Contribution of electrolyte pH and deposition potentials to the magnetic anisotropy of electrodeposited nickel films

Article abstract of DOI:10.1016/j.jmmm.2009.11.038

The influence of electrolyte pH and cathode potential on the magnetic properties of single layer Nickel films electrodeposited on polycrystalline titanium substrates was studied. The films were deposited at the electrolyte pH=3.5±0.1, 2.5±0.1 and 2.1±0.1 by varying the deposition potentials (-1.2, -1.5 and -1.8 V vs saturated calomel electrode, SCE) applied in continuous waveform. The structural analysis by X-ray diffraction revealed that the films have face-centred cubic structure. Results of the magnetic measurements obtained by vibrating sample magnetometer (VSM) indicated that the magnetic properties were affected by the electrolyte pH and the cathode potentials in terms of magnetic anisotropy. At the highest pH the films deposited at the lowest potential had in-plane magnetic anisotropy. As the electrolyte pH decreased from the high (pH=3.5±0.1) to low (pH=2.5±0.1), which is aided by increasing the potentials, resulted in an almost magnetic isotropy in the films. However, isotropic magnetic behaviour was observed for the film deposited at the low pH combined with the high potential (-1.8 V vs SCE). Magnetic thickness profile of the samples obtained by VSM revealed that the isotropic films have a smoother magnetic variation across the film from one edge than the anisotropic ones. This is also verified with a microscopic observation by an optical microscope and the surface of the isotropic films is observed to be smoother than that of the anisotropic ones. Furthermore, all films were found to have planar magnetic anisotropy irrespective of the pH's and the potentials.

Full text of DOI:10.1016/j.jmmm.2009.11.038

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 322 (2010) 1088–1091
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Contribution of electrolyte pH and deposition potentials to the magnetic
anisotropy of electrodeposited nickel films
Hakan Kockar a,n, Mursel Alper b, Oznur Karaagac a, Nuri Nakiboglu c, M. Celalettin Baykul d
a Physics Department, Science and Literature Faculty, Balikesir University, 10145, Balikesir, Turkey
b Physics Department, Science and Literature Faculty, Uludag University, Go¨ru¨kle 16059, Bursa, Turkey
c Chemistry Department, Science and Literature Faculty, Balikesir University, 10145 Balikesir, Turkey
d Physics Department, Science and Literature Faculty, Osmangazi University, 26480 Meselik, Eskisehir, Turkey
a r t i c l e i n f o
a b s t r a c t
Available online 27 November 2009
The influence of electrolyte pH and cathode potential on the magnetic properties of single layer Nickel
films electrodeposited on polycrystalline titanium substrates was studied. The films were deposited at
the electrolyte pH=3.570.1, 2.570.1 and 2.170.1 by varying the deposition potentials (À1.2, À1.5
and À1.8 V vs saturated calomel electrode, SCE) applied in continuous waveform. The structural
analysis by X-ray diffraction revealed that the films have face-centred cubic structure. Results of the
magnetic measurements obtained by vibrating sample magnetometer (VSM) indicated that the
magnetic properties were affected by the electrolyte pH and the cathode potentials in terms of
magnetic anisotropy. At the highest pH the films deposited at the lowest potential had in-plane
magnetic anisotropy. As the electrolyte pH decreased from the high (pH=3.570.1) to low
(pH=2.570.1), which is aided by increasing the potentials, resulted in an almost magnetic isotropy
in the films. However, isotropic magnetic behaviour was observed for the film deposited at the low pH
combined with the high potential (À1.8 V vs SCE). Magnetic thickness profile of the samples obtained
by VSM revealed that the isotropic films have a smoother magnetic variation across the film from one
edge than the anisotropic ones. This is also verified with a microscopic observation by an optical
microscope and the surface of the isotropic films is observed to be smoother than that of the anisotropic
ones. Furthermore, all films were found to have planar magnetic anisotropy irrespective of the pH’s and
the potentials.
Keywords:
Electrodeposition
Magnetic anisotropy
Other ferromagnetic metal and alloy
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
displayed a magnetic anisotropy and were observed to have
smoother surfaces than the ones obtained at highest pH.
Nickel (Ni) films are one of the most frequently studied
ferromagnetic materials due to their scientific and technological
importance [1–3]. In the electrodeposition technique, the film
properties are mainly dictated by a number of factors including
the deposition potentials and the electrolyte pH. Understanding
the magnetic properties of the magnetic films offers a wide
potential for the development of new applications especially in
sensors and recording media [3,4]. Therefore, the purpose of this
study is to deposit the Ni films from an electrolyte containing Ni2+
ions at various electrolyte pH by varying the cathode potentials
applied in the continuous waveform and then to investigate the
magnetic anisotropy of the films as a function of the pH and the
potential. The films deposited at the lowest electrolyte pH
2. Experimental procedure
In this study, single Ni films were grown from an electrolyte
containing Ni+2 ions under potentiostatic conditions. Electrode-
position was performed in an electrochemical cell with three
electrodes using a potentiostat/galvanostat (EGG Model 362). The
electrolyte composed of 2.0 M Ni(SO3HN2)2 Á 4H2O, and 0.5 M
H3BO3. All chemicals were reagent grade and dissolved in distilled
water. A platinum (Pt) sheet with an area of 6 cm2 was used as
counter electrode. The reference electrode was
a saturated
calomel electrode (SCE). All potentials are pronounced with
respect to the SCE. Titanium sheet was used as the substrate.
Prior to deposition, the substrates were mechanically polished,
and then washed in 10% H2SO4 followed by distilled water.
The pH value of freshly made electrolytes was 3.570.1. The
electrolyte pH decreased after each deposition. This is because the
current leads to a reduction of metal ions at the cathode and their
n Corresponding author. Tel.: +90 266 612 1278; fax: +90 266 612 1215
E-mail addresses: hkockar@balikesir.edu.tr, hakankochar@hotmail.com
(H. Kockar).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.11.038

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replacement by hydrogen ions, lowering the electrolyte pH [5,6].
It is also well known that pH of electrolytes can be adjusted to the
desired value by the metal deposition using an inert anode [5].
Continuing in the same way, the desired pH values were obtained
by flowing current through the electrolyte. In order to study the
pH effect, the electrolyte pH values were varied as 3.570.1,
2.570.1 and 2.170.1 for each group of specimens. The polarisa-
tion curve of the electrolyte (current-potential) was obtained to
select the proper potentials for deposition. According to the curve,
the films were deposited at room temperature using the cathode
potentials of À1.2, À1.5 and À1.8 V, which were applied in
continuous waveform. The nominal thickness of the films was
fixed at 4 mm. The charge amount required for the film thickness
was calculated according to Faraday’s law by assuming 100%
current efficiency for each film deposited at different potential.
After the deposition is completed the films were peeled of their
substrates.
X-ray diffraction measurements were carried out using a
Phillips diffractometer type PW1820 with a beam of Cu-K
a
radiation. A commercial vibrating sample magnetometer, VSM
(The DMS Model EV9, ADE Technologies) was employed to
determine the magnetic anisotropy and thickness distribution.
Ni foil was used to calibrate the VSM. Hystersis loops were
measured at arbitrary angles in the film plane and also
perpendicular to the film plane. In the film plane, the total
angular range of measurement was 1801 with 301 intervals,
chosen in a way that the 01 and 901 equal the easy and hard axis
direction, respectively. Magnetic anisotropy was studied by taking
hysteresis loops on 5 mm disk samples. The thickness uniformity
of the films was obtained from the saturation magnetisation
intensity, by taking measurements on samples cut into strips
(1 cm long  1 mm wide) across the width of films from one edge.
In addition, an optical microscope was used to closely examine
the surfaces of the films. All production and measurements were
carried out at room temperature.
Fig. 1. An example of the XRD patterns for single Ni film deposited at
pH=3.570.1 and –1.8 V.
3. Results and discussion
Fig. 2. An example of anisotropic single Ni film deposited at pH=3.570.1 and
À1.2 V. Hysteresis loops were measured at 01 and 901 in the film plane and also
perpendicular to the film plane.
The crystalline texture of polycrystalline Ni films were
examined by X-ray diffraction ranging 401o2y o801, where y
is the Bragg angle. As an example, the XRD pattern of the film
deposited at pH=3.5 with À1.8 V is illustrated in Fig. 1. Bragg
reflections were observed at the angles of 2y=43.91, 51.11, 75.91
that correspond to the (1 1 1), (2 0 0) and (2 2 0) planes of face-
centred cubic (fcc) Ni. Analysis of the XRD patterns revealed that
the films have fcc structure irrespective of deposition conditions.
An average crystallite size of the films was calculated to be about
20 nm from the Scherrer formula [7].
Hk was taken, as the magnetic field required saturating the film
specimen along the hard axis. The Hc decreases when the
measurement angle is rotated from 01 to 901. The Hc is different
from the Hk. This means that when the field is applied along the
easy axis, the magnetisation does not rotate uniformly but there is
a nucleation and propagation of domain walls [10–12]. The Hc
reaches 3.80 kA/m, indicating that the film has a planar magnetic
anisotropy when the field is applied perpendicular to the film
plane.
In-plane magnetisation loops in Fig. 2 closely resemble the
behaviour predicted by the Stoner–Wohlfart (SW) model for
coherent magnetisation rotation in materials with isolated single
domain particles. Single Ni films deposited in this study do not
consist of isolated domain particles, however, some success in
using this model as a basis deriving hysteresis loops has been
reported in the past for various film compositions [10–12]. The
ratio of remenant magnetisation to saturation magnetisation, Mr/
Ms and the coercivity to anisotropy, Hc/Hk for in-plane directions
spanning 1801 is shown in Fig. 3. Both the coercivity and
remanence ratio have peak values confirming the presence of a
uniaxial magnetic anisotropy. The coercivity is maximised in the
easy direction and minimised along the hard direction as
Magnetisation measurements were preformed to examine in
plane and out of plane magnetic anisotropies by taking the
hysteresis measurements of Ni films using the VSM. Repeated
depositions yielded the maximum film coercivity, Hc ranging from
0.50 to 1.25 kA/m and saturation magnetisation, Ms=0.40 kA/m,
which is close to the reported values, e.g. 0.48 kA/m of Bulk Ni
[8,9]. The average measured coercivities are almost consistent
with the published value for bulk nickel of 0.80 kA/m [9].
Hysteresis loops of single Ni film (deposited at pH=3.5 and
À1.2 V) measured at 01 and 901 in the film plane and also
perpendicular to the film plane are shown in Fig. 2. When the field
is applied at 01, the loop is almost square with a higher Hc of
1.07 kA/m. When the field is applied along 901 in the film plane,
the loop exhibits a smaller Hc of 0.78 kA/m than the anisotropy
field, Hk=6.35 kA/m, but not equal to zero. To obtain the Hk, the
extrapolation of the in-plane and perpendicular hysteresis loops
back to the saturation magnetisation level were carried out. The

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distributions whereas the anisotropic ones have poor thickness
uniformity across the film confirming the findings of the magnetic
thickness profile.
In the electrodeposition process of the films, hydrogen
evolution may be defined as the ratio of the actual amount of
metal deposited to that expected theoretically according to the
Faraday law [13,14]. Therefore, the thickness of the films could be
lower than the nominal one such as the films deposited at the
pH=3.5 and À1.2 V. At this point, the water decomposition
reaction leading to hydrogen formation can have a significant
contribution to the measured current resulting in retarded
current efficiency. Consequently, the roughness of the layer can
be expressed to evolve with the thickness as roughness is
proportional to the thickness as electrolyte pH decreased. In
other words, as it is well known that the low electrolyte pH and
the high cathode potential favour the hydrogen evaluation. The
hydrogen formation is expected to lead to the reduction of metal
ions at the cathode and their replacement by hydrogen ions in the
electrolyte, and hence the change in the ratio of ions may result in
changes in the thickness uniformity across the deposits. However,
unlike our expectations, as the pH increased to its lowest value of
pH=2.170.1 and the potential increased À1.8 V, the isotropic
magnetic behaviour was observed, which was verified by
magnetic thickness profile and microscopic observation of the
film surface across the film. It should be also noted that although
hydrogen evaluation leads to a decrease in current efficiency, it
may also tend to improve the throwing power of the cell that
governs the thickness uniformity across the film [13,14]. The
possible explanation for the findings may be found in the latter
that probably happens in this study since electrolyte with high
throwing power produces only small changes in the thickness
uniformity across the film samples whereas solutions with low
throwing power can produce large thickness variations in the
films, as reported in [13–16].
Fig. 3. Normalised remanence and coercivity ratios as a function of arbitrary
chosen angles between applied field and magnetisation easy axis in the film plane
for single Ni film deposited at pH=3.570.1 and À1.2 V.
predicted in the SW model. In the easy direction the
magnetisation reversal switches very quickly, indicating that
there is strong magnetostatic and/or exchange coupling forces
between crystal grains. However, unlike the SW model, the hard
axis in the films was not equal to zero. This probably happens
because the magnetisation reversal in these films arises from
domain wall motion, domain formation and domain rotation, only
the latter is considered in the SW model.
The loops in Figs. 2 and 3 indicate that the film has well-
defined uniaxial in-plane anisotropy. To see the effect of the lower
electrolyte pH on the deposits, the films were produced at
pH=2.570.1 and 2.170.1 keeping the potentials constant at
À1.2 V. Although the feature characteristics of the in-plane
anisotropy for the films produced at pH=2.570.1 were observed,
the loops corresponding to the angles are different from the loops
in Fig. 2. When the field is applied at 901, the loop is not as square
as in Fig. 2, and when the applied field is at the 01 direction the
remanence ratio decreases. Therefore, it should be noted that the
lower pH of 2.570.1 makes the uniaxial magnetic anisotropy less
well-defined. In the case of Ni films deposited at 2.170.1,
although there was a slight variation between the loops, this
can be expressed as the film shows almost magnetic isotropy. It is
indicated that the uniaxial in-plane anisotropy changes towards
isotropy as the electrolyte pH decreased.
In order to investigate the magnetic properties of films, further
single layer Ni films were deposited at higher cathode potentials
of À1.5 and À1.8 V by varying the electrolyte pH. The films
produced at pH=2.570.1 had almost an isotropy, and the degree
of magnetic isotropy increased with increasing cathode poten-
tials. In the same series of the films, the Ni films deposited at
pH=2.170.1 and À1.8 V showed isotropic magnetic behaviour.
Measurements of the magnetic thickness uniformity of the
films were also performed. 10 mm  10 mm size of the samples
was cut from one edge (1 mm  10 mm). Magnetic measurement
sample were performed on each sample using VSM. The thickness
profile of the films was obtained from the saturation magnetisa-
tion intensity using the formula Mint=M0V, with Mint obtained
from the VSM, V is the volume of the sample and M0 is the
saturation magnetisation per unit volume of bulk Ni [9]. It is seen
that the films have a smoother magnetic variation than the
anisotropic ones across the film. Further verification of magnetic
anisotropy study was carried out using an optical microscope. The
microscopic investigation at 600 Â magnification revealed that
the smoothness in the film surface increased with the decrease of
electrolyte pH at the cathode potential of À1.2 V, and degree of
the smoothness on the surface of the films was observed at lowest
pH values with the increase of the cathode potential. It can be
expressed that the isotropic films have uniform thickness
4. Conclusions
A series of single layer Ni films were grown as a function of the
electrolyte pH and cathode potential by electrodeposition.
Magnetic measurements showed that the magnetic properties of
the films are very sensitive to the pH and the potentials. The
higher pH values (o3.0) combined with lower cathode potentials
(41.5 V) leads to anisotropic behaviour in the films. Magnetic
thickness profile of the samples by VSM and the microscopic
observation by an optical microscope indicated that the isotropic
films have a smoother variation across the film from one edge
than the anisotropic ones. The results obtained in this study have
shown that anisotropic and isotropic magnetic materials could be
deposited by choosing the proper deposition parameters for their
potential applications as sensors and recording media materials.
Acknowledgments
This work is partly supported by Balikesir University, Turkey
under Grant no. BAP 2005/18. The authors also received support
from State Planning Organisation, Turkey under Grant no.
2005K120170 for VSM system and the Scientific and Technical
Research Council (TUBITAK) of Turkey under Grant no. TBAG-
1771 for electrodeposition system.
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