Microstructures and magnetic properties of as-deposited and annealed FexCo1-x alloy nanowire arrays embedded in anodic alumina templates

Article abstract of DOI:10.1016/j.physb.2010.03.012

FeCo nanowires of 30 nm diameter were fabricated by ac electrodeposition in nanopores of alumina templates at different electrolyte concentrations and various electrodeposition frequencies. The effect of annealing on microstructures and magnetic properties of the nanowires fabricated under various conditions was investigated. Although magnetic properties of as-deposited nanowires were frequency independent, after annealing some variations were seen in those properties. The maximum coercivity of as-deposited Fe_xCo_1-x nanowires along the nanowire axis was found at 50 at% Fe. The crystalline structure of the nanowires was concentration independent and revealed a bcc structure for the Fe_xCo_1-x containing at least 10 at% Co. A coercivity of 2950 Oe was obtained for 1000 Hz-ac electrodeposited Fe_0.64Co_0.36 nanowires, annealed at 570 °C.

Full text of DOI:10.1016/j.physb.2010.03.012

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Physica B 405 (2010) 2620–2624
Contents lists available at ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Microstructures and magnetic properties of as-deposited and annealed
FexCo1Àx alloy nanowire arrays embedded in anodic alumina templates
M. Almasi Kashi a,n, A. Ramazani a, F. Es’haghi a, S. Ghanbari a, A.S. Esmaeily b
a Department of Physics, University of Kashan, Kashan, Iran
b Institute of Nanoscience and 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:
FeCo nanowires of 30 nm diameter were fabricated by ac electrodeposition in nanopores of alumina
templates at different electrolyte concentrations and various electrodeposition frequencies. The effect
of annealing on microstructures and magnetic properties of the nanowires fabricated under various
conditions was investigated. Although magnetic properties of as-deposited nanowires were frequency
independent, after annealing some variations were seen in those properties. The maximum coercivity of
as-deposited FexCo1Àx nanowires along the nanowire axis was found at 50 at% Fe. The crystalline
structure of the nanowires was concentration independent and revealed a bcc structure for the
Received 4 January 2010
Received in revised form
4 March 2010
Accepted 4 March 2010
Keywords:
Coercivity
FexCo1Àx containing at least 10 at% Co.
A coercivity of 2950 Oe was obtained for 1000 Hz-ac
Magnetic properties
Ordered array
ac electrodeposition
electrodeposited Fe0.64Co0.36 nanowires, annealed at 570 1C.
& 2010 Elsevier B.V. All rights reserved.
1. Introduction
attention has been shifted towards preparing alloy arrays such as
CoNi [16], FeCo [17] and understanding of magnetization process.
The ordered FeCo alloy shows excellent soft magnetic properties
with almost negligible magnetocrystalline anisotropy. Therefore
the magnetic properties of FeCo nanowire arrays are mainly
predominated by shape anisotropy of the nanowires [18].
There is an interest in FeCo alloy nanowires due to their high
saturation magnetization and Curie temperature compared with
other alloy systems [19–21]. Magnetic properties of nanowires
such as coercivity and squareness are changed easily by tuning
and adjusting the alloy composition. The aim of this work is to
determine the influence of electrolyte compositions and frequen-
cies on electrodeposition as well as its impact on chemical
composition, structure, morphology and magnetic properties of
FexCo1Àx nanowire arrays. The influence of annealing on magnetic
properties and microstructure of nanowires was also investigated.
Highly ordered, densely packed arrays of nanowires have been
widely investigated due to their various applications. Not only are
nanowire arrays interesting for data storage but also for more
applications related to their use in magnetoresistive nanosensors,
and biomedical, environmental and energy safety [1–6].
A simple method to develop self-ordered arrays of nanowires
is to use anodized alumina membranes as templates. These
templates can be filled homogeneously with magnetic elements
via a well-controlled electrodeposition process.
Most of the studies have been focused on fabrication and
characterization of magnetic materials such as Fe [7], Co [8] and
Ni [9]. Although the electrodeposited Co nanowires generally
exhibit hcp polycrystalline structure with random c-axis orienta-
tion [10,11] it is reported that the electrolyte acidity rotates the
c-axis of Co towards the wires axis [12]. It has been mentioned that
Fe nanowire arrays fabricated by electrodeposition techniques are
polycrystalline with
a structure having preferential (1 1 0)
2. Experiments
orientation [13–15]. These kinds of material present wider
significance in the high density and plumb magnetism recording
mediums. Large squareness and high coercivities have been
measured parallel to the wire axis in these types of nanostruc-
tures and shape anisotropy has been found to be the dominant
mechanism in determining the magnetic properties. Recently,
Arrays of FeCo nanowires were prepared by electrodepositing
into porous aluminum oxide (PAO) templates. The highly
ordered PAO templates were fabricated by two-step anodizing
of high-purity aluminum in 0.3 M oxalic acid solution at 40 V dc
and about 17 1C [22]. Before anodization, the Al sheets were
ultrasonically cleaned for 15 min in acetone and were then
annealed in air at 500 1C for 2 h in order to increase the size
of the crystalline grains and remove any mechanical stress
induced in the samples during the cutting process of the strips.
n Corresponding author.
E-mail address: almac@kashanu.ac.ir (M. Almasi Kashi).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2010.03.012

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The samples were then electropolished in a solution of perchloric
acid and ethanol at room temperature and current density of
100 mA/cm2 for 3 min.
spectroscopy (EDX) was used to determine the composition of
samples.
The FeCo nanowires were then ac electrodeposited with
graphite as counter-electrode and PAO template with aluminum
plate as working electrode at room temperature. The electrolyte
consisted of various compositions of CoSO4 Á 7H2O and
FeSO4 Á 7H2O. To facilitate the deposition procedure, 45 g/l boric
acid was used. The PH value of the electrolyte was maintained at
about 5.0, and the ac electrodepositions were conducted at 25, 50,
200, 400, 700, 1000 and 1250 Hz deposition frequencies. The
3. Results and discussion
Current and voltage were recorded during the whole experi-
mental period. For brevity only a period of 16 ms of the current–
time curve of a sample fabricated with same reductive and
oxidative voltage is shown in Fig. 1. As seen in this figure, the
current and voltage of the reductive half cycle are in the same
phase but in the oxidative half cycle they are not so. This is an
indication of dominating resistive impedance, e.g. along with the
electrodeposition procedure the anodization takes place,
increasing the thickness of the barrier layer.
Fig. 2(a) shows AFM micrograph of the porous aluminum oxide
with the typical self-ordered cell configurations that were formed.
An almost ideally arranged hexagonal cell configuration was
observed with a cell and pore size of approximately 100 and
32 nm, respectively. A typical cross-section SEM micrograph of
the FeCo nanowires is shown in Fig. 2(b) using back-scattered
electrodeposition voltage was 30 Vp–p
.
The samples were then annealed at various temperatures
using Ar gas. The optimum temperature was seen to be about
570 1C. Before gas feeding, the vacuum in the annealing furnace
was 5 Â 10À5 Pa. The structural and magnetic behaviour of the
electrodeposited samples were analyzed by means of scanning
electron microscopy (SEM), X-ray diffractometer (XRD) and
vibrating sample magnetometer (VSM). Energy dispersive X-ray
25
1.0
Current
20
Voltage
15
10
5
0.8
Fe
Co
0.6
0.4
0.2
0
-5
-10
-15
0
2
4
6
8
10
12
14
16
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Deposition time (ms)
Fe+2 Concentration (M)
Fig. 1. Instantaneous current trace, corresponding to pulse cycle recorded at
16 ms deposition current (solid line) and the deposition voltage (dotted line) with
15/15 V reductive/oxidative voltage.
Fig. 3. Atomic percentages of iron and cobalt of FeCo nanowires as a function of
Fe2+ concentration in the deposition electrolyte.
Fig. 2. (a) AFM micrographs of the top surface of anodized alumina templates after the two-step anodization process and (b) cross-section view SEM micrograph of
FeCo-filled alumina matrix electrodeposited with 15/15 V oxidative/reductive voltage.

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As-deposited
Annealed
As-deposited
Annealed
0.006
0.004
0.002
0.000
-0.002
-0.004
-0.006
-6000 -3000
0
3000
6000
-7000 -3500
0
3500 7000
Applied Field (Oe)
Applied Field (Oe)
Fig. 4. Typical hysteresis loops of deposited and annealed Fe0.38Co0.62 nanowires fabricated at 400 Hz anodization frequency, for fields applied parallel and perpendicular
to the wires axis.
3000
50 Hz
25 Hz
2700
2400
2100
1800
1500
As-deposited
Annealed
1200
3000
200 Hz
400 Hz
2700
2400
2100
1800
1500
1200
3000
1000 Hz
700 Hz
2700
2400
2100
1800
1500
1200
3000
2700
2400
2100
1800
1500
1200
1250 Hz
0.0
0.2
0.4
0.6
0.8
1.0
Fe atomic percent (x)
Fig. 5. Coercivity dependence of the FeCo nanowires as a function of Fe content.

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electrons to maximize the atomic number contrast. The cross-
section view presents the well developed nanopores in profile,
indicating non-intercrossing and parallel holes of high aspect
ratio. The structures of PAO membranes and FeCo nanowires are
seen in this cross-section.
30 at% and then decreases [23,24]. Although the Ms of the
nanowires may decrease compared with bulk state materials
(since the wires are nanoscaled and have imperfections) the effect
3000
As seen in Fig. 3, the atomic percentage of iron as well as that of
cobalt in FeCo nanowires changes almost linearly with Fe2+ and
Co2+ concentrations in the electrolyte. Based on these values, the
component of FexCo1Àx alloy nanowires can be easily controlled by
adjusting the relative concentration of Fe2+ and Co2+ in electrolyte.
The magnetic properties of the FeCo nanowires were studied
by means of magnetization measurements carried out at room
temperature. Fig. 4 shows typical hysteresis loops obtained for the
as-deposited and annealed Fe0.38Co0.62 nanowires produced at
400 Hz anodization frequency, for fields applied along parallel and
perpendicular directions to the axis of the wires. The relative
magnetization increases after annealing but coercivity of samples
increased only for fields applied parallel to the wire axis.
Analogous results were observed in all studied samples.
The coercivity of as-deposited and annealed samples as a
function of FeCo content was determined through magnetic
measurements with the external field applied along the wire’s
axis and is shown in Fig. 5. Increasing the Fe content causes a
relatively rapid increase in coercivity for all deposition
frequencies, reaching a maximum and then reducing. In fact the
maximum coercivity of FexCo1Àx is seen for 0.38oxo0.57. The
increase of coercivity with increasing Fe content (0.38oxo0.57)
may be caused by addition of spontaneous magnetization (Ms) in
this region as reported by Slater [23] and Pauling [24]. The Slater–
Pauling curve predicts that the spontaneous magnetization Ms of
FeCo alloy in bulk states increases with increasing Co content till
As-deposited
2700
Annealed
2400
2100
1800
1500
Fe0.18Co0.82
1200
3000
2700
2400
2100
1800
1500
Fe0.38Co0.62
1200
3000
2700
2400
2100
1800
1500
FeCo (110)
As-depodite
1400
Fe0.46Co0.54
1200
3000
1200
Fe0.84Co0.16
1000
Fe0.64Co0.36
2700
2400
2100
1800
1500
1200
3000
2700
2400
2100
1800
1500
1200
800
Fe0.51Co0.49
600
Fe0.36Co0.64
400
Fe0.18Co0.82
200
Fe0.67Co0.33
FeCo (110)
Annealed
1200
Fe0.84Co0.16
1000
Fe0.64Co0.36
800
Fe0.51Co0.49
600
Fe0.36Co0.64
400
Fe0.18Co0.82
80 90
Fe0.83Co0.17
40
50
60
70
0
200 400 600 800 1000 1200 1400
Deposition Frequency (Hz)
2θ°
Fig. 6. XRD patterns of the as-deposited and annealed FeCo nanowires as a
Fig. 7. Influence of deposition frequencies on the as-deposited and annealed FeCo
function of Fe content.
nanowires containing different amounts of Fe.

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of imperfections can be expected to be the same for nanowires
fabricated of the same diameter. Therefore the results of bulk
specimens can be generalized to the nanowires and it may be said
that the coercivity of the nanowire increases with an increase in
its spontaneous magnetization.
deposition frequency. The variation in the magnetic properties
of Co nanowires obtained is attributed to modifications in the
microstructure of the nanowires. On the other hand crystallinity
enhancement was seen to improve the magnetic properties of ac
electrodeposited Fe nanowires [22,27].
The abrupt change in coercivity with a small addition of iron
might also be associated with the transition in the crystalline
structure from a highly anisotropic hcp lattice to cubic lattices
with higher symmetry. This fact is supported by the change in
squareness along the wire length, from 65% for the Co nanowire
sample to average values larger than 85% for the other FeCo
nanowire samples (not shown).
In contrast with the above results for both Co and Fe
nanowires, the coercivity of as-deposited FeCo samples contain-
ing different amounts of Fe was seen to be almost frequency
independent. The same structure of FeCo nanowires fabricated at
different deposition frequencies in the present work may be a
reason for independency of magnetic properties to the deposition
frequency. Composition non-variation of FeCo nanowires electro-
deposited at various deposition frequencies in electrolyte with
same Fe and Co ion concentrations may be another reason for the
coercivity of as-deposited samples remained almost unchanged.
As evidence the Fe content of FeCo nanowires prepared with
electrolyte mixtures of 0.3 M Fe2+ and 0.3 M Co2+ at 50, 700 and
1000 Hz deposition frequencies changed only 1 at%.
The highest coercivity of as-deposited FeCo nanowires with
50 nm pore diameter was seen in Fe0.3Co0.7 nanowires as reported
by Qin et al. [25]. In their work on FexCo1Àx nanowires, the
optimum region in which nanowires show higher coercivity was
shifted towards the Co-rich region. On the other hand, reducing
the pore diameter of FexCo1Àx to 20 nm causes the optimum
region shift towards the Fe-rich FexCo1Àx nanowires [26].
Considering the results of magnetic measurement reported by
Qin and Chen along with our experimental data, i.e. nanowires
with 20, 50 and 30 nm pore diameter, convinced us to say that the
increase in coercivity may be caused by increasing of Ms.
In order to study the annealing effects on FexCo1Àx alloy
nanowire arrays, all samples are annealed at different temperatures
and high vacuum in Ar atmosphere. The optimum annealing
temperature was almost 570 1C. It is found that the coercivities of
all samples increased after annealing and coercivity of about 2950 Oe
and squareness as high as 0.95 were obtained in Fe0.64Co0.36 annealed
at 570 1C compared with 2083 Oe for the as-deposited sample. It may
be said that thermal annealing relieves internal stress in the samples
induced by rapid deposition of iron and cobalt ions and high degree of
crystallinity is obtained, thereby increasing the Ms of annealed
samples and leading to excess coercivity.
Coercivity was also seen to change by a small amount with
frequency for the annealed samples. Fig. 7 shows the influence of
deposition frequencies on the as-deposited and annealed samples.
4. Conclusions
Highly ordered FexCo1Àx alloy nanowire arrays have been
fabricated in anodic alumina templates. Fe content and annealing
influenced the magnetic properties of FexCo1Àx nanowire. The highest
coercivity was seen for FeCo nanowires containing about 50 at% Fe
and coecivity of 2951 Oe was found for the annealed Fe0.36Co0.64
nanowire. The crystalline structure of the nanowires is concentration
independent and shows
a bcc structure for all the FexCo1Àx
nanowires. The maximum coercivity was obtained for 1000 Hz ac
electrodeposited Fe0.36Co0.64 nanowires, annealed at 570 1C.
Fig. 6 shows the XRD patterns of the FexCo1Àx nanowires in
which x ranges from 0.18 to 0.84. The XRD investigation revealed
that the as-deposited samples have body centered cubic structure
with preferred /1 1 0S orientations along the axis of the wires. It
may be said that cobalt content has no effect on the crystalline
structure of FexCo1Àx nanowires. The results show the same
crystal structure for all FexCo1Àx nanowires. Hence it may be said
that it has no 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 magnetic properties with respect to these crystalline
variations was unexpected. These results are almost confirmed by
Bozorth [18], who found that the ordered FexCo1Àx alloy
(0.3oxo0.7) shows excellent soft magnetic properties with
negligible magnetocrystalline anisotropy K1. The rapid change in
coercivity of the samples containing more than 85 at% Co might
be associated with the transition in the crystalline structure from
hcp lattice to cubic lattices. Hence the magnetic properties of
FexCo1Àx nanowire arrays are mainly predominated by shape
anisotropy of the nanowires.
The crystalline structures of annealed samples containing less
Fe are very similar to those of the as-deposited ones while the
peak intensity of FeCo containing more Fe reduces after annealing.
The lower peak intensity of annealed FeCo nanowires containing
more Fe atoms may be a consequence of more Fe oxidation than
that of Co due to the higher oxygen affinity of Fe than that of Co. It
may be said for FeCo nanowires with higher Fe content, the
annealing procedure led to the formation of nanocrystalline
grains, thereby increasing the coercivity.
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In our previous researches, the magnetic properties of the Fe
and Co nanowires were seen to change remarkably with