Fabrication and magnetic properties of nickel nanowires

Article abstract of DOI:10.1016/j.jmmm.2004.05.023

By a combined technique of anodic anodization and direct current (DC) electrodeposition, we have successfully synthesized nickel nanowires by using an ordered porous alumina film as a template. In the article, the Ni nanowires with different diameter and coercivity have been found. Moreover, the Ni nanowires exhibit high perpendicular anisotropy.

Full text of DOI:10.1016/j.jmmm.2004.05.023

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 282 (2004) 28–31
www.elsevier.com/locate/jmmm
Fabrication and magnetic properties of nickel nanowires
Ã
S.W. Lina, S.C. Changa, R.S. Liua, , S.F. Hub, N.T. Janc
aDepartment of Chemistry and Center for Nano Storage Research, National Taiwan University, Taipei 106, Taiwan
bNational Nano Device Laboratories, Hsinchu 300, Taiwan
cProcoat Technology Co., Ltd, Linkou, Taipei 244, Taiwan
Available online 15 June 2004
Abstract
By a combined technique of anodic anodization and direct current (DC) electrodeposition, we have successfully
synthesized nickel nanowires by using an ordered porous alumina film as a template. In the article, the Ni nanowires
with different diameter and coercivity have been found. Moreover, the Ni nanowires exhibit high perpendicular
anisotropy.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.30.Gw; 75.75.+a; 81.07.Vb
Keywords: Magnetic anisotropy; Anodic anodization; Porous alumina film; Electrodeposition; Ni nanowires; Hysteresis curve;
Coercivity
1. Introduction
present paper, we use a combined technique of
anodic anodization and direct current (DC)
electrodeposition and have successfully synthe-
sized Ni nanowires in the porous alumina film.
The structure and magnetic properties were also
investigated.
The preparation and the magnetic properties of
metal nanowires electrodeposited in alumina
templates have been investigated for a long time.
Metal nanowires are very important both for basic
research and for their potential applications in
high-density recording media [1–4] and optical
label [5]. They can also be considered as model
systems to investigate the interaction process in
one-dimensional magnetic structures [6]. In the
2. Experimental procedures
High-purity aluminum foils were used as sub-
strate to synthesize porous alumina films which
were taken as the template to deposit nickel metal
into its nanopores. Prior to anodizing, the
aluminum foils were annealed under nitrogen
Ã
Corresponding author. Tel.: +886-2-23690152x148; fax:
+886-2-23636359.
E-mail address: rsliu@ntu.edu.tw (R.S. Liu).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.05.023

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S.W. Lin et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 28–31
29
atmosphere at 500 1C for 2 h in order to enhance
the grain size in the metal and to obtain homo-
geneous conditions for pore growth over large
area. Then, the foils were ultrasonically cleaned in
isopropanol for 10 min to remove the grease that
existed on the surface. Subsequently, the alumi-
num foils were electropolished in a 4:4:2 weight
mixture of H3PO4, H2SO4 and H2O, respectively,
for 10 min to obtain a smooth surface.
The aluminum foils were then treated in a two-
step anodization process to obtain porous alumina
films. The foils were first anodized under a
constant voltage 30 V in a 0.3 M H2C2O4 solution
at 10 1C for 30 min. The formed alumina was then
removed by 5 wt% NaOH at 25 1C for 2 min, and
the Al sheet was reanodized under the same
condition as the first step for 12 h. After anodizing,
the back side aluminum was removed in saturated
HgCl2 solution for 1–2 h. A subsequent etching
treatment was achieved in a 5 wt% H3PO4 solution
to remove the barrier layer on the bottom side of
alumina film. Prior to the Ni electrodeposition, a
conductive layer of Au was deposited on one side
of the through-hole alumina film by vacuum
evaporation to serve as working electrode, and
stainless steel was selected to be the counter
electrode. The Ni nanowires were electrodeposited
under a constant current in a mixture of 200 g/l
NiCl2 ꢀ H2O, 120 g/l NiSO4 ꢀ 7H2O and 50 g/l
H3BO4 at 25 1C, and the pH was adjusted to 4
by adding NaOH solution. The Ni nanowires were
liberated from the alumina template by dissolving
in 5 wt% NaOH. The morphology of the alumina
films and the Ni nanowires was investigated by a
field emission scanning electron microscope (SEM,
Hitachi S-4000) and a transmission electron
microscope (TEM; Hitachi H-7100). The magne-
tization curves of Ni nanowires were measured by
superconducting quantum interference device
magnetometer (SQUID; Quantum Design
MPMS7).
Fig. 1. SEM image of the as-prepared porous alumina film.
Fig. 2. SEM micrograph of the Ni nanowires.
of 30 V, and the average pore size was 30 nm.
From this figure, we can easily realize the pore
arrangement and the pore diameters of the porous
alumina films. The SEM micrograph of the Ni
nanowires which were synthesized by DC electro-
deposition is shown in Fig. 2. After removing the
upper part of porous alumina film, individual Ni
nanowires with a diameter of 30 nm were ob-
served. The Ni nanowires synthesized in the
porous alumina film show as well the ordered
arrangement as the porous alumina film prepared.
3. Results and discussion
In Fig. 1 we show the SEM morphology of the
as-prepared porous alumina film. The porous
alumina film was synthesized at a constant voltage

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S.W. Lin et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 28–31
Fig. 3 exhibits typical TEM images of the Ni
nanowires and the corresponding electron diffrac-
tion (ED) pattern. The TEM photographs of a
bundle of nanowires and a single nanowire are
presented in Figs. 3a and b, respectively; one can
see that the Ni nanowire is long and continuous,
and a bundle of nanowires resulted from incom-
plete dissolving. The diameter of the Ni nanowires
is about 30 nm, which corresponds to the diameter
of the pores in the alumina film. Moreover, it is
very interesting that the Ni nanowire is formed by
single crystalline Ni cylinders packing along the
long axis of the wire. It reveals that the Ni
nanowire exhibits a polycrystalline bamboo-like
structure. The crystal (indicated by white arrow)
size is consistent with the diameter of the porous
alumina film. The ED pattern of the Ni nanowire
is shown in Fig. 3c. The pattern is composed of
several diffraction rings, indicating the polycrystal-
line property and the FCC crystalline structure of
the Ni nanowire.
The hysteresis curves of the Ni nanowires with
diameters of 30, 50 and 200 nm that were prepared
by adjusting experimental parameters are shown in
Fig. 4. The Ni nanowires are measured under a
field applied perpendicular to the alumina film
surface, and the coercivities of the Ni nanowires
are 72 Oe (200 nm), 273 Oe (50 nm), and 378 Oe
Fig. 3. TEM images of: (a) a bundle of nanowires and (b) a
single nanowire (c) electron diffraction pattern of the Ni
nanowire. The crystal of the single Ni nanowire is indicated by
white arrow.
Fig. 4. Hysteresis curves of the Ni nanowires with diameters of
30, 50 and 200 nm at 5 K under a field applied perpendicular to
the alumina film surface. The coercivity of the Ni nanowires
increases with decreasing wire diameter.

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31
field is removed, the remanent magnetization
measured along the long axis is greater than which
measured in the short axis and higher squareness is
acquired along the long axis.
In summary, we have successfully synthesized
Ni nanowires by a combined technique of anodic
anodization and DC electrodeposition. The struc-
ture of the Ni nanowires is observed by TEM and
is also characterized by ED pattern to be
polycrystalline. The one-dimensional nanowire is
composed of several single crystals and is formed
by packing of the single crystals. The magnetic
properties of the Ni nanowires are investigated by
SQUID, showing high perpendicular anisotropy
and the relationship between the nanowire dia-
meter and the coercivity. This template-directed
synthetic method is a simple, inexpensive and
versatile strategy for the fabrication of metal,
semiconductor and alloy nanowires.
Fig. 5. Ni nanowires measured under applied field either
parallel (HJ) or perpendicular (H ) to the surface at 300 K.
?
The squareness of the Ni nanowires is greater when the applied
field is perpendicular (S ) to the surface than parallel (SJ) to it.
?
(30 nm), respectively. It is clear that the coercivity
of the Ni nanowires increases with decreasing wire
diameter. This should be attributed to the ten-
dency toward single domain nature [7]. The results
of the Ni nanowires measured under the applied
Acknowledgments
We thank the financial supports form the
National Science Council of Taiwan under the
grant number 92-2113-M-002-036 and the Minis-
try Economic Affairs of Taiwan under the grand
number 92-EC-17-A-08-S1-0006.
field either parallel (HJ) or perpendicular (H ) to
?
the surface are presented in Fig. 5. The squareness
(remanent magnetization/saturated magnetiza-
tion) of the hysteresis curve is greater when the
applied field is perpendicular (S ) to the surface
?
References
than parallel (SJ) to it, the squareness is 0.16 (S )
?
and 0.05 (SJ), respectively. The shape anisotropy
of ferromagnetic nanowires can be responsible for
high magnetic anisotropy perpendicular to the
substrates [8]. In general, magnetic materials with
rod-like shape will exhibit a higher demagnetiza-
tion field along the long axis than along the short
axis. In consequence, when the external magnetic
[1] S.Y. Chou, et al., J. Vac. Sci. Technol. B 15 (1997) 2897.
[2] L. Wang, et al., Thin Solid Film 288 (1996) 86.
[3] S. Dubois, et al., Appl. Phys. Lett. 70 (1997) 396.
[4] Y.G. Guo, et al., Chem. Mater. 15 (2003) 664.
[5] S.R. Nicewarner-Pena, et al., Science 294 (2001) 137.
[6] R. Hertel, J. Appl. Phys. 90 (2001) 5752.
[7] T.M. Whitney, et al., Science 261 (1993) 1316.
[8] S.Z. Chu, et al., Chem. Mater. 14 (2002) 4595.