Magnetic properties of Co-Cu nanowire arrays fabricated in different conditions by SC electrodeposition

Article abstract of DOI:10.1016/j.ssc.2012.05.016

Magnetic Co-Cu alloy nanowires with low Cu content were prepared by SC electrodeposition in pores of anodic aluminum oxide templates. The as-deposited Co-Cu nanowires, with a diameter of 15 nm, show distinctive magnetic anisotropy as an applied magnetic field parallel to the axis of nanowires. With increase in the molar ratio of Co and Cu, the coercivity along nanowire axis increases and reaches a maximum value of 1977.5 Oe at the Co/Cu molar ratio of 60:1, but the maximum value of coercivity increases to 1743.6 Oe with the decrease of frequency to 2 Hz. Crown

Full text of DOI:10.1016/j.ssc.2012.05.016

Solid State Communications 152 (2012) 1585–1589
Contents lists available at SciVerse ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Magnetic properties of Co–Cu nanowire arrays fabricated in different
conditions by SC electrodeposition
Xiaohui Lin a, Guangbin Ji a,n, Tingting Gao a, Junwu Nie b, Youwei Du c
a Department of Applied Chemistry, College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, PR China
b Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China
c Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetic Co–Cu alloy nanowires with low Cu content were prepared by SC electrodeposition in pores
of anodic aluminum oxide templates. The as-deposited Co–Cu nanowires, with a diameter of 15 nm,
show distinctive magnetic anisotropy as an applied magnetic field parallel to the axis of nanowires.
With increase in the molar ratio of Co and Cu, the coercivity along nanowire axis increases and reaches
a maximum value of 1977.5 Oe at the Co/Cu molar ratio of 60:1, but the maximum value of coercivity
increases to 1743.6 Oe with the decrease of frequency to 2 Hz.
Received 30 December 2011
Received in revised form
7 May 2012
Accepted 21 May 2012
by Xianhui Chen
Available online 26 May 2012
Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.
Keywords:
A. CoCu nanowire
B. Electrodeposition
C. Magnetic properties
1. Introduction
ideal template for the fabrication of nanostructured materials
[16]. Typically, electrodeposited M–Cu (M¼Co, Permalloy, Ni)
Metallic nanowires have attracted wide attention due to their
exciting electrical, optical, catalytic, and magnetic properties,
with potential applications in optoelectronics [1,2], high-density
magnetic memories [3], and sensors [4]. For the applications in
perpendicular recording, nanowire is a new promising candi-
date [5]. Nanowire-based sensors allow for a higher sensitivity,
higher capture efficiency, and faster response time due to their
large adsorption surface and small diffusion time [6]. Magnetic
Cu–Co alloys are known to exhibit giant magnetoresistance
(GMR) because of the smaller saturation magnetization and
relatively larger giant magnetoresistance effect [7–10]. Chi et al.
[11] have fabricated Cu/Co multilayer nanowire arrays and
investigated GMR of the samples. There are other investigators
studied the magnetic properties and GMR of Co/Cu multilayer
flims/nanowires [12–14]. To the best of our knowledge, the
microstructures and magnetic properties of the solid solution
Co–Cu nanowire arrays have rarely been reported [15].
nanowires are grown using the so-called single bath technique in
which a single electrolyte contains both the magnetic and Cu ions
and the concentration of Cu ions is quite low [17]. As Co has large
magnetocrystalline anisotropy and may be grown with different
crystallographic structures, such as cubic or hexagonal, in order to
control their specific magnetic properties, the concentration of
non-magnetic Cu ions should be low during the electrodeposition
procedure. It has been shown that nearly pure magnetic layers
could be obtained by keeping the Cu concentration very low
(o2%) [18]. In this paper, by keeping the Cu concentration lower
than 10%, Co–Cu nanowires grown into porous anodic aluminum
oxide templates were successfully fabricated via electrodeposi-
tion with square-wave voltages. The magnetic properties of the
nanowire arrays with different molar ratio of Co/Cu (10:1, 20:1,
40:1, 50:1, 60:1) were widely investigated. The frequency of
voltage which is a key factor of electrodeposition and the
influence of voltage were studied as well.
Among various synthetic processes, the electrodeposition
within the nanoporous membranes has been proved to be a
promising technique for the preparation of nanowire arrays.
Porous anodic aluminum oxide, with high pore density, uniform
pore distribution and small diameter of the pores, makes it an
2. Experiment
Nanoporous anodic aluminum oxide (AAO) templates were
prepared by the process described previously [19]. The anodizing
process was carried out at ꢀ0 1C with H2SO4 as electrolyte for 6 h,
and the DC anode voltage is 10 V. All the templates were immersed
in 0.3 M H3PO4 for 5–10 min to dissolve the oxide layer, and then
n Corresponding author. Tel.: þ86 25 52112902; fax: þ86 25 52112626.
E-mail address: gbji@nuaa.edu.cn (G. Ji).
0038-1098/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ssc.2012.05.016

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X. Lin et al. / Solid State Communications 152 (2012) 1585–1589
washed by distilled water. The Co–Cu nanowires were electrode-
posited into the pores by square-wave pulse current (SC) electrolysis
in an electrolyte consisting of Co(Ac)2 ꢁ 4H2O (1 mol/L), CuSO4 ꢁ 5H2O
(0.1, 0.05, 0.025, 0.02 and 0.017 mol/L), and H3BO3 (10 g/L). SC
electrodeposition was conducted at 20–26.9 V with a frequency of
25 Hz at room temperature, and the duty cycle was set as 50%.
Moreover, another series of electrodeposition were conducted at
different frequency (2, 10, 25, 50, 100, 150 and 200 Hz) with the
constant Co/Cu ratio of 50:1. The bottom aluminum layer was
removed in CuCl2 solution, and the nanowires were released from
the AAO templates by NaOH solution. The as-deposited nanowires
gained from the electrolyte with different CoCu ratios were denoted
as Rn (n¼10, 20, 40, 50, 60), here ‘‘R’’ is an abbreviation for ‘‘ratio’’,
‘‘n’’ represents for the different CoCu ratio of electrolyte. And for the
nanowires deposited at different frequencies of voltage, they were
denoted as Fn, here ‘‘F’’ is an abbreviation for ‘‘frequency’’, ‘‘n’’
represents for the value of frequency.
were measured by a vibrating sample magnetometer (VSM,
Lakeshore, Model 7400 series).
3. Results and discussion
3.1. Characterization of the Co–Cu nanowires
Fig. 1 shows the SEM and TEM images of the Co–Cu nanowires.
Fig. 1(a) reveals that the nanowires with a length of ꢀ3
mm are
well-aligned and orderly along one direction. From the top view,
it can be seen that the nanowires arrays congregate to some
bundles in areas without the dependence of AAO template, and
this may attribute to the magnetic force between nanowires and
the specific surface energy caused by the high surface area of
nanowires. Fig. 1(b) shows an enlarged SEM image of Co–Cu
nanowires, and it is clear that the nanowires of a diameter of
ꢀ15 nm are highly parallel to each other and uniform throughout
the whole length which mainly depends on the well-ordered
nanochannels of the AAO templates. Some particles have been
found to attach to the surface of nanowire which may be induced
by the residual AAO template after dissolving by NaOH solution.
The broken nanowires also can be seen from Fig. 1(b), which
The obtained nanowires were characterized by field emission
scanning electron microscopy (FE-SEM, Hitachi S-4800) and
transmission electron microscopy (TEM, JEOL JEM-2100). The
crystal structures of the nanowire arrays embedded within AAO
templates were examined by using an X-ray diffractometer (XRD,
Bruker D8 ADVANCE), and the magnetic properties of the samples
Fig. 1. FE-SEM images of Co–Cu nanowires (a, b); TEM images of Co–Cu nanowires (c, d, e, f).

X. Lin et al. / Solid State Communications 152 (2012) 1585–1589
1587
caused by two main reasons: the defects of AAO template and the
1.0
0.5
electrodeposition progress. Fig. 1(c, d, e, f) show the TEM images
of Co–Cu nanowires which have been released from the AAO
templates. From Fig. 1(c), it also illustrates that the lengths of all
the nanowires are larger than 2 mm, and the nanowires are
continuous with a uniform diameter throughout their entire
length. In some regions, amounts of nanowires arranged along
one direction and parallel to each other which was contributed to
the arrangement of channels of AAO template. The semitranspar-
ent sheets around the nanowires were probably the undissolved
template. Fig. 1(d) gives the enlarge image of few nanowires, and
it can be seen that the wires gathered and several intensive
growth parts appeared in the single nanowire which often
occurred in electrodeposition. During the electrodeposition pro-
cess, even a small change to the parameters such as voltage,
current density and concentration of electrolyte will affect the
components and structure of nanowires. Fig. 1(e) shows a single
0.0
-0.5
parallel to nanowires
perpendicular to nanowires
-1.0
-10000
-5000
0
5000
10000
Hc(Oe)
nanowire with a diameter of about 15 nm and the length of 2 mm,
the aspect ratio of the nanowire can be easily gotten from the
image which should be over 100. The edge of the nanowire is not
smooth and straight enough because of the imperfection of the
AAO template. The aluminum used for anodization and the
operating temperature are the two main factors which decide
the morphology of AAO template, therefore the incomplete
annealing of aluminum and the increase of temperature both will
result in the imperfect nanochannels of AAO templates. The
diffraction spot of Co–Cu nanowires is shown in Fig. 1(f), few
bright spots can be seen in the image. Although they are not
obvious which due to the weak crystallization of Co–Cu nano-
wires, it can prove the nanowires are polycrystalline.
1800
1200
600
perpendicular to nanowire
parallel to nanowire
3.2. The effect of molar ratio of CoCu electrodeposition solution on
nanowires
0
20
40
60
Co/Cu
Fig. 2 shows the XRD patterns of the nanowire arrays which
deposited at the Co/Cu ratio of 50:1 and 10:1. There are two
diffraction peaks corresponding to the fcc (111) and (101) plane
0.9
0.6
0.3
0.0
of Co at the 2y of 44.31 and 47.61, respectively, which means the
grains grow along Co (111) and Co (101) crystal directions, and
the preferential orientation of the nanowires may attribute to the
confined growth of the nanowires within the porous alumina film.
The magnetic properties of the nanowires synthesized with
different Co/Cu ratio are shown in Fig. 3. Fig. 3(a) shows the
perpendicular to nanowire
parallel to nanowire
0
20
40
Co/Cu
60
50:1
10:1
Fig. 3. (Color online) The typical hysteresis loop of the nanowires synthesized
with Co/Cu ratio at 60:1 (a); the change in Hc, Hc// (b); and squareness (Mr/Ms)
(c) of the nanowires versus the Co/Cu ratio of the electrolyte.
hysteresis loop of the nanowires synthesized with Co/Cu ratio of
60:1. It is found that the nanowires possess shape anisotropy and
the easy magnetic axis parallel to the axis of the nanowires. The
change in Hc and Hc// of the nanowires as a function of Co/Cu
?
ratio of the electrolyte are shown in Fig. 3(b, c). In the process of
electrodeposition, with the increase in the proportion of Co–Cu,
the content of Co increases. In the Co–Cu alloy, the magnetism of
the nanowires is mostly determined by the Co. Therefore, the
higher content of Co will contribute to the larger influence of
45
60
2-Theta
75
Fig. 2. (Color online) XRD pattern of as-synthesized Co–Cu nanowires with the
deposited Co/Cu at the ratio of 50:1 and 10:1.

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X. Lin et al. / Solid State Communications 152 (2012) 1585–1589
magnetism. From the figures we can see that both Hc (Hc// and
can be achieved at Co/Cu ratio of 60:1. Saturation magnetization
increases with the increase of Co content in the system, so the
Hc ) and squareness (Mr/Ms) of the Co–Cu nanowires are
?
enhanced with the increase in Co/Cu ratio of the electrolyte and
maximum value of coercivity (1977.5 Oe) and squareness (0.92)
shape anisotropy energy (Edemag¼p
Ms2) also increases, which
leads to the enhancement of energy barrier during magnetization
reversal. As a result, the coercivity is enhanced with the increase
of Co content.
3.3. The effect of frequency on nanowires
Fig. 4 shows the XRD patterns of as-synthesized Co–Cu
nanowires under different supply frequency with the Co/Cu ratio
of 50:1. It is found that the nanowire arrays are mainly Co
crystalline phase. The diffraction peaks corresponding to the
100Hz
2Hz
(100), (111) and (101) plane of Co can be observed at the 2y of
41.51, 44.31 and 47.61 respectively in 100 Hz. While the Co
crystalline phase structure obtained in 2 Hz is not obvious, which
means that there are no typical diffraction peaks of CoCu alloy.
A typical hysteresis loop of the nanowires synthesized at the
frequency of 2 Hz (a) is shown in Fig. 5(a). From Fig. 5(b), it can be
found that the Hc of the nanowires decreased by increasing the
supply frequency when they are parallel to the applied magnetic
field, however, the values of the Hc// are kept constant when the
supply frequency is above 50 Hz. This tendency may be caused
due to the change in Co content. Meanwhile, the Hc can be
influenced by intergranular exchange coupling interactions. In
lower frequency electric deposition, the influence of Co content
on Hc plays a leading role, with the increase of the frequency, the
45
60
75
2-Theta
Fig. 4. (Color online) XRD pattern of nanowires wires with different supply
frequency.
1.0
1.0
0.5
20:1
0.5
0.0
0.0
20V
23V
26.9V
-0.5
-0.5
parallel to nanowire
perpendicular to nanowire
-1.0
-1.0
-10000
-10000
-5000
0
5000
10000
-5000
0
5000
10000
Hc(Oe)
Field(G)
1.0
perpendicular to nanowire
parallel to nanowire
1600
50:1
0.5
0.0
1200
800
20V
23V
26.9V
-0.5
-1.0
0
100
200
-10000
-5000
0
5000
10000
Frequency(Hz)
Hc(Oe)
Fig. 5. (Color online) The typical hysteresis loops of the nanowires synthesized at
the frequency of 2 Hz (a); and the variation tendency of Hc on frequency of
electrodeposition (b).
Fig. 6. (Color online) The hysteresis loops of the nanowires synthesized at
different voltages at the electrolyte with different Co/Cu ratio.

X. Lin et al. / Solid State Communications 152 (2012) 1585–1589
1589
grain may be changed, and the effect of exchange coupling
interaction should be considered. From Fig. 5, one can see that
the Hc reached the maximum value at 10 Hz, when the magnetic
field perpendicular to the axis of the nanowires.
result was the combined action between the content of Co
deposited in nanowires and the exchange coupling effect.
Acknowledgments
3.4. The effect of voltage on magnetic properties of the nanowires
Support by the Aeronautics Science Foundation of China
(2011ZF52064), National Natural Science Foundation of China
(51172109) and Natural Science Foundation of Jiangsu Province
of China (no.: BK2010497) is gratefully acknowledged.
As compared to direct voltage deposition, the influence of
alternating voltage and square-wave voltage on the deposition of
nanowires may not be so distinct. In order to investigate the
effects on the magnetic properties of the nanowires, three
different applied voltages (20, 23 and 26.9 V) have been selected.
For the electrolyte with different Co/Cu proportion (20:1 and
50:1), it is obvious that with the increase in voltage, some
changes can be found for the values of Ms and Hc as shown in
Fig. 6. The reasons for the changes can be attributed to the
different voltages lead to produce different Co and Cu precipita-
tion amounts. It is well known that the electrochemical potential
of Co2þ is higher than Cu2þ, so in the lower voltage, the
deposition rate of Cu2þ is faster than Co2þ. On the contrary, the
deposition rate of Co2þ is faster. As a result, with the increase in
voltage, the precipitation amount of Co is more than Cu.
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4. Conclusion
Magnetic Co–Cu alloy nanowires were successfully grown in
small pore size AAO templates by SC electrodeposition. The
nanowires have polycrystalline structure, showing clearly mag-
netic anisotropy with the preferred magnetization direction
parallel to the axis of nanowires. Both the coercivity and square-
ness of Co–Cu nanowires increased with the increase of Co/Cu
ratio in electrolyte and fairly large coercivity (1977.5 Oe) and
squareness (0.92) can be achieved. On the other hand, the
coercivity decreased with the frequency of power and kept
constant when the frequency was higher than 50 Hz, and this
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