Magnetic properties of Co-Pt alloy nanowire arrays in anodic alumina templates

Article abstract of DOI:10.1016/j.jmmm.2005.05.038

Microstructure and magnetic properties of ferromagnetic Co9Pt nanowire arrays electrodeposited into self-assembled anodic alumina templates have been investigated. X-ray diffraction shows that as-prepared Co-Pt nanowires are of FCC polycrystalline structure. The nanowires with diameter d<80nm are mainly of (1 1 1) preferred orientation along the wire axis. For a specific diameter, the coercivity and the remanent ratio with the external magnetic field parallel to the nanowire axis, i.e., the parallel geometry, are larger than those of the perpendicular geometry. With temperature increasing from 90 to 600K, above two physical quantities in the perpendicular geometry decrease monotonically, while those of the parallel geometry change little. The dependence of the coercivity and the remanent ratio on temperature and the nanowire diameter can be explained after the shape and the magnetocrystalline anisotropies, and the dipolar magnetic interaction are considered. For the parallel and perpendicular geometries, the coercivity decreases with increasing diameter d as a linear scale of 1/d2 and approaches each other in the two geometries for large d. The degradation of coercivity in Co-Pt nanowires with increasing diameter is suggested to come from the curling mode of magnetization reversal process.

Full text of DOI:10.1016/j.jmmm.2005.05.038

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 300 (2006) 471–478
www.elsevier.com/locate/jmmm
Magnetic properties of Co–Pt alloy nanowire arrays in
anodic alumina templates
Ã
T.R. Gaoa, L.F. Yina, C.S. Tiana, M. Lub, H. Sangb, S.M. Zhoua,b,
aSurface Physics Laboratory (National Key Laboratory) and Department of Physics, Fudan University, Shanghai 200433, China
bState Key Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
Received 15 October 2004; received in revised form 30 May 2005
Available online 1 July 2005
Abstract
Microstructure and magnetic properties of ferromagnetic Co9Pt nanowire arrays electrodeposited into self-assembled
anodic alumina templates have been investigated. X-ray diffraction shows that as-prepared Co–Pt nanowires are of
FCC polycrystalline structure. The nanowires with diameter do80 nm are mainly of (1 1 1) preferred orientation along
the wire axis. For a specific diameter, the coercivity and the remanent ratio with the external magnetic field parallel to
the nanowire axis, i.e., the parallel geometry, are larger than those of the perpendicular geometry. With temperature
increasing from 90 to 600 K, above two physical quantities in the perpendicular geometry decrease monotonically, while
those of the parallel geometry change little. The dependence of the coercivity and the remanent ratio on temperature
and the nanowire diameter can be explained after the shape and the magnetocrystalline anisotropies, and the dipolar
magnetic interaction are considered. For the parallel and perpendicular geometries, the coercivity decreases with
increasing diameter d as a linear scale of 1=d2 and approaches each other in the two geometries for large d. The
degradation of coercivity in Co–Pt nanowires with increasing diameter is suggested to come from the curling mode of
magnetization reversal process.
r 2005 Elsevier B.V. All rights reserved.
PACS: 75.50.ꢀy; 75.60.ꢀd; 75.75.þa
Keywords: Nanowire; Coercivity; Curling mode; Magnetocrystalline anisotropy
1. Introduction
Porous anodic alumites are considered as
particularly attractive template materials for fab-
Ã
Corresponding author. Physics Department, 220 Handan
road, Shanghai 200433, PR China. Tel.: +862165642316; fax:
rication of nanowires, because the pore density is
high, the pore distribution is uniform and the
+862165104949.
E-mail address: shimingzhou@yahoo.com (S.M. Zhou).
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2005.05.038

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diameter of the pores is small [1,2]. Due to the
shape anisotropy, the magnetic easy axis will be
aligned along the magnetic nanowires with strong
perpendicular magnetic anisotropy if the wire
length is much larger than its diameter. Therefore,
the magnetic nanowire arrays are expected to have
great potential applications in high-density per-
pendicular magnetic recording [3]. Many kinds of
magnetic metals and alloys, such as Fe, Co, Ni,
and their alloys Fe–Co and Fe–Ni have been
prepared by electro-deposition into self-assembled
anodic alumina oxide (AAO) templates [4–10]. For
magnetic nanowire arrays, microstructure, mag-
netic interaction, and magnetization reversal pro-
cess have been studied extensively. For most
nanowires, the squareness of hysteresis loops
along the nanowire axis, i.e., the parallel geometry,
is far below one unit despite large shape aniso-
tropy. Two possible reasons have been proposed.
First, the magnetostatic interaction between neigh-
boring wires has a serious influence on the
magnetization reversal process of magnetic nano-
wires. Secondly, in addition to the shape aniso-
tropy, the magnetocrystalline anisotropy also has
an influence on the magnetization reversal process.
If the easy axis of the magnetocrystalline aniso-
tropy is not aligned along the nanowire axis, the
squareness along the nanowire will be reduced. It
is noted that attention has recently been focused
on the magnetocrystalline anisotropy of nanowires
and on the correlation between the microstructure
and the magnetic properties. Therefore, studies of
the coercivity in the parallel and perpendicular
geometries are required.
Cobalt–platinum alloys have been extensively
studied because of importance in both basic
research and potential applications [11–16]. First,
well-known, single-phased, disordered Co–Pt al-
loys can be formed in a wide composition range.
For some specific compositions, the transforma-
tion from disorder to order has been observed
after annealing at high-temperatures [11]. Sec-
ondly, disordered and ordered Co–Pt alloys are
easy to have perpendicular anisotropy and the
latter ones can have magnetic perpendicular
anisotropy as large as about 4 ꢁ 107 erg=cm3 [12].
Thirdly, the Co–Pt alloy films have large polar
Kerr effect at short wavelengths [13]. Finally, they
have good chemical stability and good corrosion
resistance [14]. Therefore, Co–Pt alloys have
potential applications in the fields of magneto-
optical and magnetic storage. In this paper, we
report on preparation of Co–Pt nanowires and
their magnetic properties. Up to now, few reports
have appeared on fabrications of Co–Pt alloy
nanowires [11,15,16]. The evolution of the micro-
structure and the magnetic properties of Co–Pt
nanowires with diameter and temperature was
studied.
2. Experimental
Co9Pt nanowires 3-mm long with different
diameters were prepared by electrodeposition. In
order to prepare AAO templates, high purity
(99.99%) Al sheets were first ultrasonically de-
greased in tricholoroethylene and then etched in
NaOH. Subsequently, the sheets were electropol-
ished for 3 min in a mixed solution of C2H2O6 and
HClO4 (volume ratio ¼ 4: 1). Then they were
anodized in different acidic solution such as
1.2 M sulfuric acid and 0.6 M oxalic acid for
60 min. After the completion of AAO templates,
the electrodeposition was conducted at 200 Hz and
13 V AC at room temperature for a duration of
5 min by using graphite as the counter-electrode.
The Co and Pt atoms were electrodeposited into
templates from an aqueous bath containing 2 g/L
H2PtCl6 ꢂ 6H2O and 11.2 g/L CoSO4 ꢂ 7H2O. 30 g/
L boric acid was added to adjust the pH value to
5.0. The composition of the Co–Pt nanowires was
adjusted by varying the volume ratio of the
H2PtCl6 ꢂ 6H2O and CoSO4 ꢂ 7H2O solutions.
With atomic absorption spectroscopy (AAS), the
Co content of the Co–Pt nanowire was found to be
about 90 at%. In this study, transmission electron
microscopy (TEM) was used to determine the
morphology of AAO films, as well as topography,
diameter, and length of nanowires. The micro-
structural characterization of nanowires was per-
formed by X-ray diffraction (XRD). The magnetic
properties of the arrays at various temperatures
were measured using vibrating sample magnet-
ometer (VSM) with a maximum external magnetic
field of 1:0 T.

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3. Results and discussion
film by dissolving the alumina layer with aqueous
NaOH. Co–Pt nanowires were found to be not
smooth and not very uniform along the nanowire
with an average length of about 3-mm, as shown in
Fig. 1(b).
Fig. 1(a) shows a plan-view TEM micrograph of
oxalic-acid anodized AAO template sample with
30 V DC for 60 min. Highly ordered nanohole
arrays were formed with an average diameter of
40 nm. In experiments, the average diameter of the
pores was found to increase linearly with anodiz-
ing voltage. For further TEM measurements,
Co–Pt nanowires were liberated from the AAO
Before X-ray measurements, Al substrates were
removed from the AAO films by using aqueous
HgCl2 to avoid the influence of the Al substrate.
Co–Pt nanowires are still kept in the AAO
templates, which have flat surfaces. Fig. 2 shows
the XRD spectra of aligned Co–Pt nanowires in
the AAO film with different diameters. The spectra
clearly indicate that Co–Pt nanowires have FCC
structure with preferred texture orientations of
(1 1 1), (2 0 0), and so on along the wire axis. For
nanowires with do80 nm, the intensity of the
(1 1 1) peak is much larger than those of others and
(
the lattice constant a0 ¼ 3.33 A for Co–Pt nano-
wires. For the diameter of 80 nm however, the
diffraction peaks of (0 0 1), (1 1 0), (2 0 0), and
(2 2 0) become stronger in comparison with small
diameters, showing a more random distribution of
grain orientations. Moreover, Co(1 1 1) diffraction
peak was detected at 2y ¼ 44:4ꢃ. In order to
verify a homogeneous distribution in composition
and structure for do80 nm, more experiments are
required.
Hysteresis loops were measured at various
temperatures for nanowires with various diameters
in the parallel and perpendicular geometries. Fig. 3
presents typical hysteresis loops for Co–Pt nano-
wires with a diameter of 60 nm in the temperature
range 90–600 K. For the present sample, the
coercivity and the squareness in the parallel
geometry are much larger than those of the
perpendicular geometry. The coercivity in the
two geometries tends to approach each other at
low temperatures.
Fig. 4 shows the coercivity in the parallel and
the perpendicular geometries HCðkÞ and HCð?Þ,
and remanent ratio Mr=MsðkÞ and Mr=Msð?Þ as a
function of temperature. In Fig. 4(a), HCð?Þ
decreases monotonically and HCðkÞ does not
decrease monotonically as T increases, which is
in accordance with report by Paulus et al. [17].
DHC ¼ HCðkÞ ꢀ HCð?Þ increases with increasing
temperature. As shown in Fig. 4(b), along the
parallel direction the remanent ratio Mr=MsðkÞ
Fig. 1. (a) Plan-view TEM micrograph of an AAO template
with 40 nm diameter holes. (b) TEM image of Co–Pt nanowires
liberated from AAO template.

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1
(111)
350K
450K ⁄⁄
⁄⁄
40nm
(110)
⊥
⊥
0
(001)
(220)
(200)
-1
1
300K
⁄⁄
200K
⁄⁄
⊥
⊥
0
60nm
(111)
-1
1
150K
⁄⁄
90K
⁄⁄
(200)
⊥
⊥
(001)
0
(110)
-1
(111)
80nm
-10
-5
0
5
10 -10 -5
0
5
10
H (kG)
H (kG)
(110)
Co(111)
(200)
(001)
Fig. 3. Magnetization hysteresis loops for an array of 60 nm
diameter Co–Pt nanowires in AAO template at different
temperatures. The solid and dash curves refer to the external
magnetic field parallel and perpendicular to the wire axis,
respectively.
(220)
Fig. 5 presents typical hysteresis loops for Co–Pt
nanowires with the diameters of 40 and 80 nm at
room-temperature. For the diameter of 40 nm, the
coercivity in the parallel geometry is larger than
that of the perpendicular geometry and the
coercivity in the two geometries approaches each
other for the diameter of 80 nm. For diameters of
40 and 80 nm however, the squareness in the
parallel geometry is larger than that of the
perpendicular geometry. Similar phenomena were
also observed in other kinds of nanowires and
attributed to the parallel alignment of the mag-
netic easy axis along the wire axis [4–10]. There-
fore, for Co–Pt nanowires with small diameters,
the easy axis of magnetic anisotropy favors to be
20
40
60
2θ (degrees)
Fig. 2. X-ray diffraction patterns of AAO films filled with
aligned Co–Pt nanowires with diameters of (a) 40 nm, (b)
60 nm, and (c) 80 nm. All diffraction peaks belong to Co–Pt,
except for Co (1 1 1) in (c).
increases with the temperature increasing while
Mr=Msð?Þ in perpendicular geometry decreases.
This indicates that the magnetic easy axis turns
away from the nanowire axis at low temperatures.

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475
1.0 (a)
(a)
1.5
1.0
0.5
⁄⁄
0.5
0.0
⊥
(kG)
C
H
⁄⁄
-0.5
⊥
-1.0
1.0
(b)
(b)
⁄⁄
0.5
0.0
⊥
⁄⁄
0.5
-0.5
⊥
-1.0
-10
-5
0
5
10
0.0
H (kOe)
200
400
T (K)
600
Fig. 5. Magnetization hysteresis loops for an array of (a) 40 nm
and (b) 80 nm diameter Co–Pt nanowires in AAO templates.
The solid and dash curves refer to the parallel and perpendi-
cular geometries, respectively.
Fig. 4. Temperature dependence of (a) coercive field and (b)
remanent ratio in two geometries. The empty and filled boxes
refer to the parallel and perpendicular geometries, respectively.
aligned along the wire axis. Fig. 6 shows typical
angular dependence of the coercivity and the
remanent ratio for the diameter of 40 nm. The
coercivity and the remanent ratio change simulta-
neously with the direction of the applied magnetic
field. However, for 80 nm the dependence is quite
different. Although the remanent ratio changes in
the same way as that of 40 nm, the coercivity does
not change much.
In order to explain above experimental results in
Figs. 3–6, several aspects must be considered. First,
because of the large aspect ratio of Co–Pt nanowires,
the shape anisotropy energy 2pM2S is expected to play
a dominant role in the effective magnetic anisotropy
and its easy axis is parallel to the wire axis, where MS
is the saturation magnetization. Secondly, the mag-
netocrystalline anisotropy must be taken into ac-
count. For present Co–Pt nanowires with small
diameters, the (1 1 1) orientation of most grains and
thus the easy axis of the magnetocrystalline aniso-
tropy are also aligned along the nanowire axis.
Although the magnetocrystalline anisotropy is in the
order of 105 erg=cm3, about one order smaller than
the demagnetization energy [16], it has an apparent
effect on the effective magnetic anisotropy. Finally,
since the squareness is still less than one unit for
small diameters, magnetostatic interaction between
neighboring nanowires should be considered [18].

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total energy approaches each other in the two
(a)
geometries for large diameters [19]. In this case,
the easy axis favors to be aligned perpendicular to
the wire axis. Therefore, the coercivity in the two
geometries crosses as a function of the diameter as
shown below. Although the shape anisotropy is
the major contribution to the magnetic anisotropy,
the evolution of the coercivity and the remanent
ratio in the parallel and the perpendicular geome-
tries results from a competition among the shape
and magnetocrystalline anisotropies, and the
dipolar magnetostatic interactions. The results in
Fig. 4 might hint that, as the temperature is
lowered, the difference of the dipolar magneto-
static interaction between the parallel and perpen-
dicular geometries is increased more rapidly than
those of the magnetocrystalline and the shape
anisotropies. It should also be pointed that other
reasons cannot be excluded, such as a distribu-
tion in the orientations of crystallographic axes
and an enhancement of the magnetocrystalline
anisotropy in small grains with respect to bulk
materials.
1.4
1.2
1.0
(kOe)
C
H
0.8
0.8
(b)
0.6
0.4
0.2
For the present Co–Pt nanowires, the room
temperature HC in the parallel and the perpendi-
cular geometries decreases rapidly as the nanowire
diameter is increased. When the diameter is
increased from 40 to 70 nm for example, HC in
the parallel geometry decreases from 2430 to
870 Oe. Moreover, the difference between the two
configurations becomes small with the diameter
increasing. More remarkably, Fig. 7 shows that
the coercivity in the two geometries scales as a
linear function of 1=R2, where R is the radius of
nanowires. As is well known, for magnetic
nanowires the magnetization reversal mechanism
strongly depends on the diameter. For a specific
material, there is a critical radius RC, where a
transition of coercivity behavior can usually be
observed. If R4RC, the magnetization reversal
process can be described by curling mode and the
coercivity decreases with increasing diameter of
nanowires. Using the spin S, Curie temperature
TC and saturation magnetization MS for bulk
Co–Pt alloy, the critical diameter can be estimated
[20]. It is smaller than 20 nm in the present case
and close to that of 14 nm for Co0:65Pt0:35
nanowires [16].
0
30
60
90
H (deg)
120
150
180
θ
Fig. 6. Angular dependence of (a) coercivity and (b) remanent
ratio for the diameter of 40 nm, where yH is the angle between
the applied magnetic field and the wire axis.
For the present Co–Pt nanowires, the total
energy consists of the demagnetization energy Ede,
the magnetocrystalline anisotropy EK, and the
dipolar magnetostatic interaction energy Edi.
Here, we have EdeðkÞoEdeð?Þ, EKðkÞoEKð?Þ,
and EdiðkÞ4Edið?Þ, where k refers to the magne-
tization parallel to the wire axis and ? to the
magnetization perpendicular to the wire axis. For
small diameters, the total energy of the nanowires
with the magnetization along the wire axis is
smaller than that of the perpendicular geometry
and the easy axis favors to be aligned along the
wire axis. As the voltage is increased during the
fabrication of AAO templates, the diameter of
pores and the density of pores are increased
simultaneously. With the density of pores increas-
ing, the dipolar interaction is increased and the

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477
by X-ray measurements in Fig. 2. It coincides with
the results in Fig. 5(b).
2.5
2.0
1.5
1.0
0.5
4. Conclusion
Arrays of Co–Pt nanowires with various dia-
meters have been successfully prepared by electro-
deposition into the pores of AAO templates. The
as-prepared Co–Pt nanowires were observed by
XRD to have FCC structure with preferred
orientation of (1 1 1) along the nanowire. The
coercivity of nanowires in the parallel geometry is
larger than that of the perpendicular geometry and
approaches each other in the two geometries for
large diameters and low-temperatures. These
experimental results can be qualitatively explained
after taking into account the shape and magneto-
crystalline anisotropies, and the dipolar magnetic
interaction. Since the diameter is larger than the
critical value, the magnetization reversal process
can be described by curling mode, and the
coercivity in the two geometries decreases with a
linear scale of 1=R2 as the nanowire diameter is
increased. The structure of the nanowires is found
to have a strong influence on the magnetic
properties.
along wire axis
(kOe)
perpendicular to wire axis
1
2
3
1/R2 (10-3 (nm)-2)
Fig. 7. Dependence of room temperature coercivity on
nanowire radius for Co–Pt nanowires in the parallel (ꢄ) and
perpendicular geometries (K). The dash line corresponds to
linear fitting results.
Acknowledgements
Since the diameter of the present nanowires is
much larger than the critical value, the magnetiza-
tion reversal process is dominated by curling
mode. Thus, the HC of nanowires is given as
follows [21]:
This work was supported by the National Science
Foundation of China Grants nos. 10174014,
60271013, 60490290, and 10021001, and the State
Key Project of Fundamental Research Grant nos.
2001CB610602 and 2002CB613504, JSNSF, and
Shanghai Nanotechnology Program Center (no.
0252nm004).
2pkA 1
2KU
HC
¼
þ
,
(1)
R2
MS
MS
where the right term 2KU=MS is the magnetocrys-
talline anisotropy field with uniaxial anisotropy
constant KU. The dependence of the coercivity on
the radius in Fig. 7 can be fitted very well using Eq.
(1), indicating that the magnetization reversal
process can be described by curling mode [22].
As shown in Fig. 7, HC for 80 nm is larger than
that of 70 nm, which may originate from existence
of Co component and more random alignment of
grain orientations in Co–Pt component as shown
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