Oriented cobalt nanowires prepared by electrodeposition in a porous membrane

Article abstract of DOI:10.1016/j.jmmm.2004.11.387

Porous alumina membranes with a uniform pore size of 100 nm have been used to prepare arrays of oriented cobalt nanowires by electrodeposition in an applied magnetic field. The hexagonal c-axis lies in the plane of the membrane, perpendicular to the nanowire axis, for material deposited in 0 T or 1.5 T, but in 5 T the c-axis lies at about 70° to the axes of the wires. The degree of field-induced crystallographic orientation is insufficient to overcome the macroscopic demagnetizing field in samples with a large volume fraction of cobalt. The effective demagnetizing factors perpendicular and parallel to the plane of the membrane are Neff⊥=0.7 and Neff∥=0.15, respectively. Pole figures of the vector magnetization measured in a small rotating field of fixed amplitude in a permanent-magnet vector vibrating sample magnetometer are used to deduce torque and hence the effective magnetocrystalline anisotropy.

Full text of DOI:10.1016/j.jmmm.2004.11.387

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1210–1213
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Oriented cobalt nanowires prepared by electrodeposition in a
porous membrane
Ã
N.B. Chaure , P. Stamenov, F.M.F. Rhen, J.M.D. Coey
SFI Nanoscience Laboratory, Physics Department, Trinity College, Dublin 2, Ireland
Available online 14 December 2004
Abstract
Porous alumina membranes with a uniform pore size of 100 nm have been used to prepare arrays of oriented cobalt
nanowires by electrodeposition in an applied magnetic field. The hexagonal c-axis lies in the plane of the membrane,
perpendicular to the nanowire axis, for material deposited in 0 T or 1.5 T, but in 5 T the c-axis lies at about 701 to the
axes of the wires. The degree of field-induced crystallographic orientation is insufficient to overcome the macroscopic
demagnetizing field in samples with a large volume fraction of cobaltk. The effective demagnetizing factors perpendicular
and parallel to the plane of the membrane are Ne?ff ¼ 0:7 and Neff ¼ 0:15; respectively. Pole figures of the vector
magnetization measured in a small rotating field of fixed amplitude in a permanent-magnet vector vibrating sample
magnetometer are used to deduce torque and hence the effective magnetocrystalline anisotropy.
r 2004 Elsevier B.V. All rights reserved.
PACS: 81.15.Pq; 82.45.Mp; 98.58.Ay
Keywords: Shape anistropy; Magnetic nanowires; Electrodeposition; Nanoporous alumina membrane; Cobalt
A promising method to obtain nanowires with large
shape anisotropy is based on membrane technology [1].
Magnetic metals such as Fe [2], Co [3–6] and Ni [7] have
been embedded into the nanoscale pores of membranes
by electrodeposition. The use of a nanoporous mem-
brane improves coercivity and squareness of the hyster-
esis loop, compared to that of thin film or bulk material.
Shape anisotropy can dominate when magnetocrystal-
line anisotropy of the material is weak. This is the case
for iron or nickel nanowires in well-separated pores,
which give square hysteresis loops with coercivity as
high as 30% of that expected for a coherent rotation
reversal mechanism, m0Ms=2 [8]. Magnetisation reversal
proceeds via nucleation and domain-wall propagation
process in the smallest wires, and by curling in wires
with greater diameters. In the case of cobalt however,
little hysterisis is found because the hexagonal c-axis,
which is the magnetocrystalline anisotropy axis tends to
lie in the plane perpendicular to the axis of the
electrodeposited nanowires.
Here, we report the effect of applying a strong
magnetic field during electrodeposition of Co nanowires
on the magnetic properties and structural texture of the
deposits.
A conventional three-electrode electrochemical cell was
used to fabricate the Co nanowires. A platinum layer of
thickness ꢀ100nm was sputtered on one side of 100nm
pore diameter ‘Anodisc 47’ alumina membrane obtained
from Whatman and used as a working electrode to
fabricate an array of cobalt nanowires. Platinum wire and
Ag/AgCl were used as the counter and reference
electrode, respectively. Co was electrodeposited at room
Ã
Corresponding author. Tel.: +353 1 608 2171;
fax: +353 1 671 1759.
E-mail address: chauren@tcd.ie (N.B. Chaure).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.11.387

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N.B. Chaure et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1210–1213
1211
temperature from an aqueous electrolyte containing
(d)
0.9 M CoSO4 7H2O, 0.11 M NaCl and 0.8 M H3BO3.
Boric acid was used to adjust the pH of the bath to 3.5.
Deposition was carried out at room temperature for a
few hours to grow 20–25 mm long nanowires with no
stirring during the deposition. The volume fraction of
ferromagnetic material in the filled membrane is about
70%. During the deposition a 1.5 T field was applied
parallel or perpendicular to the membrane using an
electromagnet or else a superconducting magnet with a
105mm room-temperature bore was employed to apply a
5 T field perpendicular to the membrane. When necessary
for characterization of the electrodeposit, the membrane
was dissolved in 1.5 M NaOH solution at room
temperature for 15 min.
(c)
(b)
(a)
The magnetic properties at room temperature have
been determined using an automated vector vibrating
sample magnetometer (VVSM) with a 2 T permanent-
magnet variable flux source. Pole figures of the vector
magnetization measured in a rotating field of fixed
amplitude ranging from 20 mT to 2 T are used to deduce
the effective demagnetising factor and the hence the
magnetocrystalline anisotropy constant K1.
40
50
60
70
80
90
100
110
2θ (deg)
Fig. 2. X-ray diffraction of the cobalt nanowires electrodepos-
ited at growth potential À1.0 V without field (a), 1.5 T parallel
(b), 1.5 T perpendicular (c) and 5 T parallel (d) to nanowire
during growth.
Fig. 1 shows cobalt nanowires which were electro-
deposited at –1.0 V relative to the Ag/AgCl reference
electrode. The SEM cross section view of the wires is
obtained after dissolving the membrane.
peak. In all the cases the cobalt c-axis lies in the plane of
the membrane. On the other hand, when a 5 T field is
applied during deposition, the texture is quite different.
The (0 0 2) peak in Fig 2d show that the c-axis is no
longer perpendicular to the axis of the wire.
Fig. 2 shows the X-ray diffraction pattern of the Co
nanowires electrodeposited at À1.0 V without field (a);
1.5 T parallel (b); 1.5 T perpendicular (c) and 5 T parallel
(d) to the nanowires during growth. As can be seen in
Fig. 2a the main peak is from (1 1 0) planes of the HCP
cobalt structure; Figs. 2b and c show an intense (1 0 0)
The texture in Fig. 1 shows that the cobalt (1 1 0) axis
tends to lie along the wire in zero field, but in 1.5 T, the
(1 0 0) peak becomes prominent. The second order (2 0 0)
peak also appears in Figs. 2b and c. In both cases the
magnetically easy c-axis is in the plane of the membrane,
tending to reinforce the in-plane shape anisotropy. The
5 T X-ray diffraction pattern indicates that the c-axis has
moved out of the plane of the membrane which does
not seem to agree with the magnetization measurement
(Fig. 3).
The slope of the magnetisation curve when the field is
applied along the wire is steeper for the sample
deposited in 0 T in comparison with the one deposited
at 5 T, which contradicts the expected development of an
easy axis along the wires (as the easy axis for a-Co is
normally collinear with the c-axis). The reason for this
inconsistency is unclear, but may be related to a similar
phenomenon observed in anisometric cobalt nanoclus-
ters (around 100 nm in size), which exhibit on easy axis
of magnetisation [10].
The coercivity is small but it has increased both along
and perpendicular to the wire. We find an increase
from 2.5 mT (0 T) to 16 mT (5 T) along the wire when
field is applied during deposition. These changes in
coercivity are associated with the appearance of (0 0 2)
reflection.
Fig. 1. Cross section of the electrodeposited cobalt wires after
the removal of the 100 nm nanoporous alumina membrane.

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1212
3.0x10-5
2.0x10-5
1.0x10-5
0.0
8
0T
5T
zero field
5 T
4
0
-1.0x10-5
-2.0x10-5
(a)
-3.0x10-5
3.0x10-5
-4
0T
5T
-8
2.0x10-5
1.0x10-5
0.0
-1.0x10-5
-2.0x10-5
-3.0x10-5
-12
0
50
100 150 200 250 300 350
(b)
Angle ꢂ, deg
Fig. 4. Torque curves measured with a 0.1 T applied field at
room temperature for a 100 nm array of cobalt nanowires. Note
that the 0 and 5 T field was applied during the growth of
nanowires.
-2
-1
0
1
2
Magnetic Field ꢀ0H,T
Fig. 3. Magnetization loops at room temperature for 100 nm
cobalt nanowires with magnetic field applied parallel (a) and
perpendicular (b) to the wire axis during measurements. The
magnetic field 0 and 5 T stand for the field applied parallel to
the wires during the deposition.
the SEM images), we estimated the primary anisotropy
to be 50 kJ/m3 and
constant using K1 ¼ m HM
?max
0
200 kJ/m3 for the samples deposited in 0 and 5 T,
respectively.
The effective demagnetizing factor Neff of the filled
membrane is given by expression [9]
In conclusion, we have demonstrated that the
magnetic anisotropy axis and crystallographic texture
can be controlled to some extent by applying an external
field during electrodeposition of cobalt in a nanoporous
alumina membrane. Shape anisotropy favours the
membrane plane for a fill factor f 42=3; and the natural
texture is with the c-axis of the nanowires in the plane of
the membrane. While it is possible to induce some c-axis
texture by depositing nanowires in 5 T magnetic field,
this was insufficient to overcome the shape anisotropy of
the membrane.
Neff ¼ N þ f ðN0 À NÞ;
(1)
where N is the demagnetizing factor of the individual
wire, N0 is the overall demagnetizing factor for the
membrane and f is the fill factor (0.7). Hence, when the
field is perpendicular to the membrane we find Ne?ff
¼
0:7; and in the parallel direction Nkeff ¼ 0:15: Therefore,
despite the structure in nanowires, shape anisotropy
forces and easy direction of magnetization to lie in the
plane of the membrane. The fill factor must be less than
2
3
for the shape anisotropy to favour an easy direction
perpendicular to the plane of the membrane.
This work was supported by Science Foundation
Ireland and by the EU IST program (M2EMS project).
The torque pattern measured as the field is rotated in
a plane containing the wire is deduced from the VVSM
experiment, as the two components of the magnetisation
are simultaneously recorded the torque may be esti-
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~
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magnetocrystalline anisotropy constant K1 and a smaller
signal with period p=2 due to K2. The misalignment
angle (ꢀ701 ) was estimated from the zeros of the torque
curves.
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