickel (both of structure fcc). Only one peak
appeared for each orientation revealing the formation of the
ternary alloy and no peaks assigned to the pure metals were
observed. An accurate zoom of each diffraction peak shows
that the position depends on the specific composition of de-
posit. Diffraction peaks moved as a function of the global
composition of the alloy (Fig. 12B) shifting to higher 2θ val-
ues when nickel percentage increases. Peaks adjustment al-
lows estimate cell parameter for each composition. The cell
4. Conclusions
In the sulphate–citrate medium at pH 4 was confirmed
the character anomalous of the codeposition of cobalt–nickel
alloys. The electrochemical study combined with composi-
tional analysis of the films formed in the first stages allows
assure the anomalous behaviour of deposition process even
at the initio of deposition. In all conditions Co/Co + Ni ra-
tio in the deposit is great than the corresponding in solution.
The anomalous character of the codeposition of ferromag-
netic materials maintains during the codeposition of ternary
cobalt–nickel–copper system.
˚
parameter a varies in the range 3.548–3.544 A as the percent-
age of Cu decreases. The position of the peaks was concor-
dant with those deduced assuming the formation of a solid
solution. All deposits show high roughness and nodular mor-
phology.
The properties of the binary Co–Ni alloys obtained from
sulphate–citrate solutions can be controlled by adjusting de-

E. Go´mez et al. / Electrochimica Acta 51 (2005) 146–153
153
position potential due to the total miscibility of the two met-
als. In the first stages of the alloy formation films around 1:1
Co:Ni ratio are formed, over which the growth of cobalt-rich
alloys occurs. A main oxidation peak of the alloy is detected
in voltammetric experiments, which slightly shifts to more
positive potentials as the cathodic limit is decreased related
to the slight variation of alloy composition. The incorporation
of nickel into deposit leads to coatings that are less oxidizable
than pure cobalt.
per was supported financially by contract MAT 2003-09483-
C02-01 with the Comisio´n Interministerial de Ciencia y Tec-
nolog´ıa (CICYT) and by the Comissionat of the Generalitat
de Catalunya under Research Project 2001 SGR 00046.
References
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morphology and structure of Co–Ni alloys. Acicular deposits
of hcp structure can be obtained with a slow growth. On the
other hand, nodular, fcc, deposits are favoured at higher depo-
sition rates. This behaviour has been previously observed for
pure-cobalt deposition in different bath compositions [26].
However, no notable differences have been observed in the
magnetic properties of the Co–Ni films as a function of these
different structures.
[2] E. Go´mez, E. Valle´s, J. Appl. Electrochem. 29 (1999) 805.
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The sulphate–citrate medium at pH 4 has been useful for
incorporating different copper percentages in cobalt–nickel
alloys. The incorporation of low percentages of copper in
Co–Ni alloy causes a great variation of alloy properties.
Lower copper content is achieved as the potential gradually
decreases. In all cases, even at very low copper percentages
fcc structure is found, regardless of the deposition rate.
Electrochemical response, both in voltammetric and in
stripping experiments, informs about alloy properties. More
oxidizable films are formed for copper-containing deposits.
Moreover, the position of the oxidation peak informs on the
copper content in the deposits. The increase of the Qox/Qred
ratio and the shift of the oxidation peak to more positive po-
tentials reveal the gradual increase of the copper percentage.
The detection of a single oxidation peak of variable position
as a function of the deposition potential also reveals the for-
mation of a solid solution, corroborated by X-ray diffraction
experiments.
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The authors thank the Serveis Cientificote`cnics (Univer-
sitat de Barcelona) for the use of their equipment. This pa-