Magnetic properties improvement through off time between pulses and annealing in pulse electrodeposited CoZn nanowires

Article abstract of DOI:10.1016/j.jallcom.2011.06.098

CoZn alloy nanowire arrays embedded in anodic aluminum oxide (AAO) template were fabricated by alternative current (ac) pulse electrodeposition. Various off times between pulses in an electrolyte with constant concentration of Co⁺² and Zn⁺² and acidity of 4 were employed. The effect of deposition parameters on the alloy contents, microstructures and magnetic properties of Co_xZn_1-x nanowires were studied. It is shown that, Co content increased by increasing the off time between pulses. This phenomenon enables us to fabricate Zn and Co-rich nanowires by adjusting the off time during the deposition procedure. Increasing the off time more than 200 ms increased the coercivity and squareness of CoZn nanowire arrays. A significant increase in the coercivity of CoZn nanowires was observed after annealing which was varied for the samples fabricated with different electrodeposition conditions. A coercivity of 1785 Oe was obtained for the annealed sample (a sample fabricated with 50 ms off time) from initially 240 Oe.

Full text of DOI:10.1016/j.jallcom.2011.06.098

Journal of Alloys and Compounds 509 (2011) 8845–8849
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Magnetic properties improvement through off time between pulses and
annealing in pulse electrodeposited CoZn nanowires
M. Almasi Kashia,b,∗, A. Ramazania,b, Z. Fallahb
a Department of Physics, University of Kashan, Kashan, Iran
b Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
CoZn alloy nanowire arrays embedded in anodic aluminum oxide (AAO) template were fabricated by
alternative current (ac) pulse electrodeposition. Various off times between pulses in an electrolyte with
constant concentration of Co+2 and Zn+2 and acidity of 4 were employed. The effect of deposition parame-
ters on the alloy contents, microstructures and magnetic properties of CoxZn1−x nanowires were studied.
It is shown that, Co content increased by increasing the off time between pulses. This phenomenon
enables us to fabricate Zn and Co-rich nanowires by adjusting the off time during the deposition proce-
dure. Increasing the off time more than 200 ms increased the coercivity and squareness of CoZn nanowire
arrays. A significant increase in the coercivity of CoZn nanowires was observed after annealing which was
varied for the samples fabricated with different electrodeposition conditions. A coercivity of 1785 Oe was
obtained for the annealed sample (a sample fabricated with 50 ms off time) from initially 240 Oe.
© 2011 Elsevier B.V. All rights reserved.
Received 14 March 2011
Received in revised form 21 June 2011
Accepted 22 June 2011
Available online 29 June 2011
Keywords:
Magnetization
Microstructure
Atomic force microscopy
1. Introduction
of the material deposited. In general, ac electrodeposition through
the barrier layer is a complicated process which depends on depo-
Conductor and semiconductor nanowires embedded in
nanoporous anodic aluminum oxide (AAO) have attracted great
interest during last decade. Thermal, mechanical and chemical
properties along with specific geometries and easy fabrication of
nanowires into the aluminum oxide template are the main reasons
for AAO to be a proper template preparing magnetic recording
media [1,2]. Another reason using AAO template is its morphology
which exhibits a lot of homogeneous parallel pores growing per-
pendicular to the surface with a narrow distribution of diameter
so that their length could be well controlled in a wide range of
distances. Both direct and alternative currents were performed
to electrodeposit nanowires into the porous alumina. Deposition
into the pores using direct current (dc) electrodeposition method
has proven to be a particularly powerful technique, enabling
substantial control over composition and crystallinity [3,4] and
easy access to compositional modulation along the wire lengths
[5]. However, none of the dc techniques reported to date are
amenable to industrial scale processing because of the laborious
preprocessing that must be performed on the AAO template before
deposition. In contrast, alternative current (ac) deposition tech-
nique requires fewer processing steps and is more amenable to
scale-up but currently provides far less control over the structure
sition conditions such as electrolyte concentration, composition
and temperature [6], as well as deposition voltage, frequency
and waveform (sine, square and triangle) [7,8]. Fabrication of
alloy nanowires using ac and pulse electrodeposition techniques
is more complicated than pure element nanowires such as Cu,
Co, Fe and Ni [8–11]. Therefore it is valuable to investigate the
effects of electrodeposition parameters on the microstructure
and magnetic properties of magnetic alloys nanowires. Among
magnetic nanowires, magnetic and nonmagnetic Co based alloys
have been more interested because of its uniaxial crystal structure.
There have been numerous reports studying the microstructures
and magnetic properties of Co based alloys such as CoFe [12], CoNi
[13], CoCu [14], CoPt [15,16], CoPd [17,18], CoPb [19,20] and CoZn
[21,22].
In the present work in a constant electrolyte, CoZn nanowires
with various Zn contents were synthesized by varying the off
time between pulses of the ac pulse electrodeposition voltage.
Performing pulse electrodeposition in a certain electrolyte con-
centration enables us to fabricate CoZn alloy nanowires with
various compositions. The effect of pulse electrodeposition condi-
tions and annealing on the microstructure and magnetic properties
of nanowires were investigated.
2. Experiments
The AAO templates have been prepared by a two step anodization process.
Highly pure aluminum foils of 0.3 mm thickness were electropolished in a 1:4 vol-
ume mixture of perchloric acid and ethanol. The foils were then anodized in 0.3 M
oxalic acid solution at 17 ◦C for 10 h at 40 V. Subsequently, the anodized foils were
∗
Corresponding author at: Department of Physics, University of Kashan, Kashan,
Iran.
E-mail address: almac@kashanu.ac.ir (M.A. Kashi).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.06.098

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M.A. Kashi et al. / Journal of Alloys and Compounds 509 (2011) 8845–8849
Fig. 1. (a) A top view AFM image of the nanopore arrays prepared by the two step
anodization method for 2 h and (b) a cross section SEM micrograph of Co0.8Zn0.2
nanowire arrays prepared with an effective electrodeposition time of 1 min. A cross
section view of nanowires after removing of alumina template is inserted.
immersed in a solution composed of 0.2 M chromic and 0.5 M phosphoric acid at
60 ◦C for 10 h to remove the anodized layer. The foils were then anodized, by iden-
tical parameters as the first step for 2 h. Following the second anodization step,
voltage was reduced systematically to promote thinning of the barrier layer. The
voltage was initially reduced by a rate of 0.064 V s−1 to 20 V and then 0.033 V s−1 to
12 V and ultimately anodization continued in the last voltage for 1 min in order to
barrier layer reaches to equilibrium.
The ac pulse electrodeposition technique in a simple electrochemical cell with
2 electrodes was performed to deposit nanowires into the nanopores. Reduction
and oxidation times were 5 ms and the off time between pulses was 10, 20, 50,
100, 200, 400 and 600 ms. A sine waveform with 18/18 V reduction/oxidation volt-
age was employed during the electrodeposition process. A solution composed of
0.3 M CoSO4·7H2O, 0.06 M ZnSO4·7H2O, 45 g l−1 H3BO3 was used as electrolyte.
The pH value of electrolyte solution was initially 3.3–3.5, which increased to 4 by
adding NaHCO3. The effective electrodeposition time (the total electrodeposition
time minus entire off times between pulses) was 60 s.
Fig. 2. EDS patterns of the CoZn nanowire arrays fabricated by (a) 10, (b) 50, (c) 400,
(d) 600 ms, off-time between pulses.
prepared with an effective electrodeposition time of 1 min. The
wires have an average length of 3.5 ␮m. The nanowires are par-
allel to each other through the entire pores. For more detail a cross
section view of nanowires after removing of alumina template is
also inserted in Fig. 1(b).
Fig. 2 shows EDS patterns of the CoZn nanowire arrays fabricated
by various off-time between pulses. The oxygen and aluminum
peaks are related to the alumina template. The Co content of CoZn
nanowire arrays is tabulated in Table 1. It is found that Co content
in fabricated nanowires has been enhanced from 58.2% at 10 ms
off time up to 80% at 600 ms off time. Consequently, it may be said
The structure and composition of the arrays of CoZn nanowires were stud-
ied by X-ray diffraction pattern and electron dispersive spectroscopy (EDS). The
magnetic properties were measured by alternating gradient force magnetometer
(AGFM) at room temperature. Scanning electron microscope (SEM) and atomic force
microscopy (AFM) were employed to investigate the surface morphology of the
samples.
3. Results and discussions
Table 1
Fig. 1(a) shows a typical AFM top view image of the nanopore
arrays prepared by the two step anodization method. Relatively
perfect hexagonally arranged nanopores with 30 nm diameter and
100 nm interpore distances can be seen. Fig. 1(b) shows a typical
SEM micrograph of the cross section of Co0.8Zn0.2 nanowire arrays
The Co content of CoZn nanowire arrays as a function of off time between the pulses.
Off time (ms)
10
50
61
400
600
80
Co Content (at.%)
58.2
73.6

M.A. Kashi et al. / Journal of Alloys and Compounds 509 (2011) 8845–8849
8847
0.015
0.010
0.005
0.000
-0.005
-0.010
a
b
d
as prepared
annealed
-0.015
0.015
c
0.010
0.005
0.000
-0.005
0.010
-
-0.015
-4000 -2000
0
2000
4000
-4000
-2000
0
2000
4000
Applied Field (Oe)
Applied Field (Oe)
Fig. 3. Hysteresis loops of both the as prepared samples and annealed ones synthesized at (a) 10, (b) 50, (c) 400 and (d) 600 ms off times.
that increasing the off time results in enhancement of Co content
in CoZn alloy nanowires. Using electrolyte with same concentra-
tion of Co and Zn ions and applying same reduction voltage for all
the samples, it was expected the fabricated CoZn nanowires show
same alloy compositions but this fact was not experimentally con-
firmed. It seems by applying ac pulse in an electrochemical cell,
metallic zinc and cobalt ions reduce in competing with each other,
but during the off time cobalt atoms considering their higher stan-
dard potential (E0 = −0.28 V) take the opportunity to replace zinc
atoms (with standard potential of −0.76). By increasing off time,
more numbers of Zn atoms are replaced with Co atoms. Therefore
with increase in the off time between pulses, the Co-rich, CoZn
nanowires were formed. On the basis of these contents the point
is; we can control nonmagnetic element content in CoxZn1−x alloy
nanowires by increasing off times in a relatively wide range.
Hysteresis loops of as prepared CoZn alloy nanowires apply-
ing external magnetic field parallel to long-axis of the nanowires
at room temperature were obtained. The magnetic properties of
annealed samples at 580 ◦C in a mixture of 15% hydrogen and 85%
argon gases were also examined. The hysteresis loops of both the
as prepared samples and annealed ones synthesized at 10, 50, 400
and 600 ms off times are displayed in Fig. 3. As shown in this figure,
coercivity, magnetization, and squareness of the samples increases
with increase in the off time between pulses. This trend was not
exactly seen for the annealed samples. Magnetization and coerciv-
ity show a peak around 50 ms off time for the annealed samples.
Coercivity of the as prepared samples with 10 ms off time increased
to 1325 Oe from initially 170 Oe after annealing. The squareness of
this sample was also increased to 0.66 from 0.36. It was also seen
the coercivity and squareness of the samples fabricated at 600 ms
off time increased to 1770 Oe and 0.87 after annealing from initially
455 Oe and 0.59 respectively. Although magnetization of samples
was increased after annealing, the rate of increasing was reduced
with increase in the off time between pulses which is accompanied
with increase in the Co content. As evidence, the magnetization of
samples prepared with 600 ms off time (CoZn nanowire arrays with
highest Co content) was not so much changed after annealing.
The variation of the longitudinal coercivity and magnetization
of as prepared and annealed samples as a function of off time
were determined with the external field applied along the wire’s
axis and displayed in Fig. 4. Although coercivity of the samples
initially (10–200 ms) shows no significant variation, in the range
of 200–600 ms off times it almost becomes more than twice. In
the range of 10–200 ms, the relative variation of coercivity for
the annealed samples was the same as that of as prepared ones
2000
a
1600
1200
as prepared
annealed
800
400
0
b
0.015
0.012
0.009
0.006
0.003
0
100
200
300
400
500
600
Off time(ms)
Fig. 4. (a) Coercivity and (b) magnetization of prepared and annealed samples, as a
function of off time between pulses, with applied field parallel to the wire’s axis.

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M.A. Kashi et al. / Journal of Alloys and Compounds 509 (2011) 8845–8849
a
Co (002)
b
1000
800
600
400
200
Zn (101)
600 ms, off time
600 ms, off time
10 ms, off time
10 ms, off time
30
40
50
60
70
80
30
40
50
60
70
80
2 Theta (degree)
2 Theta (degree)
Fig. 5. X-ray diffraction pattern of (a) as prepared and (b) annealed samples fabricated with 10 ms and 600 ms off times.
while coercivity variation of the annealed samples prepared with
200–600 ms off time is less than that of as prepared samples. From
200 to 600 ms off time coercivity variation of annealed samples was
nearly 25% while it was more than twice for the prepared samples
as mentioned before. An almost 2.6 times increase in the magneti-
zation was observed for the samples prepared with 50 ms off time
after annealing while the magnetizations of as prepared samples
at 600 ms off time and annealed ones were almost the same. It is
anticipated that increasing off time replaces the Zn with Co atoms
through electroless procedure forming a mixed phases in which,
the size of magnetic grains increases to a critical size and causes
an increase in both the coercivity and magnetization. The increase
in magnetization of annealed nanowire arrays may caused by the
changes of Co particles in the annealing process. It may be said that
increase of saturation magnetization is related to the formation of
larger Co cluster during the annealing process.
For samples with less Co content, the magnetic atoms were
individually distributed between nonmagnetic Zn atoms and could
not show their real contribution in the magnetization. Formation
of magnetic Co clusters during annealing procedure enhances the
saturation magnetization and coercivity of these samples. In sam-
ples with higher Co content relatively larger magnetic grains were
formed and annealing process has less effect on the magnetic clus-
ter formation and less increase in the magnetization was observed.
For theses samples, during annealing procedure, CoZn alloy phase
is transferred to solid-Co and liquid-Zn and Co grains with uniform
size recrystallize in liquid Zn. It may be said that the resulting Co
is polycrystalline and are segregated by the Zn particles. The resid-
ual Zn solidified when the samples were cooled down and became
pins between Co particles. Higher coercivity might be resulted from
nanocrystalline Co grains uniformly distributed in the nanowires
and also pinning effect.
To estimate the influence of the off time on the microstruc-
ture of the nanowires, X-ray diffraction pattern of as prepared and
annealed samples were performed at room temperature. In order
to omit the back ground, coming from the aluminum substrate, the
aluminum was removed as we mentioned elsewhere [23]. As seen
in Fig. 5(a) the samples made at 10 ms off time has an almost amor-
phous structure as reported by Xu et al. [22]. Small peak observed
between 40 and 45◦ may be an indication of very fine crystal grains
of Co and Zn formed in this sample. Increasing the off time to 600 ms
increases the crystallinity of the samples thereby intensifies the
Zn and Co peaks located around 43.18 and 44.46◦, respectively.
Although because of more Co content in these samples the peak
intensity of (0 0 2) Co is much higher than that of the (1 0 1) Zn.
Increase in the degree of crystallinity with increase in off time may
be resulted from tranquil electrodeposition during a relatively long
deposition time. It may be also said that replacing more Zn with Co
in the CoZn alloy nanowires during 600 ms off time forms a crys-
talline magnetic Co phase. Crystal structure of samples obtained
with 10 ms did not change after annealing unless a relatively broad
peak was seen for these samples around 44◦ which may be an indi-
cation of very fine crystal formation within amorphous phase (see
Fig. 5(b)). X-ray diffraction pattern of the sample fabricated with
600 ms shows a quite different structure after annealing. Although
as prepared sample with 600 ms has quite distinct peak, it shows
peaks with relatively lower intensity after annealing. A small peak
around 51.33 may be an indication of (1 1 1) Co fcc formation in this
sample. The treatment of CoZn nanowires may therefore be demon-
strated as following. Since the annealing temperature (580 ◦C) is
much higher than melting point of Zn (the melting point of bulk
Zn is about 420 ◦C), as noted above, the Co atoms aggregated dur-
ing the annealing procedure and formed Co clusters with Zn atoms
in the boundaries so that increased both the coercivity and mag-
netization. This phenomenon is also observable when the off time
reduces down below 200 ms. Adding the off time more than 200 ms,
increases the Co content of as deposited samples to a relatively high
amount so that annealing has no significant effect on the compo-
sition variation of CoZn nanowires. On the other hand, although
coercivity of high Co content nanowires after annealing was high,
their crystal structures were the same as those samples prepared
with 10 ms off time.
4. Conclusion
CoxZn1−x nanowire arrays were fabricated by ac pulse electrode-
position with various off times between pulses changing from 10
to 600 ms.
Our investigation on the effect of off time between pulses on
the composition, microstructures and magnetic properties of CoZn
nanowire arrays enables us to reach the following conclusions:
It was found that Co content increased by increasing the off time
between pulses, using an electrolyte with a constant concentration
of Co+2 and Zn+2. This phenomenon enables us to fabricate Zn-rich
and Co-rich nanowires by adjusting the off time during the deposi-
tion procedure. Increasing the off time between pulses more than
200 ms increased the coercivity and squareness of CoZn nanowires
arrays.
A significant increase in the coercivity of CoZn nanowires was
observed after annealing which was related to the electrodeposi-
tion conditions.
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