Dual behaviors of magnetic CoxFe1-x (0≤x≤1) nanowires embedded in nanoporous with different diameters

Article abstract of DOI:10.1016/j.jmmm.2012.05.036

Co_xFe_1-x nanowire arrays with various diameters and different composition were fabricated by ac electrodeposition using porous alumina template. Coercivity along the easy axis reaches to a maximum at 2330 Oe, for Co_xFe_1-x nanowires containing about 40 at% Co. The crystalline structure of the nanowires was concentration-independent and shows a bcc structure. The critical diameter for transition from coherent rotation to curling mode is 35 nm for CoFe containing less than 40 at% Co while it is 30 nm for those with more than 40 at% Co. Optimizing the magnetic properties of CoFe with different Co content was seen to be dependent on the diameter of nanowires. For 25 nm diameter, the optimum was shown below 50 at% Co while it was seen above 50 at% for nanowires with 50 nm diameter. The angular dependence of the coercivity with nanowires diameter were also studied.

Full text of DOI:10.1016/j.jmmm.2012.05.036

Journal of Magnetism and Magnetic Materials 324 (2012) 3193–3198
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Dual behaviors of magnetic CoxFe1ꢀx (0rxr1) nanowires embedded
in nanoporous with different diameters
A. Ramazani a,b,n, M. Almasi Kashi a,b, S. Ghanbari a, F. Eshaghi a
a Department of Physics, University of Kashan, Kashan, Iran
b Institute of Nanoscience & Nanotechnology, University of Kashan, Kashan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
CoxFe1ꢀx nanowire arrays with various diameters and different composition were fabricated by ac
electrodeposition using porous alumina template. Coercivity along the easy axis reaches to a maximum
at 2330 Oe, for CoxFe1ꢀx nanowires containing about 40 at% Co. The crystalline structure of the
nanowires was concentration-independent and shows a bcc structure. The critical diameter for
transition from coherent rotation to curling mode is 35 nm for CoFe containing less than 40 at% Co
while it is 30 nm for those with more than 40 at% Co. Optimizing the magnetic properties of CoFe with
different Co content was seen to be dependent on the diameter of nanowires. For 25 nm diameter, the
optimum was shown below 50 at% Co while it was seen above 50 at% for nanowires with 50 nm
diameter. The angular dependence of the coercivity with nanowires diameter were also studied.
& 2012 Elsevier B.V. All rights reserved.
Received 21 January 2012
Received in revised form
23 April 2012
Available online 29 May 2012
Keywords:
Magnetic model
AC electrodeposition
Nanowire
1. Introduction
which is more suitable for high temperature applications. Since
the magnetocrystalline anisotropy of CoFe nanowires is almost
Ferromagnetic nanowire arrays have been widely investigated
to understand their microstructures, magnetic properties and
their applications in microwave circulator [1] recording media [2],
magnetic nanodevices [3] and nanosensors [4]. To produce nanowire
arrays, advanced lithographic techniques [5], electroless procedures
[6] and electrodeposition techniques in nanoporous templates have
been developed [7–9]. Among these methods, the electrodeposition
in alumina templates which is a simple low-cost technique has been
widely used [10,11]. The advantage of the alumina templates over
other porous membranes is the possibility to fabricate these ordered
nanopores, using pre-patterning [12] or multiple anodization steps
[13,14]. Most of the studies have been focused on the fabrication
and characterization of magnetic materials such as Fe [15], Co [16]
and Ni [17]. Since the shape anisotropy is the dominant mechanism
in determining the magnetic properties, high coercivities and large
squareness have been obtained in these types of nanostructures.
Recently attention has been shifted towards preparing and
characterizing of alloy nanowire arrays such as CoNi [18], FeCo
[19–22]. It is easy to change the coercivity and squareness of
recording media to reach the requirements of real device by
adjusting the components of alloy. CoFe-based alloys have received
much attention because of high saturation magnetization and low
magnetic crystalline anisotropy and elevated Curie temperature
negligible, the magnetic properties of these nanowires are mainly
predominated by shape anisotropy of the nanowires. It was reported
that alloy composition of CoFe nanowires has an effective role to
optimize the magnetic properties. In these reports, the magnetic
properties of CoFe nanowires with 20 and 50 nm diameters with
respect to the Co content was seen to be quite different [23,24].
In our previous work the effect of ac electrodeposition frequency
and post annealing on the nanowires with various compositions
embedded into the porous alumina with 30 nm pore diameter
was investigated [25].
In the present work CoxFe1ꢀx (0rxr1) nanowires embedded in
porous aluminum oxide with pore diameters ranging between 25 to
50 nm were fabricated. The microstructures and magnetic proper-
ties of nanowires were studied and the degree of conformity of the
coherent rotation and curling model on the magnetic behavior of
CoFe nanowires with different Co content was investigated.
2. Experiments
Highly ordered porous aluminum oxide (PAO) templates with
pore diameter of about 25 nm were prepared by two-step
anodizing process. The process was carried out in 0.3 M sulfuric
acid solution at a constant anodization voltage of 25 V for 5 h.
After dissolving the formed alumina, the patterned aluminum
substrate was re-anodized under the same conditions as the first
step for an anodization time of 30 min. Subsequently the pores of
the anodic alumina were widened by immersing the anodized
n Corresponding author at: Institute of Nanoscience & Nanotechnology, University
of Kashan, Kashan, Iran. Tel./fax: þ98 3615552935.
E-mail address: rmzn@kashanu.ac.ir (A. Ramazani).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmmm.2012.05.036

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A. Ramazani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3193–3198
templates in 6 wt% phosphoric acid at 30 1C for 4, 8, 16 and 20 min
yielding a series of porous alumina templates with different pore
diameters. Following the widening process, the voltage was sys-
tematically reduced to 8 V to promote thinning of the barrier layer.
The samples were then ac electrodeposited in electrolyte with
different concentrations of CoSO4 and FeSO4. A 45 g/L boric acid
was used as buffer. The electrolyte acidity with the NaHCO3
solution was adjusted to be at 5. The peak to peak deposition
voltage and frequency of the sine waveform were 30 V and 200 Hz,
respectively.
The crystal structure of the nanowire arrays was examined by
X-ray diffraction (XRD). Scanning electron microscopy (SEM) was
used to confirm the morphology of the samples. A vibrating
sample magnetometer (VSM) was used to measure the magnetic
properties of the samples at room temperature. Energy dispersive
X-ray spectroscopy (EDX) was used to determine the composition
of samples.
almost the same as that of Co2þ. As seen in Fig. 2, the atomic
percent of cobalt nanowires initially is slightly more than that of
Coþ2/(Coþ2þFeþ2) percentage in the electrolyte but it decreases
slightly for x more than 0.2. Based on these values, the component
of CoxFe1ꢀx alloy nanowires can be easily controlled by adjusting
the relative concentration of Co2þ and Fe2þ in electrolyte. These
results are consistent with those reported by Qin et al. [24].
The hysteresis loops for Co0.18Fe0.82 and Co0.43Fe0.57 in 25 nm
diameter are shown in Fig. 3(a)–(d), with the applied field parallel
and perpendicular to the wire’s axis. The hysteresis loop (parallel
to the wire’s axis) of the nanowires with 25 nm pore diameter is
evidently different from that of 50 nm (see Fig. 3a, c, e and g).
For nanowire arrays with 25 nm diameter, a square hysteresis
loop with high coercivity and large squareness is observable.
Increasing the pores diameter to 50 nm, the squareness and
coercivity become comparatively small. Reducing the Co content
(Co0.18Fe0.82 nanowire) causes a reduction in the coercivity and
squareness of the nanowire arrays. In fact the nanowires have
grown with axis perpendicular to the sample surface and the easy
axis loops was performed with applied field along to the wires
axis. On the other hand, since the samples had a circular shape it
may be said that the nanowire array is magnetically isotropic in a
plane perpendicular to the nanowires axis.
3. Results and discussions
Fig. 1 shows SEM micrographs of top surface of anodized
alumina templates after pore widening process. The interpore
distance was 65 nm and the pore diameters were 25, 30, 35, 45
and 50 nm for the samples widened at 0, 4, 8, 16 and 20 min,
respectively. Here, the electrodeposition velocity of Fe2þ was
Fig. 4 shows coercivity and squareness as a function of
Co content of the CoxFe1ꢀx nanowires with different diameters.
With an increase in x to 0.18, the obtained coercivity (with the
applied field parallel to nanowires axis) initially decreases, reaches
a minimum and then almost linearly increases with increasing x to
0.45–0.6 and then reduces and reaches to about 1000 Oe in the
case of pure Co. The minimum and maximum squareness of the
alloy CoFe (but not pure Co) nanowires with 25 nm diameter is
almost 0.9 and 1 while for 50 nm, it is 0.26–0.49, respectively.
However, average coercivity and squareness were about 300 Oe
and less than 0.1, respectively when the applied field was perpen-
dicular to the nanowires axis (not shown). As shown in these
figures, all CoxFe1ꢀx nanowires containing almost 18 at% Co exhibit
fewer coercivity. The maximum coercivity for the nanowires with
25 nm diameter was seen for the 43 at% Co (Hc¼2330 Oe) while it
was seen at 58 at% Co for the nanowires with 50 nm diameter
(Hc¼1660 Oe).
Crystalline structure of the samples was examined by X-ray
analysis. A D5000 X-ray diffractometer was used, which utilizes a
standard Cu tube source run at a voltage of 40 kV and filament
current of 40 mA. A Cu (K ) radiation with wavelength of
a
0.1540496 nm is produced by this system. All
y–2y
scans were
90
Experimental data
Linear data fitted
80
70
60
50
40
30
20
10
0.0
0.2
0.4
0.6
0.8
1.0
Co+2/(C0+2+Fe+2
)
Fig. 1. SEM micrographs of the top surface of anodized alumina templates after
the two-step anodization process, widened at (a) 0, (b) 4, (c) 8, (d) 16, and
(e) 20 min.
Fig. 2. Co content of CoFe nanowires as a function of Coþ2/(Coþ2þFeþ2) in the
electrolyte.

A. Ramazani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3193–3198
3195
1.2
0.8
Co0.18Fe0.82
Co0.18Fe0.82
0.4
0.0
D=25 nm
-0.4
-0.8
-1.2
1.2
Co0.43Fe0.57
Co0.43Fe0.57
0.8
0.4
0.0
D=25 nm
D= 25 nm
-0.4
-0.8
-1.2
1.2
Co0.18Fe0.82
Co0.18Fe0.82
0.8
0.4
0.0
D=50 nm
D=50 nm
-0.4
-0.8
-1.2
1.2
Co0.43Fe0.57
Co0.43Fe0.57
0.8
0.4
0.0
D=50 nm
D=50 nm
3000
-0.4
-0.8
-1.2
-6000 -4000 -2000
0
2000 4000 6000
-9000
-6000
-3000
0
6000
9000
Applied Field(Oe)
Applied Field(Oe)
Fig. 3. Hysteresis loops of CoFe nanowires with different Co content. The applied field was parallel to the wires axis in (a), (c), (e) and (g) and perpendicular
in (b), (d), (f) and (h).
made from incident beam angles of 351 to around 801 of the
surface with detector increments of 0.11 every 20 s. The incident
beam to the sample, the diffracted beam from the sample, and the
normal to the sample surface were coplanar.
In order to distinguish the peaks of nanowires, the remaining
pure aluminum metal was removed from behind the samples.
Sample was seated on an amorphous substrate, aluminum was
then removed from behind the samples, using a mixture of HCl and
saturated CuSO4. Considering the high X-ray activation volume of
alumina, it was expected that the whole nanowire length would be
exposed by the X-ray beam applied from behind the samples.
Fig. 5 shows the XRD patterns of the CoxFe1ꢀx nanowires with
various diameters in which x ranging from 0.5 to 0.9. The XRD
analysis revealed that they all are body-centered cubic structure

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A. Ramazani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3193–3198
2500
2000
1500
1000
500
1.4
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
D=40 nm
2000
D=25 nm
1.2
1500
1000
500
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Hc
Mr/Ms
0
0
2500
D = 45 nm
D=30 nm
1500
2000
1500
1000
500
1000
500
0.0
1.4
0
2500
D= 35 nm
D= 50 nm
1500
1000
500
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2000
1500
1000
500
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Co content (x)
Co content(x)
Fig. 4. Coercivity and squareness of CoFe nanowires with different diameters as a function Co content.
and the main diffraction peak can be assigned to be preferential
(110) CoFe. It may be said that nanowires diameter and cobalt
content have no effect on the crystalline structure of CoxFe1ꢀx
nanowires.
the demagnetization factor along the hard axis (Na and Nb), is
equal to 2p, while it is equal to 0 along the easy axis Nc. As shown
in Fig. 6 the length of the wires is almost 700 nm and the wires
diameter is ranging between 25 and 50 nm. The aspect ratio of
prepared nanowires then changes from 28 to 14. Since the
infinitely long cylinder approximation can be used for nanowires
having an aspect ratio greater than about 10, the shape anisotropy
has the same effect on the magnetic properties of our prepared
nanowires. The variations of magnetic properties then could be
due to magnetostatic interaction between the nanowires and/or
be because of appearance of incoherent effects in the nanowires
with larger diameter (transition from coherent to curling mode).
Coercivity of CoxFe1ꢀx nanowires with various diameters
shows dual behavior (see Fig. 4); they almost follow the coherent
rotation and curling models. The coercivity variation of the CoFe
nanowires containing more than 40 at% Co is negligible when the
diameter of the nanowires increases from 25 to 35 nm, while it is
considerable for those nanowires containing less than 40 at% Co.
It is seen that the coercivity of the CoFe nanowires with 43 at% Co,
changed from 2330 to 2280 Oe (almost 2 at% reduction) when the
diameter increased from 25 to 35 nm while with the same
variation of the diameter, it increases from 1950 to 2165 Oe and
then reduces to 1740 Oe for CoFe nanowires containing 25 at% Co.
It may therefore be said that the small change in coercivity due to
change in diameter of the nanowires with more than 40 at% Co, is
in agreement with coherent model in which the coercivity is
diameter independent [29] while in those containing less than
40 at% Co, a considerable reduction of coercivity due to diameter
is an indication of greater consistency of the curling model with
the magnetic behavior of these alloys [30]. It should be noted that
the coercivity of nanowires which almost follows the curling
model inversely changes with diameter square.
As reported, the CoFe ordered alloys show small magnetocrys-
talline anisotropy K1 [26]. The results show crystalline anisotropy
in the CoxFe1ꢀx nanowires is negligible and can be omitted and
could not have a significant effect on the coercivity variations.
If any variation occurred in the preferential direction of the bcc
structure due to the predomination of shape anisotropy in these
nanowires, the variation of the magnetic properties with respect
to these crystalline variations was unexpected. It should be noted
that nanowires were embedded into the parallel nanopores
therefore all are parallel and have the same crystalline orienta-
tion. On the other hand although all the X-ray diffraction pattern
show preferential (110) orientation, it is not possible to say those
are single crystals. Many reports confirmed ac electrodeposited
nanowires formed with polycrystalline structure [27,28]. The
rapid change in coercivity of the samples containing more than
85 at% Co might be associated with the transition in the crystal-
line structure from cubic to hcp lattices. This conclusion is
supported by the abrupt change in squareness of the samples
containing more than 85 at% Co.
A nanowire with a high aspect ratio (an aspect ratio m¼c/
ao10) can be considered as a prolate spheroid. In this case,
according to the following equations [29]:
"
!#
pffiffiffiffiffiffiffiffiffiffiffiffiffi
m
2ðm2ꢀ1Þ
"
1
mþ m2ꢀ1
pffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢁ ln
pffiffiffiffiffiffiffiffiffiffiffiffiffi
Na ¼ Nb ¼ 4
p
ꢁ
mꢀ
ꢁ ln
,
2
m2ꢀ1
mꢀ m2ꢀ1
!
#
pffiffiffiffiffiffiffiffiffiffiffiffiffi
1
m
mþ m2ꢀ1
pffiffiffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffiffi
m2ꢀ1
Nc ¼ 4
p
ꢁ
ꢀ1
ðm2ꢀ1Þ
2
mꢀ m2ꢀ1

A. Ramazani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3193–3198
3197
1.2
1.0
0.8
0.6
0.4
0.2
0.0
D=30 nm
D=50 nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(110)
D=30 nm
Fig. 6. Cross section SEM micrograph of FeCo nanowire arrays embedded in AAO
template.
2000
1800
1600
1400
D=50 nm
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
1200
experimental data
1000
D=50
D=40
1/D2 fitted curve
800
2400
2200
2000
1800
1600
1400
D=25
30
40
50
60
2 Theta
70
80
Fig. 5. XRD patterns of the CoFe nanowires with various diameters containing
(a) 81, (b) 58 and (c) 43 at% Co.
0.0004
0.0008
Dp-2(nm-2)
0.0012
0.0016
For more clarification, the effect of diameter on the coercivity of
Co18Fe82 and Co50Fe50 nanowires is displayed in Fig. 7. As shown in
Fig. 7(a) the coercivity of Co18Fe82 nanowires are well fitted with
radial inverse square curve to 0.00111 nmꢀ2 (diameter¼30 nm),
while in Fig. 7(b) the fitted curve include less points and approaches
to 0.00111 nmꢀ2 (diameter¼35 nm), for the Co50Fe50 nanowires.
As a rule of thumb it may be said that the coercivity of nanowires
which initially is independent of the diameter, decreases with
further increase in diameter according to radial inverse-square
law [30]. With an increase in the Fe content, the transition from
coercivity independency to the radial inverse-square dependency
occurs for the nanowires with larger diameters. Anyway, the
experimental data of Fig. 4 represent the magnetic behavior of all
CoFe nanowires with more than 35 nm diameter is more consistent
with curling model.
Fig. 7. Coercivity of (a) Co18Fe82 nanowires and (b) Co50Fe50 nanowires as a
function of 1/D2p along with fitted radial inverse square curve.
0.98 and 0.49 for our nanowires with 25 and 50 nm diameters and
same interpore distances of 65 nm, respectively. The difference
between our results and what was reported by Chen and Qin may
be a direct consequence of magnetostatic interaction between
nanowires. The diameter to interpore distance ratio of nanowire
arrays reported by Chen and Qin is 0.33 and 0.5, which is an
indication of low magnetostatic interaction between nanowires.
In the present work, this ratio varies from 0.38 to 0.76 for 25 to
50 nm diameters, respectively. In comparison, the former (0.38) is
almost the same as what is reported by them while the later is
almost twice than what they reported and implies a large inter-
wire magnetostatic interaction resulting in a 50 at% reduction in
squareness.
The coercivity reduction with increase in diameter may have
another source. As reported by Chen and Qin the squareness of
nanowire arrays with 20 and 50 nm diameter and interpore
distance of 60 and 100 nm was almost 1 while it was about

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A. Ramazani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3193–3198
the behavior of nanowires (with a wide range of diameters) with
2500
2000
1500
1000
500
less than 40 at% Co. Maximum coercivity for the nanowires with
25 nm diameter was seen in Co0.43Fe0.57, while it was seen for
nanowire with 50 nm diameter Co0.58Fe0.42. The study of angular
dependence of the coercivity with diameter CoFe nanowires
shows similar treatment with angular dependence of the coerciv-
ity with packing density of Fe nanowires.
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By anodizing aluminum in sulfuric acid and immersing template
into phosphoric acid for different widening times, the diameters of
the subsequently deposited nanowires were varied.
Highly ordered CoxF1ꢀx alloy nanowire arrays were produced in
anodic aluminum templates. Coercivity and squareness of the CoFe
alloy nanowires are improved over the properties of similar nanos-
tructures composed of pure Co and Fe. The crystalline structure of
the nanowires is concentration independent and shows a bcc
structure for CoxFe1ꢀx nanowires containing only a small amount
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