Fabrication and magnetic properties of Ni-Zn nanowire arrays

Article abstract of DOI:10.1016/j.jmmm.2009.06.062

Ni-Zn nanowire arrays, with diameters of approximately 60 nm and lengths of around 40 μm, were fabricated by electrodeposition in porous anodic aluminum oxide templates at different electric potentials. X-ray diffraction observations demonstrated that the

Full text of DOI:10.1016/j.jmmm.2009.06.062

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Journal of Magnetism and Magnetic Materials 321 (2009) 3511–3514
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Fabrication and magnetic properties of Ni–Zn nanowire arrays
a,b
b,c
a,b
a,b
a,b
a,b,Ã
Lihu Liu , Haitao Li , Shenghua Fan , Jianjun Gu , Yaopeng Li , Huiyuan Sun
a
College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang 050016, China
b
Key Laboratory of Advanced Films of Hebei Province, Shijiazhuang 050016, China
c
Department of Physics, Hebei Chengde Teachers College, Chengde 067000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ni–Zn nanowire arrays, with diameters of approximately 60 nm and lengths of around 40 mm, were
Received 13 January 2009
Received in revised form
fabricated by electrodeposition in porous anodic aluminum oxide templates at different electric
potentials. X-ray diffraction observations demonstrated that the isolated nanowires had polycrystalline
structure and that their phases changed with the deposition potential. The amount of deposited zinc in
the nanowires increased with the deposition potential, whereas the amount of nickel decreased.
Magnetic measurements showed that there was a gradual change of magnetism from isotropic to
anistropic with increasing potential amplitude and that the coercivity reached a maximum value in the
nanowire deposited at ꢀ1.35 V.
2
0 March 2009
Available online 25 June 2009
Keywords:
Nanowire
Electrodeposition
Magnetic property
&
2009 Elsevier B.V. All rights reserved.
1
. Introduction
Following anodization, the remaining aluminum was removed
using a saturated CuCl solution. Etching treatment was then
carried out in 6 wt% H PO at room temperature for 2 h to remove
the barrier layer on the bottom side of the AAO and obtain a
template with holes passing through the entire thickness of the
foil. This process resulted in AAO templates with a pore length of
2
Recently, one-dimensional and quasi-one-dimensional nanos-
tructured materials have attracted considerable interest because
of their applications in optics and high-density perpendicular
magnetic recording media and much effort has been expended on
the synthesis of these materials [1–4]. Among the different
preparation procedures, the most widely used is the template
method [5–7]. Compared with other templates [8,9], the anodic
aluminum oxide (AAO) template is ideal for preparing ordered
nanotube arrays using electrochemical methods because of its
uniform and nearly parallel porous structures.
3
4
about 40 mm and a nearly uniform diameter of about 60 nm.
Afterwards, a 300 nm Cu layer was sputter-deposited onto one
side of the AAO, this served as the working electrode for the
following electrodeposition.
The pH of an aqueous bath containing 0.1 M NiCl
2
ꢁ 6H
2
O, 0.1 M
ZnCl and 0.01 M glycin was adjusted to 2.5 by adding appropriate
2
In this paper, we investigate the preparation of an array of
highly ordered Ni–Zn nanowires by DC electrodeposition into
porous AAO templates [8,10]. The structure of the nanowires has
been investigated using conventional X-ray diffraction (XRD),
transmission electron microscopy (TEM) and scanning electron
microscopy (SEM), while the magnetic properties have been
measured using a vibrating sample magnetometer (VSM).
amounts of dilute HCl. Electrodeposition was carried out at a
constant potential of ꢀ0.9 to ꢀ1.46 V in a conventional geometry
consisting of a graphite rod counter electrode and an Ag/AgCl
reference electrode. The porous alumina membranes, with pore
diameters of 60 nm and thickness 40 mm, were used as the
working electrode. All measurements were carried out at room
temperature.
Depending on the deposition potential used, the electrodepo-
sition process lasted 60–120 min. After electrodeposition, a piece
of the AAO template now containing Ni–Zn nanowires was
dissolved using 5 wt% NaOH, and washed 3–5 times alternatingly
with distilled water and ethanol. Afterwards, the nanowires were
detached from the substrate by ultrasonic dispersion in 2–3 ml
ethanol. A drop of the solution was then placed on a Cu grid with a
carbon film and air dried prior to electron microscope analysis.
2
. Experimental procedure
Porous anodic aluminum oxide templates were prepared using
a two-step anodizing process [11]. Aluminum foils 0.28 mm thick
and with purities of 99.999% were anodized for 10 h in 0.3 M
oxalic acid solution under a constant potential of 40 V at 10 1C.
3. Results and discussion
Ã
Corresponding author at: College of Physics Science & Information Engineer-
ing, Hebei Normal University, Shijiazhuang 050016, China. Tel.: +86 31186268312;
fax: +86 31186268314.
Fig. 1(a) shows an AFM surface view of an alumina film
E-mail address: huiyuansun@126.com (H. Sun).
prepared by the two-step anodization described above. From the
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.06.062

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Fig. 1. (a) SPM image of the AAO template with hole diameter of about 60 nm and
b) TEM image of isolated Ni–Zn nanowire liberating from the alumina template.
(
Fig. 2. XRD patterns of aligned Ni–Zn nanowires in the AAO template prepared at
different potentials:(a) ꢀ0.9 V; (b) ꢀ1.12 V; (c) ꢀ1.24 V; (d) ꢀ1.35 V and (e) ꢀ1.46 V.
( ) denotes the peak for Ni; (.) denotes the peak for Zn and (K) denotes the peak
r
for NiZn alloy.
image we can see that the pores have almost identical diameters
of 60 nm and spacings of about 35 nm. The highly ordered white
spots correspond to the pores in the AAO template. This special
structure of the alumina film makes it promising as a template for
fabricating an ordered nanowire array over a large area. Fig. 1(b) is
a TEM image of an isolated NiZn nanowire after removing the
alumina prepared by electrodeposition for 80 min. The diameter
of the nanowires is about 60 nm, which corresponds to the pore
diameter in the AAO template shown in Fig. 1(a).
Typical room-temperature hysteresis loops (M–H) of NiZn
nanowire arrays deposited under different electric potentials are
shown in Fig. 3((a)–(e)). The saturation magnetization is
underlined in each figure. We designate the corresponding
magnetic fields as H
J ?
and H . For the nanowires deposited at
ꢀ0.9 V, M–H loops in the parallel and perpendicular geometries
are almost the same, and there is no apparent magnetic
anisotropy (Fig. 3(a)). At low potential, the current between the
electrode and electrolyte is low and correspondingly, the whole
nanowire is homogeneous. On the other hand, only nickel can be
detected in the sample according to the XRD results mentioned
above. As a result, we can conclude that the effective anisotropy of
the magnetic nanowires results from shape anisotropy, and the
magnetocrystalline anisotropy of the nanowires gives only a small
contribution in this case [14]. For wire arrays prepared at higher
potential, a different behavior can be observed (Fig. 3(b)–(e)).
Thus, nanowire arrays show apparent magnetic anisotropy with
the increasing potential, which may be due to structural changes
caused by the increased concentration of zinc in the nanowires
[15]. In particular, the deposition of cations into the alumina
template will accelerate with increasing deposition potential [16],
and many dispersed structures will form in the wires at high
deposition potential. Consequently, intrinsic stress and defects are
high in the Ni–Zn nanowire arrays, which leads to an increase in
magnetocrystalline anisotropy. In this regard, we suggest that the
changes in the magnetic properties are related to structural
changes under different deposition potentials.
Table
synthesized under different deposition potentials. We can see
that the squareness SQ (SQ ¼ M
/M where M denotes remanence
and M the saturation magnetization) of parallel hysteresis loops
decreases from 0.55 to 0.23. On the other hand, a distinct change
is found in the SQ of the hysteresis loops measured in a
perpendicular field. With increasing deposition potential, Zn will
be deposited preferentially. The growth process and texture of Ni
will then be influenced by tiny regions of hcp-phase Zn, where the
magnetocrystalline anisotropy makes a contribution which favors
magnetization parallel to the nanowire arrays. Under these
conditions the uniformity of nanowires will be destroyed, and
Fig. 2 shows the XRD pattern of aligned NiZn nanowire arrays
prepared at different deposition potentials in the AAO templates.
For XRD measurements, the Al and AAO substrates were etched
2
away by an amalgamation process using saturated CuCl and
NaOH aqueous solutions and finally washed thoroughly with
distilled water. From the pattern we can see that the Ni–Zn
nanowire arrays show different phase structures with different
deposition potentials. At low potential, where we adjust the
deposition potential to ꢀ0.9 V, there is only the face-center-cubic
(fcc) Ni phase with the (111) pattern evident (Fig. 2(a)). However,
a peak (101) of the hexagonal-closed-packed (hcp) Zn phase
appears with increasing deposition potential. For the sample
prepared at ꢀ1.35 V, another interesting feature can be seen.
There is a broadening of the peak near 2
y ¼ 431, which may be
due to the appearance of a NiZn alloy phase (JCPDS card, 065-
203). When the potential rises to ꢀ1.46 V, there are only six
diffraction peaks in the XRD pattern, corresponding to the (0 0 2),
10 0), (101), (10 2), (10 3) and (110) peaks of hcp Zn. No more
3
(
extra peaks such as those due to the NiZn alloy or elemental Ni
were detected. This indicates that the amount of deposited zinc in
the nanowires increases with the deposition potential, whereas
the amount of nickel decreases. Consequently, we can conclude
that the texture configuration of binary alloy nanowire arrays can
be adjusted by changing the deposition potential. From Fig. 2(e)
we observe that the intensity of the Zn hcp (101) peak is the
largest, indicating that for Zn nanowires deposited in porous AAO
membranes, growth is preferentially along the Zn hcp (101)
direction [12].
In order to investigate more clearly the influence of the doping
conditions, magnetic characterization measurements of the
hysteresis loops in configurations with the magnetic field
parallel(J) and perpendicular(?) to the NiZn long axis nanowires
were carried out. Measurements performed on the AAO templates
containing the nanowire arrays were made at room temperature
using a Lake Shore 7310 vibrating sample magnetometer in
applied magnetic fields ranging up to 5 kOe. The properties of the
nanowires are expected to be dependent on the diameter of
the nanowires. In our experiments, in order to eliminate the
effects of the diameter, all nanowires were fabricated with the
same diameter by adjusting the pore size of AAO template [13].
1 shows the magnetic parameters of nanowires
r
s
r
s
? J
we can see that SQ decrease more rapidly than does SQ .
Fig. 4 shows the variation in saturation magnetization and
coercivity of Ni–Zn nanowire arrays as a function of the deposition
potential. The external magnetic field was parallel and
perpendicular to the long axes of the Ni–Zn nanowire arrays as
indicated. The values of HcJ and Hc? show different variations
as functions of the deposition potential. When the external field is
parallel to the long axis of the nanowire, the coercivity first

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Fig. 3. Magnetic hysteresis loops for nanowire arrays deposited at different potentials with the external field parallel and perpendicular to the nanowire: (a) ꢀ0.9 V; (b)
1.12 V; (c) ꢀ1.24 V; (d) ꢀ1.35 V and (e) ꢀ1.46 V.
ꢀ
increases with increasing deposition potential to a maximum at
1.35 V and then decreases sharply. From the discussion above,
we know that, at low potential, a small quantity of zinc cannot
change the geometric shape of the nanowire, so shape anistropy
predominates here. However, the addition of more zinc during the
deposition induces defects and may divide the whole nanowire
into tiny magnets, according to whether Zn forms continuous or
quasi-continuous nanosegments. These defects disperse in the
ꢀ

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Table 1
only a small quantity of Zn is deposited into the nanowire, and
cannot produce a greater change in the magnetization. We find
that the saturation magnetization of samples prepared at ꢀ1.46 V
Magnetic parameters of NiZn nanowires of equal diameter, synthesized under
different deposition potentials.
3
is only 7.27 emu/cm , about half that of the sample prepared at
Sample
Deposition Potential (ꢀV)
H
cJ (Oe)
H
c? (Oe)
SQ
J
SQ
?
ꢀ
0.9 V. This can be explained by the deposition of more
S1
S2
S3
S4
S5
0.9
656
654
714
871
538
562
481
324
194
143
0.55
0.55
0.45
0.33
0.23
0.47
0.11
0.08
0.05
0.03
nonmagnetic zinc in nanowires with increasing deposition
potential. This will reduce the magnetic moment per unit volume,
i.e., the saturation magnetization.
1.12
1.24
1.35
1.46
4. Conclusion
Fabrication of Ni–Zn nanowires was successfully carried out
through electrodeposition in AAO templates. XRD patterns
indicate that the phase structures of the nanowires change
significantly with the deposition potential. The nanowire arrays
deposited at low potential display notable magnetic isotropy, but
the samples show obvious magnetic anistropy with the increasing
potential, which may be due to structural changes. The changes in
coercivity are regarded as being related to the phase changes
in the nanowires. The system, as it stands, is suitable for
producing arrays of segmented magnetic nanowires by depositing
metallic components at different potentials from the same
electrodeposition bath.
Acknowledgments
Fig. 4. Electrodeposition potential dependence of coercivity and saturation
magnetization for NiZn nanowire arrays with the external field perpendicular or
parallel to the nanowires as indicated.
This work has been financed by the Natural Science Foundation
of Hebei Province under Grant no. A2009000254, and the Ph.D.
fund from the Hebei Normal University under Grant no.
L2006B10. The authors wish to thank Dr. Norman Davison for
helpful discussion.
nanowire in a disordered fashion. This blocks domain wall
movement and increases the magnetocrystalline anisotropy. The
enhanced coercivity HcJ of the NiZn nanowires array may be
attributed to the high concentration of defects into NiZn
nanowires and the additional contribution of magnetocrystalline
anisotropy, caused by zinc doped in the nickel nanowires.
However, the content of Zn in the nanowire increases with
the deposition potential, and according to the phase diagram of
the binary Ni–Zn system, a mixed structure of fcc and hcp phases
will be formed in NiZn alloy. When the potential is higher than
Reference
[
[
[
1] J.O. Lee, H.K. Kim, G.H. Kim, W.Y. Jeung, J. Appl. Phys. 99 (2006) 08B704.
2] X.T. Tanga, G.C. Wang, M. Shima, J. Appl. Phys. 99 (2006) 123910.
3] J. Sanchez-Barrigaa, M. Lucasb, G. Riveroa, P. Marina, A. Hernandoa, J. Magn.
Magn. Mater. 312 (2007) 99.
[
4] K. Nielsch, R.B. Wehrspohn, J. Barthel, J. Kirschner, U. G o¨ sele, S.F. Fischer, H.
Kronm u¨ ller, Appl. Phys. Lett. 79 (2001) 1360.
[5] J. Zhang, W.X. Li, G.A. Jones, T.H. Shen, J. Appl. Phys. 99 (2006) 8Q502.
ꢀ
1.35 V, Zn atoms will be the main component of the nanowires,
as demonstrated by XRD. HcJ then decreases.
[6] H.N. Hu, H.Y. Chen, J.L. Chen, G.H. Wu, Physica B 368 (2005) 100.
[7] S.W. Lin, S.C. Chang, R.S. Liu, S.F. Hu, N.T. Jan, J. Magn. Magn. Mater. 282 (2004)
28.
On the other hand, Hc? decreases monotonically with increas-
ing deposition potential. As with the anisotropy, the coercivity
differences in different nanowires are thought to be caused by
factors such as competition between shape anisotropy and
magnetocrystalline anisotropy, the crystalline structure and local
morphological changes in the nanowires.
[
8] G.W. Meng, A.Y. Cao, J.Y. Cheng, A. Vijayaraghavan, Y.J. Jung, M. Shima, P.M.
Ajayan, J. Appl. Phys. 97 (2005) 064303.
[9] V. Badri, A.M. Hermann, Int. J. Hydr. Energy 25 (2000) 249.
10] X.W. Wang, G.T. Fei, K. Zheng, Z. Jin, L.D. Zhang, Appl. Phys. Lett. 88 (2006)
[
[
173114.
11] H. Masuda, K. Fukuda, Science 268 (1995) 1466.
[12] Y.J. Chen, B. Chi, H.Z. Zhang, H. Chen, Y. Chen, Mater. Lett. 61 (2007) 144.
13] O. Jessensky, F. Muller, U. Gosele, Appl. Phys. Lett. 72 (1998) 1173.
14] J.B. Shi, Y.C. Chen, C.W. Lee, Y.T. Lin, C. Wu, C.J. Chen, Mater. Lett. 62 (2008) 15.
15] N.B. Chaure, J.M.D. Coey, J. Magn. Magn. Mater. 303 (2006) 232.
[
[
[
The saturation magnetization of samples prepared at ꢀ0.9 and
3
ꢀ
1.12 V is almost the same; the former is 12.01 emu/cm and the
3
latter is 11.96 emu/cm , which is consistent with the result that
[16] W. Luo, Q.P. Deng, B.Q. Liu, X.P. Zhao, J. Funct. Mater. 39 (2008) 1011.