Structure and magnetic properties of metastable Co-Cu solid solution nanowire arrays fabricated by electrodeposition

Article abstract of DOI:10.1002/pssa.200521463

Nanowire arrays of the metastable Co_xCu_1-x (0.20 ≤ x ≤ 0.85) solid solution system which can not be obtained by equilibrium methods, were prepared by electrodeposition in pores of anodic aluminum oxide (AAO) template, and subsequentl

Full text of DOI:10.1002/pssa.200521463

phys. stat. sol. (a) 203, No. 10, 2426–2431 (2006) / DOI 10.1002/pssa.200521463
Structure and magnetic properties of metastable Co–Cu solid
solution nanowire arrays fabricated by electrodeposition
*
Tao Wang , Fashen Li, Ying Wang, and Lijing Song
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou Univesity,
Lanzhou 730000, P.R. China
Received 21 November 2005, revised 24 April 2006, accepted 17 May 2006
Published online 27 June 2006
PACS 61.46.–w, 75.30.Gw, 75.30.Hx, 75.75.+a, 81.15.Pq
Nanowire arrays of the metastable Co Cu1–x (0.20 ≤ x ≤ 0.85) solid solution system which can not be ob-
x
tained by equilibrium methods, were prepared by electrodeposition in pores of anodic aluminum oxide
(AAO) template, and subsequently annealed at different temperatures. The as-deposited samples all show
single phase of fcc structure, and lattice parameters decrease with the increase of Co content and funda-
mentally accord with Vegard’s law. The phase transition with heat treatment was investigated by X-ray
diffraction and differential thermal analysis (DTA) which further confirmed the formation of solid solu-
tion. With Co content increasing, the coercivity along nanowire axis for as-deposited samples increases,
but it decreases for the annealed samples at 700 °C. This phenomenon was explained considering the in-
teraction of Co particles through Cu in nanowires after phase separation.
©
2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Heterogeneous metastable alloys, e.g. Co–Cu, Fe–Cu, or Fe–SiO , with a much larger supersaturation
2
far from equilibrium have been widely investigated due to their remarkable mechanical, optical and
magnetoelectrical transport properties, especially giant magnetoresistance effect (GMR) found in these
materials [1–4].
The equilibrium phase diagram of the Co–Cu system shows virtually no solubility of Co in Cu or Cu
in Co below 500 °C [5]. It is difficult to produce a solid solution by equilibrium process. Therefore, some
special preparation methods were used to obtain a metastable solid solution of Co–Cu system. Gente
et al. and Modder et al. have prepared Co–Cu solid solution alloys by mechanical alloying [6, 7];
Kneller reported many years ago that Co–Cu films evaporated at room temperature formed metastable
fcc solid solution [8]; In addition, some other nonequilibrium techniques such as rapid quenching, sput-
tering technique have also been used to prepare Co–Cu metastable alloys with different Co content
[9, 10]. Recently the electrodeposition technique (ED) has also been used successfully in the production
of this kind of alloys [11]. This technique has been demonstrated to be very convenient because of its
simplicity and low cost in comparison with the methods mentioned before. On the other hand, nanowire
systems have been studied widely. However, only a few studies focused on heterogeneous alloys, espe-
cially Co–Cu alloys, in nanowire systems. In this paper, we fabricated nanowire arrays of Co Cu solid
x
1–x
solution by electrodeposition method based on AAO template with Co content from 20% to 85%, and
studied magnetic properties of the as-deposited and annealed samples.
*
Corresponding author: e-mail: wangt01@st.lzu.edu.cn
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2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 203, No. 10 (2006)
2427
2
Experiments
A 0.25 mm thick aluminum foil (99.999%) was used as starting material. To obtain highly ordered pores,
a two-step anodization process was employed. The detailed preparation procedure and properties of the
AAO template have been published elsewhere [12]. Co Cu wires were deposited into pores of AAO
x
1–x
template at room temperature with an AC voltage of 12 V, 50 Hz using a graphite counter electrode from
an electrolyte consisted of CoSO ·7H O, CuSO ·5H O, and boric acid. The compositions of Co Cu
1–x
4
2
4
2
x
2+ 2+
nanowires were adjusted by varying the Co :Cu ion ratio in the electrolyte. Meanwhile pure Co nano-
wire array was also fabricated with the same deposition condition. After electrodeposition the ordered
nanowire arrays were separated from the Al substrates by immersion in a saturated HgCl solution.
2
Co Cu nanowire arrays embedded in AAO were annealed at different temperatures (450 °C, 550 °C,
x 1–x
650 °C, 700 °C) for 30 minutes under a flowing nitrogen atmosphere.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to
confirm the morphology of AAO film and nanowires. Atomic percentages of Co and Cu in the nanowires
were determined by atomic absorption spectroscopy (AAS). The crystal structures and phase evolution of
Co–Cu nanowire arrays were examined by X-ray diffraction (XRD) and differential thermal analysis
(DTA). Vibrating sample magnetometer (VSM) was used to measure the magnetic properties of the
samples at room temperature.
3
Results and discussion
Figure 1(a) shows a SEM micrograph of AAO film with average pore size of 70 nm. One can see the
distribution of pores is almost uniform. Co and Cu were electrodeposited into the pores of anodic alumi-
num oxide, and the compositions of Co Cu nanowires were obtained by atomic absorption spectros-
x
1–x
copy analysis (AAS). Results show that Co content is in the range of 20 at% to 85 at%. Figure 1(b)
shows a typical TEM image of Co Cu nanowires with diameter of 70 nm after removing alumina. The
55
45
length is several micrometers and aspect radio of nanowires is about 50.
Figure 2 shows the X-ray diffraction patterns of Co, and Co Cu1–x (0.20 ≤ x ≤ 0.85) nanowire arrays
x
embedded within the AAO film. From the patters we can see pure Co nanowire array consists of a phase
mixture of hexagonal close-packed (hcp) and face-centered cubic (fcc) crystals. But when nonmagnetic
Cu was added, the diffraction peak of hcp structure disappeared, only fcc peaks of Co–Cu alloy existed
in the diffraction patterns of Co Cu (0.20 ≤ x ≤ 0.85) nanowire arrays. Instead of hcp and fcc structure
x
1–x
of Co nanowire, the Co Cu (0.20 ≤ x ≤ 0.85) nanowires crystallized into single fcc structure. The dif-
x 1–x
(a)
(b)
Fig. 1 (a) SEM micrograph of AAO film with average pore size of 70 nm, and (b) TEM image Co Cu
45
5
5
nanowires with diameter of about 70 nm.
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Tao Wang et al.: Structure and magnetic properties of Co–Cu nanowire arrays
fcc(111)
3.62
3
.60
x=0.20
x=0.40
3.58
Vegard' law
x=0.55
x=0.73
3.56
hcp(100)
x=0.85
x=1.00
80
3.54
3
0
40
50
60
70
0.0
0.2
0.4
0.6
0.8
1.0
2Theta (degree)
x
Fig. 2 X-ray diffraction patterns of Co Cu nanowire
x
Fig. 3 Lattice parameters depend on Co content
of Co Cu nanowire arrays.
1–x
arrays.
x
1–x
fraction peaks of all the Co–Cu nanowire arrays are observed on higher-angle side than the peaks of
pure Cu, and the peak of the (111) plane shifts to the higher angle as the Co ratio increases. Lattice pa-
rameters estimated from the diffraction peaks of Co–Cu nanowire arrays are shown in Fig. 3. They are
all smaller than that of pure Cu, and larger than that of pure Co respectively. We also can see the lattice
parameters decrease with Co content increasing, and fundamentally accord with Vegard’ law, which
shows that the supersaturated solid solution was formed for Co Cu (0.20 ≤ x ≤ 0.85) system [5].
x
1–x
Since Co and Cu are almost immiscible below 700 °C, the heat treatment will result in the recystalli-
zation of this metastable alloy into separate phases [13], which can lead to the formation of small ferro-
magnetic Co particles and nonmagnetic Cu particles in nanowires. Phase evolution of typical Co Cu
55
45
nanowire array with anneal treatment at different temperatures examined by XRD is shown in Fig. 4. The
111) peak of fcc Co Cu nanowire array preserves up to an anneal temperature of 550 °C. With further
(
55
45
annealing at 650 °C, the peaks of fcc Co and Cu appear, indicating the metastable Co–Cu alloy recrys-
tallized into fcc Co and fcc Cu. As anneal temperature increased to 700 °C, the diffraction peaks of (111)
planes become obviously narrower due to the growth of particle size and a more complete phase separa-
tion. The other Co content samples of Co Cu (0.20 ≤ x ≤ 0.85) nanowire arrays have the same phase
x
1–x
evolution during anneal treatment besides Co Cu
55 45.
fcc(111)
(
a)
b)
c)
(d)
(
(
Cu(111) Co(111)
(e)
30
40
50
60
70
80
2
Theta (degrees)
Fig. 4 X-ray diffraction patterns of (a) Co Cu nanowire array, and Co Cu nanowire arrays annealed
45
5
5
45
55
at (b) 450 °C, (c) 550 °C, (d) 650 °C, (e) 700 °C.
©
2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
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phys. stat. sol. (a) 203, No. 10 (2006)
2429
Fig. 5 DTA curve of Co Cu nanowire array in nitro-
5
5
45
gen with a heating rate of 15 °C/min.
To investigate the phase transition behavior in Co–Cu nanowires further, DTA measurement has also
been carried out in nitrogen. Figure 5 shows a typical DTA curve of Co Cu nanowire array with a
55
45
heating rate of 15 °C/min up to 700 °C. A broad exothermic peak is seen at about 500 °C. This broad
peak mainly originates from the decomposition of the fcc Cu–Cu solid solution. The DTA result also
indicates that a heating rate of 15 °C/min is too fast to complete the decomposition of Co–Cu alloy na-
nowires at 700 °C. Although the exothermic peak of DTA curve is seen at about 500 °C, the sample
annealed up to 550° still reveals no peaks of separated Co and Cu in XRD patterns. This surprising result
can be explained by the course of the decomposition reaction. Due to the thermodynamically unstable
state of the Co–Cu solid solution, short-range change may have started at or below 500 °C. This led to
the obvious exothermic peak. However, the short-range change can not lead to large-scale phase separa-
tion, and only when obvious phase separation completed, the separated diffraction peaks could be ob-
served from XRD pattern, but the exothermic reaction may have approached the end at this time. Since
DTA measurement is extremely sensitive to phase transition [6], the result of DTA is a clear evidence for
the formation of Co–Cu solid solution nanowire arrays.
Figure 6 illustrates the coercivities for Co Cu nanowire arrays annealed at different temperatures
55 45
with applied field parallel to nanowire. As the annealing temperature increases from room temperature to
50 °C, the coercivity of the sample increases slowly. Then, it increases dramatically when the annealing
temperature is up to 650 °C and the largest coercivity was observed after the annealing at the highest
temperature applied.
5
2000
1500
1000
Fig. 6 Coercivities for Co Cu nanowire array an-
5
5
45
nealed at different temperatures with the applied field
parallel to the nanowire axis.
500
0
200
400
600
800
O
Annealing temperature ( C)
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2430
Tao Wang et al.: Structure and magnetic properties of Co–Cu nanowire arrays
2
1
1
000
600
200
2000
1600
1200
800
8
00
0.0
0.2
0.4
0.6
0.8
1.0
x
Fig. 7 Coercivities for Co Cu (0.2 ≤ x ≤ 0.85) nanowire arrays of as-deposited (᭡) and annealed at
x
1–x
700 °C (᭹) samples as a function of Co content with the applied field parallel to the nanowire axis.
The coercivities along nanowire axis of as-deposited and 700 °C-annealed samples with different Co
contents are shown in Fig. 7. From the figure we can see the coercivities of annealed samples are all
larger than those of as-deposited samples for the same Co content. With Co content increasing, the coer-
civity of as-deposited sample increases, while it decreases for the sample annealed at 700 °C.
For a ferromagnetic nanowire with great aspect ratio (L/d > 20:1), demagnetizing factor along
w
nanowire axis is approximate zero, magnetic moments tend to align along nanowire axis [14, 15]. Coer-
civity along nanowire axis mainly depends on energy barrier during magnetization reversal.
For as-deposited Co Cu nanowire arrays, saturation magnetization increases with the increase of Co
x
1–x
2
content in this system [16]. So the shape anisotropy energy (Edemag = πMs ) 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 (Fig. 7).
Because of the changes of ferromagnetic metal of Co Cu (0.20 ≤ x ≤ 0.85) nanowire arrays in the
x 1–x
annealing process, saturation magnetizations of these samples increase with the increase of annealing
2
temperature [17], and as a result, the shape anisotropy energies (Edemag = πMs ) also increase. So, the
coercivities for Co Cu (0.20 ≤ x ≤ 0.85) nanowire arrays annealed at 700 °C are all larger than those of
x 1–x
as-deposited samples (Fig. 7). From Fig. 6, we can also see the coercivity of Co Cu nanowire array
55 45
annealed at 650 °C has a remarkable increase. This phenomenon is consistent with XRD pattern of the
sample annealed at the same temperature where Co and Cu particles obviously recrystallize from metast-
able Co Cu alloy, and saturation magnetization of this sample increases dramatically at this point.
45
55
After Co Cu (0.20 ≤ x ≤ 0.85) nanowire arrays were annealed at 700 °C, contrary to as-deposited
x 1–x
samples, the coercivities along nanowire axis display a decrease with the increase of Co content. With
the content of magnetic Co particles decreasing, the saturation magnetization of the nanowire decrease,
which leads to smaller shape anisotropy energy. But the coercivities exhibit an increase. So, besides
shape anisotropy energy, the other factor also affects the energy barrier during magnetization reversal.
After phase separation, the nanowires are not uniform but composed of magnetic Co particles and non-
magnetic Cu particles. In this heterogeneous nanowire arrays system distributed in a non-magnetic ma-
trix, magnetic particles are distributed in nonmagnetic particles. We assume the interaction of Co parti-
cles through Cu can also enhance the energy barrier during magnetization reversal. As Co particle con-
tent decreases, there are more Co particles separated by Cu particles, which may increase the interaction
effect on Co particles, consequently leading to the enhancement of energy barrier during magnetization
reversal although the shape anisotropy energy decreases, and as a result, coercivity is enhanced. Al-
though this phenomenon was explained by assuming that the interaction of magnetic Co particles
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2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
phys. stat. sol. (a) 203, No. 10 (2006)
2431
through Cu can also affect the energy barrier besides shape anisotropy energy, the physical mechanism
of magnetization reversal in this type nanowire array consists of magnetic particles and nonmagnetic
particles is not yet clear.
4
Conclusion
In summary, nanowire arrays of metastable Co Cu (0.2 ≤ x ≤ 0.85) solid solution were prepared by
x 1–x
electrodeposition in pores of anodic aluminum oxide (AAO) templates. The lattice parameters of as-
deposited samples decrease with Co content increasing and fundamentally accord with Vegard’s law.
When the as-deposited samples were annealed up to 650 °C, phase separation resulted in the formation
of small ferromagnetic Co particles and nonmagnetic Cu particles in nanowires. Coercivity along
nanowire axis of as-deposited sample increases with the increase of Co content, but decreases for the
700 °C-annealed samples. This phenomenon was explained considering the interaction of Co particles
through Cu in nanowires besides the contribution of shape anisotropy energy after phase separation.
References
[1] A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. P. Young, S. Zhang, F. E. Spada, F. T. Parker, A. Hutten, and
G. Thomas, Phys. Rev. B 68, 3745 (1992).
[
[
[
[
[
[
[
[
2] J. Q. Xiao, J. S. Jiang, and C. L. Chien, Phys. Rev. B 68, 3749 (1992).
3] F. K. LeGoues and H. I. Aronson, Acta Metall. 32, 1855 (1984).
4] H. Wendt and P. Haasen, Scr. Metall. 19, 1053 (1985).
5] X. Fan, T. Mashimo, and X. S. Huang, Phys. Rev. B 69, 094432 (2004).
6] C. Gente, M. Oehring, and R. Bormann, Phys. Rev. B 48, 13244 (1993).
7] I. Modder, E. Schoonderwaldt, G. Zhou, and H. Bakker, Physica B 245, 363 (1998).
8] E. Kneller, J. Appl. Phys. 33S, 1355 (1962).
9] M. G. M. Miranda, E. Estévez-Rams, G. Martínez, and M. N. Baibich, Phys. Rev. B 68, 014434 (2003).
[
[
[
[
[
[
[
[
10] Q. Xiao John, J. Samuel Jiang, and C. L. Chien, Phys. Rev. Lett. 68, 3749 (1992).
11] H. Zaman, A. Yamada, H. Fukuda, and Y. Ueda, J. Electrochem. Soc. 145, 565 (1998).
12] D. J. Sellmyer, M. Zheng, and R. Skomski, J. Phys.: Condens. Matter 13, R433 (2001).
13] T. Liu, T. D. Hu, Y. N. Xie, and W. D. Wang, J. Magn. Magn. Mater. 192, 467 (1999).
14] F. S. Li, T. Wang, L. Y. Ren, and J. R. Sun, J. Phys.: Condens. Matter 16, 8053 (2004).
15] Y. Peng, H. L. Zhang, S. L. Pan, and H. L. Li, J. Appl. Phys. 87, 7405 (2000).
16] J. R. Childress and C. L. Chien, Phys. Rev. B 43, 8089 (1991).
17] G. B. Ji, L. Tang, B. X. Gu, and Y. W. Du, J. Phys. Chem. B 108, 08862 (2004).
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