Fabrication and magnetic properties of ordered 20 nm Co-Pb nanowire arrays

Article abstract of DOI:10.1016/j.ssc.2004.03.013

Ordered Co-Pb nanowire arrays embedded in anodic alumina template were successfully fabricated by electrodeposition. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) observations revealed that the Co-Pb nanowires were polycrystalline with uniform diameters around 20 nm and lengths up to several micrometers. Magnetic measurements showed that the coercivity and remanence of the as-deposited Co-Pb nanowires decreased with the increase of the Pb content. After annealing the Co-Pb nanowires present higher coercivities (2.4-2.5kOe) than that of pure Co nanowires (2.1kOe) and the dependence of coercivity and remanence on the Pb content is inconspicuous. A phase separation of Co and Pb occurred after annealing. The familiar pinning model was employed to explain the above experimental results.

Full text of DOI:10.1016/j.ssc.2004.03.013

Solid State Communications 130 (2004) 541–545
www.elsevier.com/locate/ssc
Fabrication and magnetic properties of ordered 20 nm Co–Pb
nanowire arrays
*
G.B. Ji, W. Chen, S.L. Tang , B.X. Gu, Z. Li, Y.W. Du
National Laboratory of Solid State Microstructure and Physics, Department of Nanjing University, Nanjing 210093,
People’s Republic of China
Received 9 January 2004; accepted 4 March 2004 by G. Luke
Abstract
Ordered Co–Pb nanowire arrays embedded in anodic alumina template were successfully fabricated by electrodeposition.
Transmission electron microscopy (TEM) and X-ray diffraction (XRD) observations revealed that the Co–Pb nanowires were
polycrystalline with uniform diameters around 20 nm and lengths up to several micrometers. Magnetic measurements showed
that the coercivity and remanence of the as-deposited Co–Pb nanowires decreased with the increase of the Pb content. After
annealing the Co–Pb nanowires present higher coercivities (2.4–2.5 kOe) than that of pure Co nanowires (2.1 kOe) and the
dependence of coercivity and remanence on the Pb content is inconspicuous. A phase separation of Co and Pb occurred after
annealing. The familiar pinning model was employed to explain the above experimental results.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 75.50.Ss; 75.60.Nt; 82.45.Qr
Keywords: A. Co–Pb nanowire arrays; B. Magnetic recording materials; D. AAO template
1
. Introduction
produce arrays of nanowires is electrochemical deposition,
which also allows production of multilayers.
There have been numerous publications about magnetic
nanowire arrays filled in porous AAO template, such as Fe,
Co, Ni, FeCo, FeNi, CoNi nanowires [5–7]. In addition,
various multilayered nanowires including Co/Cu, Ni/Cu,
NiFe/Cu, CoNi/Cu and Ag/Co [8–10] have been studied
intensively due to their interesting magnetic and transport
properties. Heterogeneous ferromagnetic–nonmagnetic
alloy films such as Co–Cu [11] and Co–Ag [12] grown
by a variety of methods have also been investigated in detail.
However, only a few studies on heterogeneous ferromag-
netic–nonmagnetic alloy nanowire systems exist in the
literature. Among them, Blythe et al. [13] have fabricated
Co–Cu alloy nanowires and presented a preliminary study
of the transport property. Fedosyuk et al. [14] reported the
granular Ag/Co deposited in porous anodic aluminium
oxide that exhibits a magnetoresistance effect at room
temperature. Recently, Wang et al. [15] have fabricated Fe–
Ag and Co–Ag nanowire arrays embedded in the AAO
A great deal of attention has been paid to the fabrication
of magnetic nanowires in recent years for their potential
application in ultra-high-density magnetic recording media
[
1,2]. Nanoscaled arrays have been suggested as recording
media to achieve recording densities of higher than
2
00 Gbit/in. , one bit of information corresponds to one
1
single-domained nanoparticle [3]. An approach to go
beyond this limit is via hexagonally arranged porous anodic
aluminum oxide (AAO) template [4]. Compared with other
templates, the size of AAO holes can be readily controlled
by properly adjusting the anodizing condition. The AAO
template is also thermally stable and shows better parallel
alignment of perpendicular pores. The general procedure to
*
Corresponding authors. Tel./fax: þ86-258-359-3817.
E-mail address: tangs1@nju.edu.cn (S.L. Tang), gbji@ufp.nju.
edu.cn (G.B. Ji).
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.03.013

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G.B. Ji et al. / Solid State Communications 130 (2004) 541–545
Fig. 1. SEM images of anodic alumina oxide template with 20 nm-
diameter pores (a) and the Co–Pb nanowire arrays embedded in
AAO template (b).
Fig. 2. TEM images of the Co–Pb nanowires with diameter about
0 nm detached from AAO template: (a) Co–Pb nanowires with the
2
inset for the SAED pattern taken from these nanowires; (b) arrayed
Co–Pb nanowires.
template and showed the variation of the coercivities vs.
annealing temperature. Up to now, there is still no report on
the study of Co–Pb nanowire arrays.
graphite as counterelectrode. The composition of Co–Pb
alloy nanowires was investigated by induction-coupled
plasma spectrometer (ICP). The percentage of Co content
varies with the variation of the concentration of the
electrolyte. The length of the as-deposited Co–Pb nano-
wires ranges from 0.5 to 20 mm and the average diameter
basically equals to that of pores of the AAO template. After
electrodeposition, the samples were annealed at 700 8C.
In this letter, we report the magnetic properties of Co–Pb
nanowrie arrays prepared by electrodeposition based on
arranged alumina templates. Enhanced coercivity (2.4–
2
.5 kOe) and perpendicular anisotropy appear after anneal-
ing. The dependence of coercivity and remanence on the Pb
content is inconspicuous.
2
. Experiment
2
.2. Experimental instrument
2
.1. The fabrication of Co–Pb nanowire arrays
Transmission electron microscopy (TEM) and scanning
Ordered porous alumina templates were prepared via the
electron microscopy (SEM) were used to characterize the
morphology of the nanowire arrays. Samples were prepared
for TEM by dissolving the AAO matrix in 0.1 M NaOH. The
resulting suspension was washed with distilled water and
ethanol and dispersed in ethanol by sonication. For
observation under SEM, the sample was dipped into a
anodizing process described in detail previously [16]. Then
the Co–Pb nanowires was electrodeposited into the
template. The electrolyte used to electrodeposit the Co–
Pb nanowires had the following composition: Co(Ac) ·4-
2
H
2
O; Pb(Ac)
2 2 3 3
·3H O; H BO . Deposition was carried out at
room temperature with an AC voltage of 40 V, 50 Hz using
3 4
solution of 5 vol% H PO for 30 min to dissolve the alumina

G.B. Ji et al. / Solid State Communications 130 (2004) 541–545
543
Fig. 3. XRD pattern of the samples annealed at 700 8C for 30 min: (a) Co; (b) Co82Pb18; (c) Co48Pb52
.
matrix. X-ray diffraction (XRD) using Cu Ka radiation was
used to investigate the crystalline structure of the samples.
Magnetic properties of the samples were tested by a
vibrating sample magnetometer (VSM).
magnetic field parallel and perpendicular to the nanowires,
respectively. For Co nanowire arrays, it is obvious that the
sample has an easy magnetic axis perpendicular to the AAO
film, i.e. parallel to the axis of the nanowires due to the
shape anisotropy. While for Co Pb nanowire arrays
4
8
52
(
shown in Fig. 4(b)) the difference between the parallel and
perpendicular direction is reducted. Fig. 5(a) shows the
M–H loops of the as-deposited Co, Co82Pb18, Co48Pb52
nanowire arrays with the external magnetic field parallel to
the nanowires. The coercivities of Co, Co Pb , Co Pb
3
. Results and discussion
3
.1. Structure properties
8
2
18
48 52
from the hysteresis loops are 1668, 1430 and 1086 Oe and
the remanences are 0.95, 0.80 and 0.52, respectively. Fig.
Fig. 1(a) shows the SEM image of ordered nanopores in
an anodic aluminum oxide template. The average diameter
of the pores is 20 nm and the center-to-center distance is
5
(b) shows the corresponding hysteresis loops of the same
samples annealed at 700 8C. The coercivities and rema-
nences are 2170, 2446 and 2472 Oe and 0.89, 0.90 and 0.89,
respectively.
6
0 nm. A typical SEM image of the Co–Pb nanowires is
shown in Fig. 1(b). The nanowire arrays fill the nanochan-
nels uniformly and the nanowires are apparently continuous.
The measured diameter of the nanowires corresponds
closely to the pore diameter. Fig. 2 shows the TEM images
of the Co–Pb nanowires. Clearly, the nanowires are both
very regular and uniform, with an average diameter of about
3
.3. Discussion of the experimental results
From Fig. 5(a) we can see that both the coercivity and
remanence of the as-deposited Co–Pb nanowires decrease
with the increase of the Pb content. This is expectable
because the inserting of Pb particles between Co particles
could weaken the interaction and thus decrease both the
coercivity and remanence. The reduction of the difference
between parallel and perpendicular directions of the
Co Pb nanowires shown in Fig. 4 (b) also supports this
2
0 nm and the length up to several micrometers. The
selected-area electron diffraction (SAED) pattern shown in
the inset of Fig. 2(a) confirms that the Co–Pb nanowires are
polycrystalline. XRD was performed to analyze the
microstructure of the samples (shown in Fig. 3). The Co
peaks were detected after annealing at 700 8C. The
evolution of XRD pattern under the annealing temperature
of 700 8C suggests a phase separation in this alloy system.
4
8
52
point of view.
Yet after annealing we can see that the coercivities of the
Co–Pb alloy nanowires are higher than that of pure Co and
thedifference betweenCo Pb and Co Pb is small, i.e. the
3
.2. Magnetic properties
8
2
18
48 52
content of Pb affects little of the magnetic properties of the
nanowires after annealing. As we know, the melting point of
bulk Pb is far below 700 8C and that of Co is above 700 8C,
Fig. 4(a) and (b) show the hysteresis loops of the as-
deposited Co and Co48Pb52 nanowires with the external

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G.B. Ji et al. / Solid State Communications 130 (2004) 541–545
Fig. 4. Representative M–H loops of the as-deposited Co nanowire
arrays (a); Co48Pb52 nanowire arrays (b) measured in a magnetic
field parallel and perpendicular to nanowires, respectively.
Fig. 5. Typical hysteresis loops of the samples before (a) and after
(b) annealing measured in a magnetic field parallel to the long axes
of nanowires.
we can expect that Pb is well melted at the annealing
temperature in our experiment while Co is still in solid state.
Because the density of the Pb is higher than that of Co and
the liquid Pb is ambulatory, the phases of the two materials
separate as shown in the XRD pattern in Fig. 3(b) and (c).
Because the resulted Co is polycrystalline and there are
always gaps between the boundaries, it is believed that there
is still a little amount of liquid Pb left between Co particles.
The residual Pb solidified when the samples were cooled
down and became pins between Co particles. The pins
cumbered the reversal of Co nanowires during the
magnetization reversal process, which led to higher
coercivity of Co–Pb nanowires than pure Co nanowires.
porous anodic aluminum oxide template. The coercivity and
remanence of the Co–Pb nanowires decrease with the
increase of the Pb content before annealing. After annealing
the Co–Pb alloy nanowires present higher coercivity than
pure Co nanowires and the dependence of coercivity and
remanence on the Pb content is inconspicuous. The above
experimental results are explained by the familiar pinning
model.
Acknowledgements
This work was supported by the Project No. 50171033 of
National Natural Science Foundation of China and a
national key project of fundamental research (973, No. G
1
999064508).
4
. Conclusion
Ordered Co–Pb nanowire arrays were successfully
prepared by AC electrodeposition with AAO as template.
The TEM and SEM observations indicate that the ordered
Co–Pb nanowire arrays are uniformly assembled into the
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