Fabrication and characterization of electrodeposited Co1-xCrx nanowires

Article abstract of DOI:10.1016/j.jmmm.2005.11.012

Co_1-xCr_x alloy nanowires with 0.0 1 < x < 0.9 3 were fabricated by electrodeposition in a porous alumina membrane from an electrolyte containing Co and Cr ions. The composition, structure and magnetic properties of the nanowires have been characterized. Cobalt-rich nanowires were electrodeposited at a potential of -1.0 V relative to Ag/AgCl and chromium-rich nanowires were deposited beyond -3.5 V. The optimized processing conditions include hydrogen annealing to give hysteresis loops for the Co₈₀Cr₂₀ nanowires with coercivity of up to 200 mT and squareness of up to 0.95. Magnetization of the Co₈₀Cr₂₀ nanowire is 77 A m² kg^-1 and the energy product of the arrays is 35 kJ m^-3.

Full text of DOI:10.1016/j.jmmm.2005.11.012

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) 232–236
www.elsevier.com/locate/jmmm
Fabrication and characterization of electrodeposited
Co Cr nanowires
1ꢀx
x
Ã
N.B. Chaure , J.M.D. Coey
Physics Department, Trinity College, Dublin 2, Ireland
Received 17 August 2005; received in revised form 19 October 2005
Available online 5 December 2005
Abstract
Co1 Cr alloy nanowires with 0:01oxo0:93 were fabricated by electrodeposition in a porous alumina membrane from an electrolyte
ꢀx
containing Co and Cr ions. The composition, structure and magnetic properties of the nanowires have been characterized. Cobalt-rich
x
nanowires were electrodeposited at a potential of ꢀ1.0 V relative to Ag/AgCl and chromium-rich nanowires were deposited beyond
ꢀ
3.5 V. The optimized processing conditions include hydrogen annealing to give hysteresis loops for the Co80Cr20 nanowires with
2
coercivity of up to 200 mT and squareness of up to 0.95. Magnetization of the Co Cr nanowire is 77 A m kg and the energy product
ꢀ1
8
0
20
ꢀ
3
of the arrays is 35 kJ m .
r 2005 Elsevier B.V. All rights reserved.
PACS: 81.15.Pq; 82.45.Mp; 98.58.Ay; 75.60.Ej
x
Keywords: Electrodeposition; Co1ꢀxCr ; Coercivity; Nanowires
1
. Introduction
Porous alumina membranes are ideal templates in which
2. Experimental details
The Co–Cr alloys were electrodeposited from an
aqueous bath containing CrCl , 6H O; CoCl , 6H O;
H BO ; HCOONH ; and H NCH COOH. Ammonium
ordered arrays of magnetic nanowires of more or less
uniform lengths and diameters are deposited. In recent
years, electrodeposition of nanostructured arrays of pure
metal nanowires [1,2], multilayered nanowires [3,4] and
alloy nanowires [5,6] have been reported. Magnetic
nanowire arrays are of potential interest for applications,
such as high-density perpendicular magnetic recording,
nanosensors, and magnetic MEMS [7–10]. For such
applications, materials with perpendicular magnetic aniso-
tropy and adequate coercivity are usually required [11–14].
Here we report the synthesis of Co1 Cr nanowires in
porous alumina membranes and the effect of annealing on
their structural and magnetic properties. Co–Cr-based
alloys are extensively used for hard disc magnetic recording
[15], but there are no previous reports of electrodeposited
ferromagnetic Co–Cr.
3
2
2
2
3
3
4
2
2
formate (HCOONH ) was used as complexing agent and
4
glycine (H NCH COOH) as a brightening agent. The
2
2
concentrations of the reagents are listed in Table 1. The
composition of the Co1 Cr alloy was adjusted by varying
the cathodic growth potential from –1.0 to –4.0 V, where
ꢀx
x
the potential is relative to the Ag/AgCl reference.
Ammonium hydroxide (NH OH) solution was added to
adjust the pH to between 5 and 6. Electrodeposition was
carried out at constant potential in a conventional three-
electrode geometry consisting of a platinum counter
electrode and an Ag/AgCl reference electrode. Porous
3
ꢀx
x
alumina membranes, with pore diameter d ffi 200 nm and
p
thickness t ffi 60 mm (Whatman anodisc), were used as the
working electrode. Prior to electrodeposition of the wires, a
150–200 nm layer of Pt was sputtered one side of the
alumina membrane. The bath temperature was maintained
Ã
Corresponding author. Tel.: +353 1 6082171; fax: +353 1 6711759.
E-mail address: chauren@tcd.ie (N.B. Chaure).
at 25 1C during the deposition. After electrodeposition the
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2005.11.012

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N.B. Chaure, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 303 (2006) 232–236
233
filled alumina membrane was annealed at temperatures
ranging from 550 to 800 1C for 40 min, either in a vacuum
of 10 bar or in a hydrogen atmosphere. First, the furnace
was heated to the desired temperature and the sample was
then pushed into the furnace. After annealing, the samples
were quenched in ice. Nanowires were extracted from the
alumina template by dissolving it in an aqueous solution of
chromic acid and phosphoric acid in a ratio 1:2. The
nanowires were investigated using X-ray diffraction (XRD)
ꢀ
6
with CuK radiation. Scanning electron microscopy (SEM)
a
was used to examine single nanowires and the cross section
of the filled alumina template. The atomic percentage of Co
and Cr in the nanowires was determined by energy-
dispersive X-ray spectroscopy (EDS). Magnetic properties
of the nanowire arrays were measured at room temperature
using a permanent-magnet vibrating-sample magnetometer
Table 1
The composition of the plating bath
Reagent
Concentration (M)
(
VSM) [16], and a 5 T SQUID magnetometer.
CrCl
CoCl
BO
HCOONH
NCH COOH (glycine)
NH OH (ammonium hydroxide)
3
, 6H
, 6H
(boric acid)
(ammonium formate)
2
O (chromium chloride)
0.4
0.2
0.7
1.8
1.0
1.0
2
2
O (cobalt chloride)
H
3
3
4
H
2
2
3. Results and discussion
3
The arrays of the Co1 Cr nanowires were prepared
x
ꢀx
with composition ranging from x ¼ 0:0–0.93 by varying the
deposition potential from –1.0 to –4.0 V. The relation
between composition and deposition potential is shown in
Fig. 1. We focus on the samples with composition x ¼ 0:20.
Compositions far from 80:20 are magnetically soft even
after annealing or less coercive field.
Co
Cr
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Fig. 2 shows a typical SEM micrograph of the cross
section of a Co Cr nanowire array prepared by
20
8
0
electrodeposition for 2 h, after dissolution of the mem-
brane. The wires have an average length of 15 mm and the
pore diameter dp is 200 nm; the center-to-center pore
distance is 250730 nm. It can be clearly seen that the
nanowires are parallel to each other through the entire
pores. The length of the nanowires is very uniform, and it
can be controlled in the range from several nanometers to a
few tens of micrometers by varying the deposition time. A
single wire separated from the alumina membrane is shown
in Fig. 2(b). Straight, single nanowires more than 10 mm in
length were used for chemical compositional analysis.
-
1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
Applied potential (V)
Fig. 1. Chemical compositions determined on single separated nanowires.
Almost pure cobalt is deposited at ꢀ1.0 V while beyond ꢀ3.5 V the
deposits are alloys with ꢁ93% Cr and 7% Co.
Fig. 2. (a) Cross-sectional SEM image of 200 nm diameter Co80Cr20 nanowires electrodeposited in the alumina template. (b) SEM image of a 200 nm
Co80Cr20 alloy nanowire removed from the alumina template.

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N.B. Chaure, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 303 (2006) 232–236
234
The XRD patterns for Co Cr nanowires deposited at
8
Co Cr thin films deposited by magnetron sputtering.
76 24
0
20
ꢀ
2.8 V and annealed in hydrogen atmosphere at various
The broadening of HCP (0 0 2) cobalt peak could be due to
the formation a mixture of cubic and hexagonal phases of
cobalt [18].
temperatures are shown in Fig. 3. The pattern of the as-
deposited wires shows the presence of an hexagonal-close-
packed (HCP) cobalt phase with (0 0 2) texture. The peaks
appearing close to 2y ¼ 521 and 771 are associated with the
Magnetic measurements of the nanowire arrays were
performed at room temperature using the VSM. Fig. 4
shows the hysteresis loops of Co Cr nanowire arrays of
(
4 3 0) and (6 0 2) reflections of the tetragonal CoCr phase.
8
0
20
As can be seen in Fig. 3(b), the HCP (1 0 0) peak of cobalt
is visible for samples annealed at temperature of 550 1C and
it remains visible as the annealing temperature is raised
from 600 to 800 1C. The samples annealed at high
temperature (4650 1C) not only show the broadening of
the cobalt HCP (0 0 2), peak but the (4 3 0) peak
corresponding to CoCr tetragonal phase disappears and
the (2 0 0) reflection of metallic chromium appears. The
CoCr and metallic Cr peaks are marked as (ꢂ) and (J),
length ꢁ10 mm annealed at 650 1C in vacuum (a) and in
hydrogen atmosphere (b) for 40 min. The external field was
applied parallel ðkÞ and perpendicular (?) to the long axes
of the nanowires, which were not removed from the
membrane. The hysteresis curves are corrected by the
effective demagnetising factor Neff of the filled membrane is
given by the expression [19]
0
Neff ¼ N þ f ðN ꢀ NÞ, (1)
where N is the demagnetizing factor of the individual wire,
respectively, in Fig. 3. A similar appearance of the Cr
0
(
2 0 0) peak is reported by Schoenmaker et al. [17] on
N is the overall demagnetizing factor for the membrane
80
60
40
20
0
field ll to wire
field _l_ to wire
(
f)
(
e)
-20
-40
-60
-80
(
d)
-
0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
(
a)
µ0H (T)
(
c)
80
60
40
20
0
field ll to wire
field _l_ to wire
(
b)
-
20
(a)
-40
-
-
60
80
4
0
50
60
70
80
90
100
-0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
2θ (deg)
(b)
µ0H (T)
Fig. 3. X-ray diffraction patterns for Co80Cr20 nanowires deposited at
2.8 V, after removal from the alumina template, (a) as-deposited and
annealed in hydrogen atmosphere at (b) 550 1C, (c) 600 1C, (d) 650 1C, (e)
Fig. 4. Room temperature magnetization measurements of arrays of
Co80Cr20 nanowires in an alumina template annealed at 650 1C for 40 min
in (a) vacuum and (b) hydrogen atmosphere. Hysteresis loops were
measured, with the magnetic field applied along (ꢂ) and perpendicular (J)
ꢀ
7
00 1C and (f) 800 1C for 40 min. (ꢂ) denotes tetragonal CoCr and (J)
denotes Cr reflections.
to the nanowire.

ARTICLE IN PRESS
N.B. Chaure, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 303 (2006) 232–236
235
and f is the fill factor (0.7). Hence, when the field is
?
160
perpendicular to the membrane we find N ¼ 0:7, and in
eff
Vacuum annealed
Annealed H ambient
eff
k
1
1
1
40
20
00
2
the parallel direction is N ¼ 0:15. The coercivities
measured along the wire for vacuum and hydrogen
annealed samples (Fig. 4a, b) are 91 and 135 mT and the
squareness is 0.82 and 0.95, respectively. Much smaller
values of coercivity and squareness are measured perpen-
dicular to the wire axis. Other samples have coercivity of
up to 200 mT but lower squareness (0.83). The anisotropy
is associated with the distribution of easy axis, which has a
tendency to lie along the wire axis. The magnetization of
the Co Cr alloys is determined from SQUID measure-
80
60
4
2
0
0
0
8
0
20
2
ꢀ1
ments to be 77 Am kg . The remanent polarization of the
filled alumina membrane is estimated as 0.32 T, and the
maximum energy product is 35 kJ m
ꢀ
3
.
Fig. 5(a) and (b) shows the dependence of coercive field
and squareness as a function of the annealing temperature
for Co Cr nanowires annealed for 40 min in vacuum and
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
Deposition potential (V)
8
0
20
Fig. 6. Variation of coercive field as a function of deposition potential
after annealing at 650 1C for 40 min in vacuum or hydrogen atmosphere.
All measurements were performed at room temperature with the field
applied along the nanowires.
160
140
120
100
Vacuum annealed
field ll el to wire
Annealed H ambient
2
in hydrogen atmosphere, when the external magnetic field
is applied along the wires. For both, vacuum and hydrogen
annealing the coercive field increases rapidly from 20 mT
and reaches a maximum for the samples annealed at
80
60
40
20
6
50 1C. As seen in Fig. 5(a) the coercive field drops by
almost half for higher (4650 1C) annealing temperatures.
The enhancement in coercive field and squareness could be
due to a better alignment between the easy axis and the
nanowire axis or a better distribution of secondary phases.
The degradation in magnetic properties noticed for higher
(4 650 1C) annealing temperature seems to be associated
0
25 500 550 600 650 700 750 800
850
with the appearance of free chromium, disappearance of
the CoCr phase as well as broadening of HCP Co (0 0 2)
peak.
o
(
a)
Annealing temperature ( C)
1
.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
.0
Fig. 6 shows the variation in coercive field after
annealing in vacuum and hydrogen atmosphere for
Vacuum annealed
field ll el to wire
0
0
0
0
0
0
0
0
0
0
Annealed H2 ambient
40 min as a function of deposition potential. The magnetic
field was applied along the nanowires during the measure-
ments. The as-deposited samples were magnetically soft.
The improvement in the coercive field is obvious for the
samples annealed in hydrogen atmosphere. It is also
observed that the wire deposited close to –2.8 V deposition
potential the coercivity increases up to 135 and 91 mT for
hydrogen and vacuum annealing, respectively. The coercive
field systematically decreases as the deposition potential
changes on either side of –2.8 V.
4. Conclusions
0
25 500 550 600 650 700 750 800 850
o
(
b)
Annealing temperature ( C)
We have developed a method for producing Co Cr
20
8
0
nanowire arrays, and single Co Cr nanowires, which
8
0
20
o c
Fig. 5. Comparison of (a) coercive field (m H ) and (b) squareness
show useful coercivity and reasonably square hysteresis
loops after annealing in vacuum or hydrogen atmosphere.
It is expected that the magnetic properties could be further
ðMr=MsÞ of vacuum and hydrogen-annealed Co80Cr20 nanowires
annealed for 40 min at different temperatures. All measurements were
performed at room temperature with the field applied along the nanowires.

ARTICLE IN PRESS
N.B. Chaure, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 303 (2006) 232–236
236
improved with additives, as for magnetic recording media.
The system, as it stands, is suitable for producing nanowire
array of hard and soft magnetic materials or segmented
magnetic nanowires by depositing at different potentials
from the same bath.
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8
[
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This work was supported by Science foundation Ireland
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[
(
M. Venkatesan for the SQUID data and Dr. F.M.F. Rhen
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