Magnetic field annealing dependent magnetic properties of Co90 Pt10 nanowire arrays

Article abstract of DOI:10.1016/j.jmmm.2009.07.081

Co₉₀ Pt₁₀ alloy and elemental Co nanowires (NWs) are fabricated by electrodeposition in the self-assembled anodic alumina templates. The fabricated NWs are subjected to magnetic field (MF) annealing under 1000 Oe applied magnetic fie

Full text of DOI:10.1016/j.jmmm.2009.07.081

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Journal of Magnetism and Magnetic Materials 321 (2009) 3984–3989
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Magnetic field annealing dependent magnetic properties of Co90Pt10
nanowire arrays
Ã
S. Shamaila, R. Sharif, J.Y. Chen, H.R. Liu, X.F. Han
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Co90Pt10 alloy and elemental Co nanowires (NWs) are fabricated by electrodeposition in the self-
assembled anodic alumina templates. The fabricated NWs are subjected to magnetic field (MF)
annealing under 1000 Oe applied magnetic field in a direction parallel to the nanowire axis at 265 1C.
The corresponding changes in the saturation magnetization, coercivity, remanent squareness, the shape
of hysteresis loops and crystal structure of NWs before and after MF annealing have been investigated.
The enhanced magnetic anisotropy has been observed in Co90Pt10 alloy NWs by MF annealing. The
elemental Co NWs have not been affected by MF annealing. The stress relief between the domains and
diffusional pair ordering of unlike atoms along the direction of external applied field are thought to be
the causes of enhanced anisotropy. Re-annealing of the samples in the absence of magnetic field at
600 1C does not completely remove the enhanced anisotropy. The shape of the NWs is concluded to play
major role in persistence of enhanced magnetic anisotropy after high temperature reannealing.
& 2009 Elsevier B.V. All rights reserved.
Received 9 January 2009
Received in revised form
15 April 2009
Available online 13 August 2009
Keywords:
Magnetic field annealing
Magnetic anisotropy
CoPt nanowires
PACS:
75.75.þa
75.60 Nt
75.50.ꢀy
1. Introduction
variations in magnetic properties of NWs in accordance with the
practical applications are an attractive topic in field of magnetic
devices. The intrinsic properties of materials such as magnetic
anisotropy (MA) and magnetostriction have been investigated for
long time [12]. Especially the modeling of anisotropy is of
technological interest since it can broaden the applications of
materials in devices. Researchers have described the application
of magnetic field (MF) during electrochemical growth of magnetic
NW arrays with an attempt to study its influence on their
structural and magnetic properties [13–15]. MF annealing is an
established technique for inducing a magnetic anisotropy in both
crystalline and amorphous materials [16–19]. Currently, there are
limited reports on systematic investigations of enhanced mag-
netic anisotropy for electrodeposited ferromagnetic NWs. In the
present report, elemental (Co) and alloy (Co90Pt10) nanowire (NW)
arrays are prepared by AC electrodeposition into nanochannels of
porous alumina membranes. The crystal structure, magnetic and
magnetization properties of the nanowires (NWs) have been
studied. For detailed analysis, Co90Pt10 and Co NWs are subjected
to magnetic field (MF) annealing and reannealing (without field)
at higher temperature. An enhanced magnetic anisotropy (MA)
and a large increase in saturation magnetization (Ms) in case
of Co90Pt10 alloy NWs have been evidenced by the changes in
hysteresis loops of the samples before and after MF annealing
processes. Post deposition MF annealing does not affect
the properties of elemental Co NWs although magnetic field has
some effects on such NWs when it is applied during the growth of
NWs [13–15].
Miniaturization of advanced devices, such as sensors and
micromotors, requires the development of nanostructures. In-
tensive efforts have been devoted in the development of
nanostructures to attain the combined goals of a high areal
density of bits and adequate thermal stability [1]. Cobalt alloys,
and in particular cobalt platinum alloys are considered as a very
promising candidate for such applications and specially for
perpendicular magnetic recording [2–5]. An alloy composed of
two kinds of materials is expected to produce a fine new magnetic
material that has both high magnetization and coercivity.
Template assisted magnetic nanowire arrays, in principle, present
an attractive potential medium because the fabricated nanowires
(NWs) by electrodepositing transitional metals or their alloys into
the nanopores of the templates comprise a highly ordered pattern
of reasonably magnetically isolated units [1]. Ferromagnetic NWs
exhibit unique and tunable magnetic properties due to the
inherent shape anisotropy and the small wire dimensions [6,7].
The material microstructure, grain size and crystallite orientations
play an important role in magnetic properties of NWs such as
hysteresis and remanent magnetization [8,9]. These microstruc-
tural features, in turn, are determined by the conditions under
which the material is prepared and treated [10,11]. The controlled
Ã
Corresponding author.
E-mail address: xfhan@aphy.iphy.ac.cn (X.F. Han).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.07.081

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2. Experimental
out under 1000 Oe magnetic field applied parallel to the nanowire
axis at 265 1C for 2 h. The samples were re-annealed without
magnetic field at 600 1C for 21 h. For comparison as deposited
Co90Pt10 NWs were also annealed in zero magnetic field at 500 1C.
X-ray diffraction (XRD) was performed to analyze the crystal
structure of the NWs. Atomic force microscopy (AFM), transmis-
sion electron microscopy (TEM) and field emission scanning
electron microscopy (FESEM) were used to characterize the
morphology and size of the templates and NWs. The contents of
the NWs were determined using induction coupled plasma
spectrometry (ICP). The analysis disclosed that the composition
of individual sample is almost homogeneous. The magnetic
properties of the same samples before, after annealing with and
without MF and then reannealing process are investigated using
vibrating sample magnetometry (VSM).
The high purity (99.99%) aluminum (Al) foil was ultrasonically
degreased in tricholoroethylene for 5 min, and etched in 1.0 M
NaOH for 3 min at room temperature (RT). It was then electro-
polished in a mixed solution of HClO4 : CH3CH2OH ¼ 1 : 4 (by
volume) for 3 min with a constant potential of about 12 volts (V).
To obtain highly ordered pores, a two-step anodization was used.
In the first anodization step the Al foil was anodized at 0 1C and
40 V dc in 0.3 M oxalic acid for about 12 h to form textures on Al
surface. The formed aluminum oxide layer was then removed by
immersing anodized Al into a mixed solution of 0.4 M chromic
acid and 0.6 M phosphoric acid solution at 60 1C. Subsequently,
the samples were reanodized for 8 h under the same anodization
conditions as in the first step. A nonequilibrium anodization
process in which the voltage was reduced to about 8 V followed
the second anodization. This led to a reduction of the barrier layer
that formed between the porous alumina and the Al substrate,
thus facilitating the electrodeposition of the metals. These self-
assembled anodic aluminum oxide (AAO) templates were used
to fabricate Co90Pt10 and Co NW arrays by AC-electrochemical
deposition method. The electrolyte contained PtCl4, CoSO4 ꢁ 7H2O,
and H3BO3 for Co90Pt10 NWs and CoSO4 ꢁ 7H2O, and H3BO3 for Co
NWs. The pH of both solutions was kept to about 3.0 to get fcc
structured NWs [20]. The AC-deposition was conducted with a
sinusoidal voltage, using a standard double electrode bath, the
AAO template was used as one electrode, and the platinum wire
was used as another. The MF annealing of the samples was carried
3. Results and discussion
Fig. 1 shows (a) AFM top view ð1 ꢂ 1
m
m2Þ, (b) AFM top view
ð0:5 ꢂ 0:5
m
m2Þ and (c) three-dimensional AFM image ð1 ꢂ 1
m
m2Þ
of the surface topography of nanopore array in anodic alumina
prepared by two-step anodization. Uniform pores with an average
pore diameter of about 40 nm and center-to-center spacing of about
100 nm have been obtained. The fabricated AAO templates contain
self-assembled pore arrays with quasihexagonal ordering. Generally,
pore diameter of AAO film is increased if the anodized voltage is
large, and length of the pores is increased with time for anodization.
Fig. 1. (a) AFM top view (1 ꢂ1
m
m2), (b) AFM top view (0.5 ꢂ 0.5
m
m2) and (c) three-dimensional AFM image ð1 ꢂ 1 m2Þ of the surface topography of nanopore array in
m
anodic alumina prepared by two-step anodization.

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Fig. 2 presents some typical TEM images of (a) Co90Pt10 alloy
The magnetic field annealing effects on the magnetic proper-
ties of Co90Pt10 NW arrays are described in Fig. 3(a–c). The inset of
Fig. 3c shows the variations in saturation magnetization (Ms) of
Co90Pt10 NW arrays. After MF annealing a large increase in the Ms
along with modest variations in magnetic properties indicates
that the magnetic anisotropy has been enhanced in Co90Pt10 alloy
NWs. The magnetic anisotropy constant (Ku) calculated for
Co90Pt10 alloy NWs is found to be increased from 3:7670:5 ꢂ
105 to 8:6670:5 ꢂ 105 J=m3 after MF annealing. The origin of this
enhanced magnetic anisotropy (MA) is the atomic pair ordering of
Co and Pt atoms and relief of stress between the grains.
The strength of the field induced anisotropy (Ku) in a binary
alloy like Co1ꢀxPtx varies as
NWs and (b) Co NWs liberated from AAO template by immersion
in an aqueous 1 M NaOH solution. The average diameter of the
NWs are about 40 nm, in agreement with that of the pores in the
AAO template. The lengths of Co90Pt10 and Co NWs observed by
FESEM before dissolving the membrane are found to be about 15
and 20 mm, respectively.
ꢀ
ꢁ ꢀ
ꢁ
2
2
1
MsðTaÞ
MsðTÞ
Msð0Þ
2
Kup
x2ð1 ꢀ xÞ
ð1Þ
kBTa Msð0Þ
where kB is the Boltzmann constant, Ta is the annealing
temperature and x represents the contents of elements in the
alloy system [12]. This expression describes the relation of Ku with
2
alloy composition as x2ð1 ꢀ xÞ , saturation magnetization after
annealing (MsðTaÞ), annealing temperature (Ta) and saturation
magnetization at measurement temperature (MsðTÞ). For same
annealing and measurement temperature, the Ku will be deter-
mined from the composition of alloy and MsðTaÞ. Furthermore for a
particular composition Ku will be proportional to the square of
MsðTaÞ. This trend is in agreement with the observed results for
Co90Pt10 alloy NWs and calculated values of Ku from the
experimental M–H curves obtained with the field applied in both
directions.
As a reference, NW arrays of elemental Co are also subjected to
MF annealing and M–H curves of Co NW arrays as deposited, after
MF annealing and after reannealing are shown in Fig. 4(a–c). The
inset of Fig. 4(c) shows that there is no change in the Ms of Co
NWs. The phenomenon of enhanced MA does not appear in case
of pure elements [12] as in the present case NWs of pure Co do not
respond to the MF annealing treatment.
The variations in the values of magnetic parameters, coercivity
(Hc) and remanent squareness (SQ) with field applied in parallel
and perpendicular to the NW axis are summarized in Tables 1 and
2 for Co90Pt10 and Co NWs, respectively. Comparison of the
parameters in Table 1 indicates that after MF annealing there is an
increase in the values of Hc with small changes in SQðMr=MsÞ of
Co90Pt10 NW arrays resulting in the enhancement of magnetic
anisotropy of alloy NWs. Table 2 presents that the changes in
Fig. 2. TEM images of isolated (a) CoPt nanowires and (b) Co nanowires liberated
from AAO template by dissolving the alumina layer with NaOH aqueous solution.
Fig. 3. Magnetic hysteresis curves of Co90Pt10 alloy nanowire arrays measured with magnetic field applied parallel and perpendicular to the nanowire axis (a) as deposited,
(b) after MF annealing and (c) after reannealing process, the inset shows the variations in saturation magnetization after the annealing processes.

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Fig. 4. Magnetic hysteresis curves of Co nanowire arrays measured with the magnetic field applied parallel and perpendicular to the nanowire axis (a) as deposited,
(b) after MF annealing and (c) after reannealing process, the inset shows the variations in saturation magnetization after the annealing processes.
Table 1
long axis of the ellipsoid and the distance between the centers of
The variations in the magnetic properties of Co90Pt10 alloy nanowire arrays due to
the MF annealing process.
two ellipsoids, and N and NJ are demagnetization factors of the
?
ellipsoid parallel and perpendicular to the axis of the NWs which
depend on the aspect ratio of the ellipsoid. This equation does not
show the dependence of Hc upon interpore distance [21]. Eq. (2)
can be adopted to explore the present results. According to the
equation, if saturation magnetization Ms increases, the Hc will
increase accordingly. In case of present samples, it is observed
that there is an increase in Hc after MF annealing but this increase
is not so large as compare to the increase in Ms because Hc also
depends on demagnetizing factor and aspect ratio, according to
Eq. (2). Due to external applied field there would be alignment of
magnetic moments or production of ordered atomic pairs of
unlike atoms along the direction of external applied field [12,20].
This phenomenon affects the demagnetization factor thus result-
ing in the small increase in Hc values (as compare to Ms).
Furthermore, after MF annealing the decrease in the value of Hs
along with an increase in the value of SQ for Co90Pt10 NW arrays is
in agreement with the enhanced MA. The NW samples are then
reannealed in the absence of magnetic field at 600 1C to erase the
effect of MF annealing and to find the actual cause of enhanced
magnetic anisotropy. Reannealing of the samples does not
completely remove the enhanced anisotropy. The relative change
in Ms, Hc, and SQ values for CoPt nanowires after reannealing as
compared with those obtained after MF annealing is very small.
This small change can be interpreted because of change in grain
size at high temperatures but that change is limited due to the
shape of NW samples.
Magnetic parameters
Co90Pt10
As deposited
MF annealing
Reannealing
HcJðOeÞ
1907
359
2033
371
2188
527
H
c?ðOeÞ
SQJðMr=MsÞ
0.80
0.16
0.83
0.15
0.87
0.17
SQ ðMr=MsÞ
?
Table 2
The variations in the magnetic properties of Co nanowire arrays due to the MF
annealing process.
Magnetic parameters
Co
As deposited
MF annealing
Reannealing
HcJðOeÞ
864
300
0.57
0.11
888
316
877
323
0.57
0.12
H
c?ðOeÞ
SQJðMr=MsÞ
0.59
0.12
SQ ðMr=MsÞ
?
magnetic parameters for Co NWs are negligible as compare to that
of Co90Pt10 NWs.
The magnetization behavior of the Co90Pt10 NWs can be
explained according to the model of prolate ellipsoid chains
[21]. According to the model, the coercivity of the nanowires can
be written as
For the structure and phase evaluation of aligned Co90Pt10 and
Co NWs, XRD patterns are taken before, after MF annealing and
reannealing. For these measurements, the Al substrates are etched
away by an amalgamation process using a saturated HgCl2
aqueous solution and washed thoroughly with distilled water.
X-ray diffraction patterns in Fig. 5 show that both Co90Pt10 and Co
NWs are mainly crystallized in the fcc structure with (111)
preferred orientation. The fcc phase of Co90Pt10 NWs is dominant
in as-deposited state with (111), (2 0 0) and (110) reflections. The
major phase, in this state, is orientated along (111) reflection
which is along the axis of the NWs. After MF annealing the fcc
structure and the preferred (111) orientation are preserved as
shown in Fig. 5(a). Since the magnetic field is applied parallel to
the NW axis during annealing, therefore more crystal grains are
expected to have their easy axis of magnetization parallel to the
NW axis that is also obvious from the increase in the intensity of
more prominent peak along the NW. Texture analysis indicates
ꢂ
ꢃ
Hc ¼
p
ð6Mn þ 2LnÞa2Ms=ð6b2Þ þ ½4
pðN? ꢀ NJÞMsꢃ
ð2Þ
where
ðnꢀ2Þ=2ojrn=2
X
3
Mn
¼
½ðn ꢀ 2jÞ=½nð2jÞ ꢃ
j¼1
ðnꢀ1Þ=2ojrðnþ1Þ=2
X
3
Ln
¼
½n ꢀ ð2j ꢀ 1Þꢃ=½nð2j ꢀ 1Þ ꢃ
j¼1
n is the number of ellipsoids in one wire, a is the length of short
axis of the ellipsoid and diameter of the NWs, b is the length of

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Fig. 5. XRD patterns of aligned (a) Co90Pt10 nanowires and (b) Co nanowires, in the AAO template before and after MF annealing and reannealing.
that MF annealing does improve the orientation distribution of
crystals in the CoPt samples indicating the alignment of domains
along the magnetic field without any phase transformation. The
XRD patterns of Co NWs show the characteristic diffraction
patterns arising from fcc phase corresponding to (111) and (2 2 0).
Hexagonal close packing (hcp) is the usual structure for
electroplated cobalt. However, the fcc cobalt can be obtained
from electrodeposition at ambient temperature by varying the
deposition conditions. In the present experiment, low pH value
ðꢄ3:0Þ [20] of the electrolyte has been adopted to get fcc structure.
The as-deposited sample and the annealed samples have fcc
structure and preferred (111) orientation along the axis of the
wire. The XRD patterns for Co NWs remain unchanged after MF
annealing. However, after 600 1C reannealing the intensity of
peaks increases due to the crystallization process. From the results
of XRD and observation of microstructure, no phase change has
been observed which predicts that the magnetic anisotropy of
CoPt NWs is enhanced by the atomic pair ordering of Co and Pt
atoms.
Fig. 6 shows the magnetic hysteresis loop of Co90Pt10 NWs as
deposited, after 2 h annealing in zero MF at 500 1C and after 2 h
MF annealing at 265 1C. Significant changes in magnetic properties
have been observed after MF annealing at 265 1C compared to
annealing without MF at 500 1C. A long period of time and high
temperature will be required to produce similar magnetic changes
by the process of annealing without magnetic field [17].
The MF annealing has affected the magnetic features by
minimizing the magnetic anisotropy energy of the system due
to the stress relief between grains and directional atomic pair
ordering. The direction of bonds of dissimilar atoms (Co, Pt) of the
samples may take on an asymmetric distribution if magnetic field,
temperature and time for the annealing are sufficient [12].
Furthermore, the MF annealing decreases the dipole magneto-
static interactions and self-demagnetizing fields of the wires due
to redistribution of pair directions perpendicular to magnetization
in each domain. This redistribution causes a large increase in the
values of Ms. The reason for persistence of enhanced anisotropy
after reannealing at 600 1C is related to some structural source like
Fig. 6. Magnetic hysteresis curves of Co90Pt10 alloy nanowire arrays measured
with magnetic field applied parallel to the nanowire axis as deposited, after
annealing without MF and after MF annealing process.
shape of the nanowires and preferred crystalline orientations.
Alignment of the grains and the magnetic anisotropy may be
further enhanced if the CoPt samples are annealed in the high
magnetic field for a longer period of time. The present work
provides a comprehensive study of the magnetic field annealing
dependent magnetic properties of Co90Pt10 alloy NW arrays.
4. Conclusions
The magnetic and microstructural behaviors of Co90Pt10 alloy
NWs and Co NWs are investigated before annealing, after MF
annealing and then reannealing of the samples. The results
presented here have been explained on the basis of model of
prolate ellipsoidal chains. The magnetic anisotropy has been

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3989
enhanced in Co90Pt10 alloy NWs by MF annealing which persists
after reannealing at higher temperature without field. Co NW
arrays do not show response to the MF annealing treatment. It is
concluded that the enhanced anisotropy, originated from diffu-
sional atomic pair ordering of unlike atoms aligned with magnetic
field, and structural source (shape of the NWs) can influence the
magnetic properties of the NWs. After reannealing at 600 1C, no
significant change in magnetic properties reveals that major role
in persistence of enhanced anisotropy is played by shape of the
NWs. In conclusion, we have demonstrated that the magnetic
anisotropy can be enhanced in alloy NWs by applying magnetic
field annealing.
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