Magnetic behavior of NixFe(100-x ) (65≤x≤100) nanowire arrays

Article abstract of DOI:10.1016/j.jmmm.2004.11.179

Arrays of magnetic nanowires with composition Ni_xFe _(100-x) (65≤x≤100), 35 nm in diameter and 2000 nm long have been prepared by electrodeposition filling of highly ordered nanoporous alumina membranes. Long-range order of nanopores, with crystalline domain size of around 2 μm² and 105 nm lattice parameter of hexagonal symmetry, is achieved by self-ordering process, and characterized by SEM and AFM. Magnetic behavior of the arrays has been determined by VSM. Maximum coercivity of around 1.23 kOe and reduced remanence (about 0.8 saturation magnetization) is observed for x=77, while minimum values are observed for x=100. Detailed AFM and MFM studies allow us to gain additional information of the filling degree of pores which can result in a distributed nanowires length that finally correlates with a deterioration of macroscopic magnetic behavior of the array.

Full text of DOI:10.1016/j.jmmm.2004.11.179

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 191–194
www.elsevier.com/locate/jmmm
Magnetic behavior of Ni Fe
x
(65pxp100) nanowire
(100ꢀx)
arrays
a,b,
Ã
a
a
, A. Asenjo , M. Jaafar , K.R. Pirota , M. Herna
a
a
lez ,
D. Navas
ndez-Ve
´ ´
a
zquez
a
R. Sanz , W. Lee , K. Nielsch , F. Batalla
b
b
a
n , M. Va
´
´
a
Instituto de Ciencia de Materiales, CSIC, 28049 Madrid, Spain
b
Max-Planck-Institut f u¨ r Mikrostrukturphysik, 06120 Halle, Germany
Available online 7 December 2004
Abstract
Arrays of magnetic nanowires with composition Ni
have been prepared by electrodeposition filling of highly ordered nanoporous alumina membranes. Long-range order of
x
Fe(100ꢀx) (65pxp100), 35 nm in diameter and 2000 nm long
2
nanopores, with crystalline domain size of around 2 mm and 105 nm lattice parameter of hexagonal symmetry, is
achieved by self-ordering process, and characterized by SEM and AFM. Magnetic behavior of the arrays has been
determined by VSM. Maximum coercivity of around 1.23 kOe and reduced remanence (about 0.8 saturation
magnetization) is observed for x ¼ 77; while minimum values are observed for x ¼ 100: Detailed AFM and MFM
studies allow us to gain additional information of the filling degree of pores which can result in a distributed nanowires
length that finally correlates with a deterioration of macroscopic magnetic behavior of the array.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.75.+a; 81.16.Dn; 75.50.Cc
Keywords: Nanowires; Nanoporous membrane; Coercivity; Magnetic force microscope
1
. Introduction
sophisticated and cheaper techniques are also used
involving templates with a given ordering thus allowing
the preparation of arrays of magnetic nanowires [6,7].
Particularly, nanoporous alumina membranes are prob-
ably one of the best examples of such templates where
magnetic nanowires can be prepared by filling the pores
by an electroplating technique [8]. Arrays with high-
order hexagonal symmetry can be then produced whose
diameter can be controlled between 20 and 200 nm and
the lattice parameter in the range from 65 to 500 nm.
Previous studies have mainly focused on pure magnetic
elements as Fe, Ni and Co [9,10].
During the past years, a huge effort is being made on
the development of arrays of highly ordered nanostruc-
tures due to the potential applications in a wide range of
areas as semiconductors, magneto-optics, biomedical
applications, and various sensor devices or magnetic
storing [1–3]. A number of methods are employed to
fabricate nanoscale magnetic structures, for example,
electron-beam, X-ray or imprint lithography [4,5]. Less
Ã
Corresponding author. Instituto de Ciencia de Materiales,
In recent works, the influence of the ordering degree
of the arrays [11,12], and of the ratio nanowire diameter
to interpore distance [13] has been reported. The
CSIC, 28049 Madrid, Spain. Tel.: +34 91 334 9000;
fax: +34 91 372 0623.
E-mail address: danavas@icmm.csic.es (D. Navas).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.11.179

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D. Navas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 191–194
192
objective of the present work has been to study the
magnetic behavior of a series of highly ordered NiFe
magnetic nanowire arrays with different compositions,
and check the influence of geometrical and pore filling
degree irregularities.
2
. Experimental techniques
Highly ordered Ni Fe
x
alloy nanowire arrays
(100ꢀx)
have been produced by electroplating and filling
the nanopores of alumina membranes. Nanoporous
membranes with long-range hexagonal ordering have
been prepared by a two-step anodization process
[
14,15]. Nanopores self-order during the first anodiza-
tion process of high-purity (99.999%) aluminum foil
Goodfellow). Parameters of the first anodization step
(
determine the characteristics of the array as pore
diameter, interpore distance and ordering degree of
pores, while second anodization determines the length
of the nanopores. NiFe nanowires are finally electro-
deposited inside the nanopores using a 300 g/l of
NiSO Á 6H O, 45 g/l NiCl Á 6H O and 45 g/l H BO
4
2
2
2
3
3
plus different amounts of FeSO
Alloy composition of Ni
between x ¼ 65 and 100. Diameter of nanowires is
5 nm, the interpore distance or hexagonal lattice
parameter is 105 nm, and the length of nanowires is
4
Á 6H
2
O as solution.
x
Fe(100ꢀx) nanowires ranges
3
Fig. 1. SEM images of the (a) surface and (b) cross-section of a
nanoporous alumina membrane.
2
000 nm.
The structure of the nanoporous membranes has been
determined by scanning electron microscope, SEM, and
atomic force microscopy, AFM, while the chemical
compositions have been analyzed with X-ray fluores-
cence analysis, XRF. Magnetic measurements of the
arrays as a whole were performed in a vibrating sample
magnetometer, VSM, whereas the magnetic state of
individual nanowires was determined by magnetic force
microscopy, MFM.
1.5
H parallel
H perpendicular
1.0
0.5
0.0
-0.5
3
. Experimental results and discussion
-1.0
Ni73Fe27
The quality of the geometrical characteristics of
nanoporous alumina membranes, templates for the
nanowire arrays, can be observed in Fig. 1.
Long-range ordering together with the existence
-1.5
-5000
-2500
0
2500
5000
H (Oe)
2
of polycrystalline domains, reaching around 2 mm , is
Fig. 2. Hysteresis loops of a Ni73Fe27 filled alumina membrane
measured with applied field parallel (’) and perpendicular (J)
to the nanowires axis.
observed in Fig. 1a. Fig. 1b shows an SEM image
of a membrane cross-section where the parallelism
between nanochannels can be confirmed as well as its
perpendicular orientation with regards to the membrane
plane.
array of nanowires with composition Ni73Fe27 (similar
results are obtained for the rest of the compositions). As
observed, perpendicular loop is nearly non-hysteretic
while the parallel loop exhibits large remanence to
magnetic saturation ratio indicating the existence of an
The effective magnetic anisotropy of the arrays can be
deduced from the comparison between hysteresis loops
with applied field parallel and perpendicular to the wire
axis. Fig. 2 shows the hysteresis loops for a particular

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D. Navas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 191–194
193
1.2
0.8
0.4
0.0
-0.4
-0.8
-1.2
-3000 -2000 -1000
0
1000 2000 3000
H (Oe)
Fig. 3. Axial hysteresis loops of NiFe nanowires arrays with
different compositions.
4
10nm
effective magnetic anisotropy with easy axis parallel to
the nanowires.
70
6
5
0
0
Fig.
3
shows the compositional dependence of
40
3
2
1
0
0
0
0
hysteresis loops corresponding to the magnetization
process parallel to the nanowire axis. For a straightfor-
ward comparison, saturation magnetization for all the
samples has been normalized.
0
0.5
1
1.5
X[µm]
As observed, although there is no dramatic composi-
tional dependence, the optimized behavior with the
larger remanence to saturation ratio, Mr=Ms ¼ 0:79;
and coercivity, Hc ¼ 1232 Oe; is observed for the
nanowire composition with x ¼ 77: It is to be particu-
larly noticed that the increase of coercivity of the NiFe
nanowire arrays in comparison with the Ni nanowires
array exhibit a reduced coercivity of Hc ¼ 780 Oe:
The reduced values of coercivity and particularly the
remanence observed for the Ni69Fe31 array should be
also mentioned. Geometrical characteristics of this array
have been analyzed in more detail in order to determine
its anomalous magnetic behavior. The AFM images in
Fig. 4 reveal the existence of filled and empty pores in
the Ni69Fe31 array. From the analysis of the AFM
profiles performed in different images corresponding to
a variety of nanowires composition, we can conclude
that the length of the nanowires is similar in all the
samples except in the case of x ¼ 69 where a length
distribution is observed. This irregular nanoporous
filling distribution is thus confirmed to significantly
affect the magnetic behavior of an array.
Fig. 4. The AFM image of Ni69Fe31 array of nanowires, and
filling profile along an indicated orientation.
4
00nm
The MFM image in Fig. 5 corresponding to the same
array shows black and dark contrast corresponding to
the wires with the magnetic moment oriented up and
down. In addition, intermediate contrast is found at the
positions of empty pores.
Fig. 5. An MFM image of Ni69Fe31 array of nanowires.
characteristics of Ni
arrays except for an optimization with regards to pure
Ni arrays. However, a significant deteriorated magnetic
x
Fe(100ꢀx) ð65pxo100Þ nanowire
In conclusion, it is firstly shown that there is no
significant compositional dependence of magnetic

ARTICLE IN PRESS
D. Navas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 191–194
194
behavior has been found for the Ni69Fe31 nanowire
array. The observed variety of nanowire lengths is
correlated to the reduced permeability, coercivity and
remanence observed for that array which are tentatively
ascribed to an increased magnetostatic coupling among
the nanowires.
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