Magnetic properties of nickel nanostructures grown in AAO membrane

Article abstract of DOI:10.1016/j.jmmm.2006.10.821

One-dimensional nanostructures can be built by performing chemical or electrochemical reactions in the pores of a suitable host or matrix material. We have developed a method of electrodeposition of nickel nanostructures inside cylindrical pores of the anodic aluminum oxide (AAO) membranes, which provides precise control of the nanostructure height. We were able to fabricate hexagonal arrays of particles in the form of spheres, rods and long wires. Magnetization measurements of these nanostructures as function of field and temperature were carried out using a superconducting quantum-interference device magnetometer. The shape of nickel nanostructures has been investigated by field emission scanning electron microscope. The coercivity of the nickel nanostructures measured with the field perpendicular to the membrane was increasing with increasing aspect ratio of the nanostructures. These experimental values of the coercivity, varying from 200 Oe for the spherical nanodots to 730 Oe for the nanowires, are in a fair agreement with our micromagnetic modeling calculations.

Full text of DOI:10.1016/j.jmmm.2006.10.821

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e827–e829
www.elsevier.com/locate/jmmm
Magnetic properties of nickel nanostructures grown in AAO membrane
Ã
S.-L. Oha, , Y.-R. Kima, L. Malkinskib, A. Vovkb, S.L. Whittenburgb,
Ã
E.-M. Kimc, J.-S. Jungd,
aDepartment of Chemistry, Yonsei University, Seoul, South Korea
bAdvanced Material Research Institute, University of New Orleans, New Orleans, LA 70148, USA
cKorea Basic Science Institute, Kangnung 210-702, South Korea
dDepartment of Chemistry, Kangnung National University, Kangnung 210-702, South Korea
Available online 16 November 2006
Abstract
One-dimensional nanostructures can be built by performing chemical or electrochemical reactions in the pores of a suitable host or
matrix material. We have developed a method of electrodeposition of nickel nanostructures inside cylindrical pores of the anodic
aluminum oxide (AAO) membranes, which provides precise control of the nanostructure height. We were able to fabricate hexagonal
arrays of particles in the form of spheres, rods and long wires. Magnetization measurements of these nanostructures as function of field
and temperature were carried out using a superconducting quantum-interference device magnetometer. The shape of nickel
nanostructures has been investigated by field emission scanning electron microscope. The coercivity of the nickel nanostructures
measured with the field perpendicular to the membrane was increasing with increasing aspect ratio of the nanostructures. These
experimental values of the coercivity, varying from 200 Oe for the spherical nanodots to 730 Oe for the nanowires, are in a fair agreement
with our micromagnetic modeling calculations.
r 2006 Elsevier B.V. All rights reserved.
PACS: 75.50.K; 81.05.Y
Keywords: Nickel nanodot; Nickel nanorod; Nickel nanowire; Magnetic property
Magnetic properties of magnetic nanoparticles depend
on many factors such as size, shape, surface roughness and
particle distribution. The particles remain single domains
below certain critical size. Larger particles are multi
domain and domain-wall movements contribute to the
magnetization process. Magnetostatic energy of the parti-
cles is controlled by the shape and roughness of the
particles. The effect of shape anisotropy on the magnetic
hysteresis loops is the most pronounced for the magneti-
cally soft particles, such as nickel or permalloy than for
hard nanomagnets where magneto-crystalline anisotropy
prevails. The magnetostatic interparticle interactions
depend on the particle arrangement in the array and they
significantly modify magnetization processes for small
interparticle distances.
In the past decades, the synthesis of nanosized mag
netic particles has extensively been studied because of
their potential applications in information storage and
biomedicine.
For example arrays of soft magnetic wires were proposed
to serve as a planar separator of bio-active particles [1].
Several methods, such as wet-chemical etching, UV
lithography or electrodeposition, have been developed to
fabricate arrays of nanoparticles [2,3] Well-defined inor-
ganic molecular sieves as porous host can be used to
synthesize one-dimensional nanostructured magnetic
material. The problem with the conventional wet-chemical
preparation methods leads to a rather broad distribution in
size and shape over a large area.
Ã
Corresponding authors. Tel.: +82 2 2123 2646.
E-mail addresses: chem1121@yonsei.ac.kr (S.-L. Oh),
jjscm@kangnung.ac.kr (J.-S. Jung).
Because of large number of parameters affecting
magnetization processes and different technologies used
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.821

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e828
to produce arrays of nanoparticles it is difficult to conclude
about the effect of the aspect ratio of the particles on their
magnetic properties based on literature data. Some earlier
studies include effect of a diameter of the nanowires [2] and
properties of nanolithograpically patterned pillars [3].
In this paper, we describe a technology that allows
fabrication of hexagonal arrays of nanostructures with
fixed diameter and well-controlled height. Arrays of nickel
nanostructures in the form of spheres, rods and long wires
were produced by electrodeposition of nickel in AAO. The
magnetization hysteresis loops and accompanying mag-
netic parameters evolve with the changing aspect ratio of
the nanoparticles, which form the arrays.
Hexagonally ordered porous AAO was formed by
anodization process as described previously [4]. The nickel
nanostructures grow from the bottom of the holes, so the
diameter of the nanostructure corresponds to the diameter
of the template hole. In Fig. 1, the diameter of nickel
nanostructures is about 100 nm. Nickel was deposited in
pores by DC electrolysis in an electrolyte of the following
composition: NiCl2 Á 6H2O.
1.0
0.5
0.0
-0.5
1.0
0.5
0.0
-0.5
1.0
0.5
0.0
-0.5
-1.0
a
b
c
Ni(H2NSO3) Á 4H2O, boric acid and ascorbic acid. The
pH value of the electrolyte was maintained at 3.5 and then
the electrolysis was conducted at 20 1C, using DC current of
3–4 mA and silver counterelectrodes. By simply controlling
the deposition time in the range from 5 to 15 min a variety
of nickel nanostructures were fabricated as seen in Fig. 1.
They had the forms of nearly spherical nanodots with the
diameter of 100 and 400 nm rods and 3.4 mm long wires.
The magnetization hysteresis loops were measured using
SQUID magnetometer with the field in the substrate plane
(transverse to the wires) and perpendicular to the
membrane, i.e. along wires or rods. The experimental
results, presented in Fig. 2, show the evolution of the
hysteresis loops with the increasing aspect ratio of the
length–to-diameter of the particles: 1:1, 4:1 and 34:1.
The most significant changes of the hysteresis loops have
been observed for the field applied perpendicular to the
substrate. Coercivities measured at 5 K for this configura-
tion of the field were 200, 630 and 730 Oe, respectively, for
spheres, rods and wires. Also the squareness of the loop
was increasing with the increasing aspect ratio. In case of
measurements with the in-plane field the shapes of the
hysteresis loops were similar and the largest coercivity of
200 Oe was found for the spherical nanodots.
-4k
-2k
0
2k
4k
H, Oe
Fig. 2. Hysteresis loops for nickel: (a) nanodots, (b) nanorods and
(c) nanowires measured at 5 K with the field applied parallel (-’-) and
J
perpendicular (- -) to the membrane plane.
The evolution of the hysteresis loops can be qualitatively
understood in terms of Stoner–Wohlfarth model for
magnetization rotations in the particles with uniaxial shape
anisotropy. However, the quantitative analysis of the
magnetization processes is far more complex in the case
of nanoparticles, as indicated by micromagnetic calcula-
tions [3,5,6]. For example, magnetization reversal occurs
through incoherent magnetization rotation, such as an
evolution of a vortex state rather than magnetization
rotation or domain-wall nucleation and domain move-
ment, which are commonly recognized mechanisms in bulk
samples. For these reasons the experimental coercivity
values were significantly lower than those expected from
the Stoner–Wohlfarth model. Therefore, we used
micromagnetic modeling code JAMM, developed by
S. Whittenburg [7], to interpret experimental hysteresis
loops. The results of the micromagnetic calculations for
isolated particles with different aspect ratios reproduced
the evolution of the shapes of the hysteresis loops and gave
reasonable agreement with the experimental values of the
coercivities. However, it is worthy to mention that
the measured coercivities were about 10% lower than the
calculated ones and squareness of the loops was larger for
theoretical curves. In addition to the aspect ratio, other
Fig. 1. The FE-SEM images of nickel: (a) nanodots, (b) nanorods and
(c) nanowires after removal of the AAO templates. The diameter, length
and the distance between the pores of the AAO template are 100 nm,
40 mm, and 210 nm, respectively.

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e829
important factors, such as interparticle interactions [8],
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This work was supported through the grant from
MOCIE(RT105-01-02, Myongji RRC), NRL(M1-0300-00-
0162), DARPA(HR001-04-1-0029) and NSF (CCF-
0403673).
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