Fabrication and magnetic properties of amorphous Co0.71Pt 0.29 nanowire arrays

Article abstract of DOI:10.1016/j.ssc.2004.07.065

Highly ordered Co_0.71Pt_0.29 alloy nanowire arrays have been fabricated successfully by direct current electro-deposition into the pores of a porous anodic aluminum oxide (AAO) template. SEM and TEM images reveal that the nanowires of array are uniform, well isolated, and parallel to one another. The aspect ratio of nanowires is over 200. XRD and EDS pattern indicates that amorphous Co_0.71Pt_0.29 structure was formed during electro-deposition. In amorphous sample, magnetocrystal anisotropy is very small, therefore, shape anisotropy plays a dominant role which leads to strong perpendicular anisotropy. High coercivity (Hc = 1.7 kOe) and squareness (Mr/Ms) around 0.7 were obtained in the samples when the field was applied parallel to the axis of the nanowires. However, when it changed to polycrystalline. structure after annealing, due to the competition of magnetocrystal anisotropy and shape anisotropy, the sample did not display perpendicular anisotropy.

Full text of DOI:10.1016/j.ssc.2004.07.065

Solid State Communications 132 (2004) 399–403
www.elsevier.com/locate/ssc
Fabrication and magnetic properties of amorphous Co0.71Pt0.29
nanowire arrays
Hua Lia, Cailing Xua, Guangyu Zhaoa, Yikun Sua, Tao Xub, Hulin Lia,
*
aCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P.R. China
bState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, P.R. China
Received 14 May 2004; received in revised form 23 June 2004; accepted 29 July 2004 by C.E.T. Gonc¸alves da Silva
Available online 14 August 2004
Abstract
Highly ordered Co0.71Pt0.29 alloy nanowire arrays have been fabricated successfully by direct current electro-deposition into
the pores of a porous anodic aluminum oxide (AAO) template. SEM and TEM images reveal that the nanowires of array are
uniform, well isolated, and parallel to one another. The aspect ratio of nanowires is over 200. XRD and EDS pattern indicates
that amorphous Co0.71Pt0.29 structure was formed during electro-deposition. In amorphous sample, magnetocrystal anisotropy is
very small, therefore, shape anisotropy plays a dominant role which leads to strong perpendicular anisotropy. High coercivity
(HcZ1.7 kOe) and squareness (Mr/Ms) around 0.7 were obtained in the samples when the field was applied parallel to the axis
of the nanowires. However, when it changed to polycrystalline structure after annealing, due to the competition of
magnetocrystal anisotropy and shape anisotropy, the sample did not display perpendicular anisotropy.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 75.30.Gw; 75.50.Ss; 75.60.Ej; 82.45.Qr
Keywords: A. Co0.71Pt0.29 nanowire arrays; B. Magnetic recording materials; D. AAO template
1. Introduction
our best knowledge, fabrication and magnetic properties of
1D amorphous binary alloy magnetic nanomaterial have not
been reported so far. But in some polycrystalline sample,
due to the competition between magnetocrystalline aniso-
tropy and shape anisotropy, there may be no perpendicular
anisotropy [15,16]. Since the magnetic properties of
nanowire arrays are related to their element components,
morphology and microstructures [17], 1D amorphous
magnetic nanoalloy are expected to exhibit the perpendicu-
lar anisotropy. Besides, the amorphous magnetic nanoma-
terials seem to be more suitable candidates for magneto-optic
recording media than crystalline magnetic nanomaterials
because of low medium noise and high signal-to-noise
ratio. Polycrystalline nanomaterials are formed by small
crystals, so boundaries between these small crystals will be
sure to cause noise of the crystal boundary when
polycrystalline nanomaterials are subjected to read/write
operations. In contrast, amorphous magnetic materials can
One-dimensional (1D) nanostructured materials, such as
nanorods and nanowires, have attracted much attention due
to their novel chemical and physical properties [1]. In
particular, 1D magnetic nanomaterials have been exten-
sively exploited not only for the fundamental interest but
also for their potential utilization in ultrahigh-density
magnetic storage devices [2,3], magneto-optic recording
media [4–6] and spintronics [7–10]. However, the most of
the work on 1D magnetic nanomaterial have been mainly
focused on the polycrystalline structure alloy [11–14]. To
* Corresponding author. Tel.: C86-931-891-2517; fax: C86-
931-891-2582.
E-mail addresses: huali@lzu.edu.cn (H. Li), lihl@lzu.edu.cn
(H.L. Li).
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.07.065

400
H. Li et al. / Solid State Communications 132 (2004) 399–403
avoid noise of the crystal boundary because of their
amorphous microstructures. Although the key synthetic
methodologies of 1D magnetic nanomaterials developed so
far include catalyzed high-temperature growth via the vapor–
liquid–solid (VLS) mechanism [18,19], molecular beam
epitaxy [20], ion-beam irradiation [21], it remains a
challenge to prepare high perpendicular magnetic anisotropy
and coercivity magnetic nanowires without high vacuum,
application of pressure, supply of temperature and external
magnetic field applied during electro-deposition. Here, we
report the first successful synthesis of an interesting 1D
amorphous magnetic Co0.71Pt0.29 alloy with obvious perpen-
dicular anisotropy by direct current electro-deposition into
the pores of a porous anodic aluminum oxide (AAO)
template at room temperature and normal pressure. The
advantages of this method in the production of high
perpendicular magnetic anisotropy nanowires are obvious
because of its simplicity, convenience, and capability of
producing large quantities of samples. Furthermore, the
amorphousCo0.71Pt0.29 alloy nanowirearrays obtained witha
strong perpendicular magnetic anisotropy, high coercivity,
large magneto-optic Kerr effect and a low medium noise are
among the most interesting candidates for ultra-high-density
recording and magneto-optic recording media.
and 1.8% H2CrO4 at 50 8C for 30 min to dissolve the
alumina matrix partly and washed by distilled water several
times. For TEM imaging, we dipped the as prepared samples
into 6% H3PO4 and 1.8% H2CrO4 at 50 8C for 180 min to
dissolve the AAO completely.
2.1. Instruments
The morphology of the AAO templates was examined by
atomic force microscopy (AFM). Transmission electron
microscopy (TEM, Hitachi 600) and scanning electron
microscopy (SEM, JEOL JSM-5600LV) were used to
characterize the morphology of nanowire arrays. The
crystalloid structure of the samples was determined by X-
ray diffraction (XRD, Rigaku, Model D/max 2400; Cu Ka
˚
radiation, lZ1.54056 A). The average composition of the
nanowires was measured from energy dispersive spectra
(EDS). Magnetic properties of the as-prepared and anneal-
ing samples were tested by a vibrating sample magnet-
ometer (VSM, TOEI-5S-15).
3. Results and discussion
For AAO film, its micrography was investigated by
AFM. The results indicate that the diameter and interpore
distances (center to center) are about 50 and 75 nm,
respectively, and pore densities about 1010 cmK2 in the
resulting AAO template prepared by two-step anodization
(see Fig. 1). Generally, the higher the anodized voltage is,
the larger the pore diameter of AAO film is; the longer the
time is, the thicker the film is and the deeper the pores are.
Fig. 2 shows the SEM images of the as-grown
Co0.71Pt0.29 nanowires. As can be seen from a low-
magnification SEM image (Fig. 2a), the sample consists of
a large quantity of straight nanowire arrays, no other
morphologies are observed in the product. Fig. 2b is a cross-
2. Experiment
Highly ordered AAO templates with pore diameter of
about 50 nm were prepared by anodic oxidation of 99.999%
pure Al sheet in oxalic acid solution under two-step
anodizing process [22,23]. The first process was carried
out at a constant voltage of 40 V in 0.3 M oxalic acid
solution at 10–15 8C for 3 h. Secondly, the oxide film was
dissolved in 0.4 M H3PO4, 0.2 M H2Cr2O4 at 50 8C. Then,
the anodization was performed for 10 h under the same
condition as the first one. The as-prepared template was
dipped into 5% phosphoric acid at 50 8C for 20 min to
eliminate the obstacle film, and then a silver film of 300 nm
thick was sputtered onto one side of the template to act as a
conductive contact.
Electro-deposition was carried out at room temperature,
using a three-electrode potentiostatic control and direct
current electro-deposition system with a saturated calomel
electrode (SEC) as reference electrode, a 1 cm!0.5 cm
platinum plate as a counter electrode and the Ag coated
AAO template as working electrode. The Co0.71Pt0.29
nanowires used in the experiments described here were
grown from a solution of [CoSO4]Z1.2 mol dmK3
[K2PtCl6]Z0.012 mol dmK3, [H3BO4]Z30–40 g dmK3
pH 2.0–3.0 at K0.8 V (vs. SCE).
,
,
After electro-deposition, the sample was annealed at
700 8C for 20 min in Ar (999.999%) atmosphere (heating
rate: 100 8C /min, cooling rate: 100 8C/min).
To obtain the SEM image of the Co0.71Pt0.29 nanowire
arrays, the sample was dipped into a solution of 6% H3PO4
Fig. 1. AFM image of AAO template.

H. Li et al. / Solid State Communications 132 (2004) 399–403
401
Fig. 3. TEM images of (a) several Co0.71Pt0.29 nanowires, (b) a
single Co0.71Pt0.29 nanowire with 50 nm in diameter.
prepared by electro-deposition for 1 h is above 10 mm and
the typical diameter is about 50 nm, i.e. the aspect ratio is
above 200. Hence, the shape anisotropy of Co0.71Pt0.29
nanowire arrays is very high in our case.
Fig. 4 shows an EDS pattern of the Co0.71Pt0.29 nanowire
arrays produced by dc electro-deposition at K0.8 V. The
average composition of the nanowire arrays was 70.69 at.%
Co, 29.31 at.% Pt. The EDS pattern also shows the peaks of
element Al, which is attributed to the AAO template. The
XRD scans were performed to examine the crystal structure
of the Co0.71Pt0.29 alloy nanowire arrays. The typical
diffraction pattern is shown in Fig. 5b, from which we can
see that no obvious reflection peak was found, indicating
amorphous-like structure of as-deposited. Whether the
deposit is amorphous or crystalline depends on the growth
conditions such as applied voltage, electrolyte concentration
and so on [24]. After annealing at 700 8C for 20 min in
99.999% Ar, the diffraction peaks are found at 39.708, 46.428,
67.588 and 81.278, corresponding to the (1 1 1), (2 0 0), (2 2 0)
and (3 1 1) planes of fcc CoPt structure (see Fig. 5a). From
Fig. 5(a) and (b), we can clearly see that upon anneal
treatment, the Co0.71Pt0.29 nanowires changed its original
amorphous phase to stable polycrystalline structure. Shown
in Fig. 6 is the Differential temperature analysis (DTA)
pattern of amorphous Co0.71Pt0.29 nanowire arrays. DTA
curve shows a strong exothermic reaction, implying the
process of crystallization of the amorphous Co0.71Pt0.29
nanowire.
Fig. 2. SEM images of uniform Co0.71Pt0.29 nanowires that were
prepared via the electro-deposition. The wires have lengths along
the longitudinal axis of up to 10 mm. (a) view from the top; (b) view
from the side.
sectional image of Co0.71Pt0.29 nanowire arrays prepared by
electro-deposition for 3 h. One can see that the nanowires of
array are very uniform, well isolated and parallel to each
other. Some wires bend together when released partly from
AAO template, which may be due to internal stress during
electro-deposition. The average length of the as-deposited
Co0.71Pt0.29 nanowires is about 10 mm. The lengths of the
nanowires can be controlled from several nm to 20 mm
according to deposition time. TEM images of the as-
prepared Co0.71Pt0.29 nanowires are shown in Fig. 3. The
nanowires prepared by AAO template synthesis are of
regular size and are continuous. All of them have uniform
diameter of about 50 nm, which basically equals to that of
pores of the AAO template. However, in the TEM picture
nanowires also appear irregular and they show different
contrast within themselves. This can be explained by that
nanowires are formed by small particles with different
composition. The average length of Co0.71Pt0.29 nanowires
Fig. 7(a) and (b) show magnetization hysteresis loops at
room temperature of as-deposited and annealed samples,
respectively, under a field applied parallel or perpendicular
Fig. 4. EDS pattern of the CoPt alloy nanowires. The composition
ratio of the nanowires was 70.69 at.% Co, 29.31 at. % Pt.

402
H. Li et al. / Solid State Communications 132 (2004) 399–403
Fig. 5. X-ray diffraction patterns of Co0.71Pt0.29 nanowire arrays (a)
as-deposited, (b) annealed at 700 8C for 20 min in 99.999%Ar.
to the length axis of the wires. As can be seen, the coercivity
and squareness (Mr/Ms) of as-prepared are about 1.7 kOe
and 0.7, respectively, when the field applied is parallel to the
axis of nanowires arrays, much larger than the coercivity
(800 Oe) and squareness (Mr/MsZ0.3) when the field
applied is perpendicular to the nanowire arrays, indicating
that the easily magnetized direction of Co0.71Pt0.29 nanowire
arrays is parallel to the nanowire arrays and that it has
obvious magnetic anisotropy. Under conditions of small
diameters about 50 nm and aspect ratio above 200, the
Co0.71Pt0.29 nanowires behave like a single domain
structure. In Fig. 7b, the squareness for the loops measured
in parallel and perpendicular to fields are 0.3 and 0.4,
respectively, implying a weak in-plane magnetic anisotropy.
These different magnetic behaviors before and after
annealing can be attributed to their different microstruc-
tures. XRD patterns indicate that the annealed samples grow
preferentially along the (1 1 1) crystal direction, i.e.
perpendicular to nanowire (in-plane). When only magneto-
crystalline anisotropy is considered, the preferred direction
Fig. 7. The magnetization hysteresis loops at room temperature (a)
as-deposited, (b) annealed at 700 8C for 20 min in 99.999%Ar. (Out
of plane means applied field is parallel to the nanowires; in plane
means field perpendicular to the nanowires).
(1 1 1) of the annealed Co0.71Pt0.29 nanowire arrays is easy
to magnetize. When shape anisotropy is also considered, the
result of the competition between magnetocrystalline
anisotropy and shape anisotropy is that magnetocrystalline
anisotropy is a little stronger than shape anisotropy, so the
sample displays a weak in-plane magnetic anisotropy. XRD
evidences show that the sample before annealing is
amorphous, but it changes to be polycrystalline after
annealing. For polycrystalline sample, magnetocrystalline
anisotropy (kmw5!105 J/m3) is comparable with the shape
anisotropy ((mo Ms2/4)w 1!105 J/m3 for infinite cylinder),
as a result, the competition of the two kinds of anisotropy
energies results in no perpendicular anisotropy of sample B.
But for amorphous Co0.71Pt0.29 nanowires of sample A,
magnetocrystalline anisotropy is very weak, therefore,
shape anisotropy makes the main contribution for total
anisotropy which favors the magnetization perpendicular to
the film plane (along the wire axis). Zeng et al. [25] also
pointed out that the magnetic reversal mechanism may be
Fig. 6. DTA pattern of Co0.71Pt0.29 nanowire arrays.

H. Li et al. / Solid State Communications 132 (2004) 399–403
[2] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80.
403
attributed to localized nucleation in such case, which will
decrease the shape anisotropy. Upon annealing the effects of
crystallization cause the decrease of superior perpendicular
magnetic characteristics for Co0.71Pt0.29 nanowires. But for
as-prepared, the competition between shape anisotropy and
magnetocrystal anisotropy is unlikely to occur, because the
latter contribution does not exist due to the amorphous
structure of these Co0.71Pt0.29 nanowires. Since a unit cell of
magnetic nanowire arrays acts as a recording cell [26], the
nanowire arrays fabricated by porous alumina film contain-
ing 1010 cells per square centimeter have a striking magnetic
recording density about (60Gbit/in2). Therefore, the electro-
deposited amorphous magnetic recording nanowire arrays
are promising for producing perpendicular magnetic
recording medium with a super-high density and magneto-
optic recording media. The magnetic properties of amor-
phous CoPt alloy nanowire arrays with different diameter
and different annealing temperature will be reported in our
later work.
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template. Magnetization measurement on the arrays of
amorphous Co0.71Pt0.29 nanowires demonstrated a notable
perpendicular magnetic anisotropy, which are mainly
attributed to shape anisotropy. Crystallization of the sample
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We thank the National Natural Science Foundation of
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