Synthesis and magnetic properties of spinel CoFe2O4 nanowire arrays

Article abstract of DOI:10.1016/j.jmmm.2009.04.036

Spinel CoFe₂O₄ nanowire arrays were synthesized in nanopores of anodic aluminum oxide (AAO) template using aqueous solution of cobalt and iron nitrates as precursor. The precursor was filled into the nanopores by vacuum impregnation.

Full text of DOI:10.1016/j.jmmm.2009.04.036

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Journal of Magnetism and Magnetic Materials 321 (2009) 2795–2798
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Synthesis and magnetic properties of spinel CoFe O nanowire arrays
2
4
a
a
a
a
b
a,
Ã
J.J. Yuan , Q. Zhao , Y.S. Xu , Z.G. Liu , X.B. Du , G.H. Wen
a
National laboratory of superhard materials, Jilin University, Changchun 130012, PR China
b
College of physics, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
2 4
Spinel CoFe O nanowire arrays were synthesized in nanopores of anodic aluminum oxide (AAO)
Received 11 December 2008
Received in revised form
template using aqueous solution of cobalt and iron nitrates as precursor. The precursor was filled into
2 4
the nanopores by vacuum impregnation. After heat treatment, it transformed to spinel CoFe O
27 March 2009
nanowires. The structure, morphology and magnetic properties of the sample were studied by X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and
vibrating sample magnetometer (VSM). The results indicate that the nanowire arrays are compact. And
Available online 21 April 2009
PACS:
7
7
7
5.50.Gg
5.60.Ej
5.75.+a
the individual nanowires have a high aspect ratio, which are about 80 nm in diameter and 10 mm in
length. The nanowires are polycrystalline spinel phase. Magnetic measurements indicate that the
nanowire arrays are nearly magnetic isotropic. The reason is briefly discussed. Moreover, the
temperature dependence of the coercive force of the nanowire arrays was studied.
Keywords:
Nanowire
Spinel
&
2009 Elsevier B.V. All rights reserved.
2 4
CoFe O
AAO
Vacuum impregnation
1
.
Introduction
to synthesize spinel CoFe
cated than electrodeposition, several ways have been introduced
to prepare spinel CoFe nanowire arrays in AAO, such as sol–gel
method [8], oxidation of CoFe alloy nanowire arrays [9] and using
spinel CoFe nanoparticles as building blocks [10]. Among them
the sol–gel method have proved to be powerful to prepare highly
stoichiometric spinel CoFe nanowires. But there are still some
limitations. First it is difficult to pour the viscous sol into the
nanopores of AAO, it often needs a long time vigorous stirring.
And the metal cations concentration in sol is low, while the
organic matters are the major content in sol. After the sol within
the nanopores changes into gel and finally burns itself out, the left
2 4
O nanowire arrays are more compli-
Recently, much effort has been made to synthesize and study
2 4
O
magnetic nanowire arrays, because of their unique physical
properties and potential applications in high-density magnetic
recording [1,2]. Among the various methods for preparing
nanowire arrays, template method was found to be effective for
preparing uniform and ordered magnetic nanowire arrays. Anodic
aluminum oxide (AAO) template was a most attractive template. It
has controllable pore diameter, narrow pore size distribution and
good chemical and thermal stability. Many magnetic metal or
alloy nanowire arrays have been prepared by electrodeposition in
AAO [3,4].
2
2 4
O
2 4
O
metal cations can hardly form long and compact spinel CoFe
nanowire arrays [8].
2 4
O
Magnetic metal oxides nanowire arrays are also of great
interest. Particularly, the study on the synthesis and magnetic
2 4
In this paper, spinel CoFe O nanowire arrays were synthesized
properties of spinel CoFe nanowire arrays has attracted much
2 4
attention. Spinel CoFe O is an important magnetic metal oxide. It
has large magnetocrystalline anisotropy, moderate saturation
magnetization, remarkable chemical stability and mechanical
hardness, which fit the requirements of a good magnetic recording
media [5–7]. But compared with the magnetic metal or alloy
2
O
4
in AAO by simply filling the nanopores with aqueous solution of
cobalt and iron nitrates and subsequent heat treatment. The
formed nanowire arrays are long and compact. The structure,
morphology and magnetic properties of the spinel CoFe
nanowire arrays were also studied.
2 4
O
2 4
nanowire arrays, the study on the preparation of spinel CoFe O
2
.
Experiment
Anodic aluminum oxide template was prepared by two-step
nanowire arrays in AAO is relatively less. Because the procedures
Ã
anodization processes as described previously [11]. Briefly, the
starting material, 99.99% pure Al foil, was first degreased in
Corresponding author. Tel.: +86 43185168211; fax: +86 43185168881.
E-mail addresses: phghwen@yahoo.com.cn, wengh@jlu.edu.cn (G.H. Wen).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.04.036

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acetone. Then it was annealed at 500 1C for 4 h under N
2
protection. The annealed Al foil was electropolished in a mixed
solution containing 100 ml perchloric acid and 400 ml ethanol at
0
1C. In the first anodization process, electropolished Al foil was
anodized in 0.3 M oxalic solution at 40 V and 0 1C for about 8 h.
Then the formed aluminum oxide layer was removed in an
etching solution composed of 0.2 M chromic acid and 0.4 M
phosphoric acid at 60 1C. Subsequently, the second anodization
process was performed under the same condition with the first
anodization process. In order to widen the pore, the AAO was
immersed in 0.3 M phosphoric acid for 30 min at 30 1C.
Then the nanopores in AAO were filled with aqueous solution
of cobalt and iron nitrates by vacuum impregnation. Firstly
3 2 3 3
Co(NO ) and Fe(NO ) with a molar ratio of 1:2 were dissolved
in distilled water to form a nearly saturated solution. And AAO
was put into a test tube with an amount of the solution. Then the
test tube was evacuated to perform vacuum impregnation. When
the air in nanopores was evacuated, the solution can easily fill the
pores. The AAO filled with the solution was taken out and dried at
8
0 1C overnight. Subsequently the AAO was annealed at 600 1C for
1
0 h in air to form spinel CoFe nanowires. The temperature was
2 4
O
2 4
Fig. 2. X-ray diffraction pattern of spinel CoFe O nanowire arrays.
ramped from room temperature to 600 1C with a rate of 1 1C/min.
The structure and morphology of the as-prepared samples
were studied by X-ray diffraction (XRD), scanning electron
microscopy and transmission electron microscopy (TEM). The
composition of the samples was analyzed by energy-dispersive
X-ray spectroscopy (EDX). For SEM observation, the upper part of
the AAO was dissolved in the etching solution to expose the spinel
CoFe
was totally dissolved and spinel CoFe
collected by a carbon-coated Cu TEM grid. The magnetic proper-
ties of spinel CoFe nanowire arrays were measured by
vibrating sample magnetometer at room temperature.
2
O
4
nanowire arrays. While for the TEM observation, the AAO
2 4
O
nanowires were
2 4
O
3.
Results and discussion
The SEM image of the AAO template is shown in Fig. 1. The AAO
template has an average pore diameter of about 80 nm. And the
nanopores are nearly uniform and hexagonally arranged.
2 4
The XRD pattern of the spinel CoFe O nanowire arrays within
the AAO template is shown in Fig. 2. Except for the amorphous
background of AAO template, only the diffraction peaks of spinel
Fig. 3. SEM image of the spinel CoFe O nanowire arrays.
2
4
CoFe
spinel CoFe
2
O
4
can be observed. All peaks in Fig. 2 can be indexed to
phase (JCPDS No. 22-1086). The XRD pattern
2 4
O
2 4
Fig. 4. TEM image of a single spinel CoFe O nanowire and corresponding electron
Fig. 1. SEM image of AAO template with an average pore diameter of about 80 nm.
diffraction pattern.

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2797
convinces that the nanowire arrays are polycrystalline spinel
CoFe phase. The average grain size in CoFe nanowires is
of the EDX analysis, this means that the nanowires are
stoichiometric. The synthesized spinel CoFe nanowire arrays
2
O
4
2
O
4
2 4
O
calculated to be about 11 nm using the Scherrer equation from the
strongest (311) reflection.
are obviously longer and compacter than that prepared by the
sol–gel method. This should be due to that the content of cobalt
and iron cations in the solution are much higher than that in the
sol, and the solution can enter the nanopores more easily.
2 4
Fig. 3 shows the SEM image of the spinel CoFe O nanowire
arrays after the AAO template has been almost dissolved away.
The nanowire arrays are compact, showing a high filling rate of
nanopores in the AAO template. The diameter of the nanowires is
about 80 nm, which closely fits the nanopore size of the AAO
2 4
TEM image of a single spinel CoFe O nanowire is shown in
Fig. 4. It can be seen that the nanowire consists of a lot of nano
grains, which is in consist with the result of XRD. The
corresponding electron diffraction pattern is shown in the inset
template, and the length is over 10
mm. Thus, the aspect ratio of
the nanowires is over 125. The composition of the spinel CoFe
nanowire arrays was analyzed by the EDX. The atom ratio of the Fe
and Co in the nanowire arrays is exactly 2:1 within the error range
2
O
4
of Fig. 4, all the diffraction rings can be indexed to spinel CoFe
phase. This further indicates that the nanowire is polycrystalline
spinel CoFe phase.
2 4
Fig. 5 shows the typical hysteresis loops of the CoFe O
2 4
O
2 4
O
nanowire arrays measured at room temperature. The external
magnetic field was applied parallel (parallel geometry) or
perpendicular to the nanowire arrays (perpendicular geometry),
respectively. The two loops do not show clear difference. This
2 4
means that the CoFe O nanowire arrays are nearly magnetic
isotropic. This is different from magnetic metal nanowire arrays,
such as Fe nanowire arrays, which show uniaxial anisotropy with
easy axis along the wire [1]. The total magnetic anisotropy of
magnetic nanowire arrays is mainly determined by shape aniso-
tropy and magnetocrystalline anisotropy. The shape anisotropy
constant can be calculated using the equation K ¼ (1/2)
2
m
0
(N
and N
axes of prolate ellipsoid. And M
the nanowires. The magnetocrystalline anisotropy constant K
a
ꢀN
c
)M
s
, by treating the nanowires as prolate ellipsoids. N
a
c
are demagnetization factors along the minor and major
is the saturation magnetization of
of
s
1
5
3
bulk Fe is 4.8 ꢁ 10 ergs/cm . And the shape anisotropy constant of
6
3
Fe nanowire arrays is calculated to be 9 ꢁ10 ergs/cm . This
determines that Fe nanowire arrays are uniaxial anisotropic.
While for the spinel CoFe
anisotropy constant comes out to be 5 ꢁ10 ergs/cm , and the
magnetocrystalline anisotropy constant K of bulk spinel CoFe
2 4
O nanowire arrays, the shape
5
3
1
2 4
O
2 4
Fig. 5. The hysteresis loops of spinel CoFe O nanowire arrays measured at room
6
3
is 2.7 ꢁ10 ergs/cm . Because the magnetocrystalline anisotropy
temperature with the applied field parallel or perpendicular to the nanowires,
respectively.
is much larger than the shape anisotropy, so the total magnetic
1
50K
0
0
0
.002
.001
.000
200K
293K
6
4
2
000
000
000
0
600K
-
-
0.001
0.002
-
20000 -10000
0
10000
20000
H (Oe)
H // nanowires
H ⊥ nanowires
100
200
300
400
500
600
T (K)
2 4
Fig. 6. Temperature dependence of the coercive force of the CoFe O nanowire arrays. Inset: hysteresis loops measured in parallel geometry at different temperature.

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J.J. Yuan et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2795–2798
anisotropy of the spinel CoFe
contributed by magnetocrystalline anisotropy. As shown in the
TEM images, the spinel CoFe nanowires are composed of a lot
of nano grains that are randomly oriented, so the CoFe
nanowire arrays do not show preferential magnetic orientation.
Hysteresis loops of the CoFe nanowire arrays were also
measured at other temperatures. At each temperature, the
hysteresis loops measured in parallel and perpendicular geometry
are similar. This consists with the result at room temperature, i.e.,
2
O
4
nanowire arrays is mainly
The formed spinel CoFe
And the CoFe nanowires fill in the nanopores of AAO nearly
uniformly. The CoFe nanowires have an average diameter of
about 80 nm and a length over 10 m. Transmission electron
microscopy and X-ray diffraction indicate that CoFe nanowires
are polycrystalline, composed of a lot of CoFe nano grains of
about 11 nm. The VSM measurements show that the CoFe
nanowires array is nearly magnetic isotropy. And the coercive
2 4
O nanowire arrays are long and compact.
2 4
O
2
O
4
2 4
O
2
O
4
m
2 4
O
2
O
4
2 4
O
2 4
O
force of the CoFe
temperature.
2 4
O nanowire arrays strongly depends on the
the magnetic anisotropy of the spinel CoFe
mainly determined by magnetocrystalline anisotropy. Fig. 6 shows
the temperature dependence of the coercive forces of the CoFe
2 4
O nanowire arrays is
2 4
O
nanowire arrays measured in parallel and perpendicular
geometry. And the inset in Fig. 6 shows several representative
hysteresis loops measured in parallel geometry. The coercive force
Acknowledgement
of the CoFe
2
O
4
nanowire arrays is about 6200 Oe at 150 K, and it
This work was supported by the National Natural Science
Foundation of China (No. 50402003).
decreases rapidly to about 1100 Oe at room temperature with
increasing temperature. Above room temperature, it slowly
decreases to zero at 600 K. This means that the CoFe
2
O
4
References
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decrease of coercive field with increasing temperature should be
due to thermal fluctuations.
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2 4
In summary, spinel CoFe O nanowire arrays were successfully
synthesized in nanopores of AAO, using aqueous solution of cobalt
and iron nitrates as precursor. This method is simple and effective.
[
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