Tailoring the morphology of CoxPt1-x magnetic nanostructures

Article abstract of DOI:10.1016/j.jmmm.2009.05.014

Co_xPt_1-x nanostructures with varying composition and controllable morphologies have been synthesized through the thermal decomposition of appropriate platinum and cobalt precursors in organic solvents. The employment of several diffe

Full text of DOI:10.1016/j.jmmm.2009.05.014

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Journal of Magnetism and Magnetic Materials 321 (2009) 3120–3125
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Tailoring the morphology of CoxPt1ꢀx magnetic nanostructures
S. Mourdikoudis a, K. Simeonidis a, K. Gloystein a, M. Angelakeris a, C. Dendrinou-Samara b,
Ã
I. Tsiaoussis a, O. Kalogirou a,
a Department of Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
b Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
CoxPt1ꢀx nanostructures with varying composition and controllable morphologies have been
synthesized through the thermal decomposition of appropriate platinum and cobalt precursors in
organic solvents. The employment of several different surfactants facilitated the production of
nanostructures with various sizes and shapes including nanowires, flower-like structures and spherical
particles. The composition of the as-prepared nanomaterials ranged between Pt-rich and stoichiometric
CoPt alloy, mainly depending on the starting ratio of the precursors. Three-dimensional structures such
as the ‘flower-like’ ones showed a net ferromagnetic behavior, even at room temperature. In certain
cases, the alloy nanostructures were annealed in order to obtain the ‘hard’ fct-CoPt phase, which
displayed high coercivity values.
Received 12 March 2009
Received in revised form
6 May 2009
Available online 15 May 2009
Keywords:
CoPt
Nanostructure
Nanoparticle
Magnetic property
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
morphologies such as spheres, wires, rods etc. For such an
interesting system as Co–Pt alloy, it is challenging to investigate
the possible correlation of morphology with the obtained magnetic
properties. Besides, tailoring the morphology could influence the
potential applications of the nanomaterial. The reported configura-
tions include nanorods [10], nanowires [11], nanopolypods [12] and
hollow spheres [13] apart from the classic spherical particles [4].
The potential advantage of shape controlled M–Pt (M ¼ Fe or Co)
nanoparticles for high-density magnetic recording media over
existing media is that their high magnetocrystalline anisotropy
and anisotropic shapes allow alignment of their magnetic easy-axis
when deposited on self-assembled films [14].
In this work we describe the preparation of CoxPt1ꢀx alloy
nanostructures with morphologies that range from nanowires,
30nm flower-like structures as well as ꢂ3 nm spherical particles,
via the thermal decomposition of cobalt precursors and platinum
acetylacetonate in organic solvents. We demonstrate that CoxPt1ꢀx
alloy nanostructures can be produced with a wide range of
compositions and shapes, according to the specific synthetic
conditions. Parameters such as the concentrations of the starting
reagents and the reaction temperature are the key factors for the
structural properties of the nanomaterials. Basic magnetic char-
acterization of as-prepared or annealed nanoalloys was performed
for a better understanding of the nature of our products.
In recent years, L10-type materials have drawn special interest
due to their potential applications for ultra-high-density record-
ing media [1,2]. In particular, various efforts have concentrated
on the production of bimetallic magnetic nanoparticles such as
FePt [3], CoPt [4] and FePd [5], aiming to nanomaterials with good
chemical stability, enhanced magnetic susceptibility and large
coercivity. More specifically, CoPt alloy is one of the candidates for
high-density storage media also due to its corrosion resistance [6].
According to the phase diagram for bulk Co–Pt alloys [7], two
atomic order–disorder transformation regions exist, namely the
allotropic CoPt and CoPt3. Near equiatomic composition CoxPt1ꢀx
alloy exhibits
a disordered (A1) phase–ordered (L10) phase
transformation below 825 1C. The L10 phase with an ordered fct
structure is a hard magnetic phase with high magnetocrystalline
anisotropy (4 ꢁ107 erg/cm3). It should be considered that the
thermal treatment of CoPt and FePt nanostructures at 600–800 1C,
which is needed so as to get the desired ‘hard’ magnetic behavior,
is often accompanied by undesirable coalescence and sintering,
which hinders the direct applicability of the nanomaterials in
magnetic recording media [8]. However, some works concerning
the partial ‘in-situ’ transformation of the particles to the
compositionally ordered L10 phase without the need for post-
stage annealing have been reported [9].
Synthetic chemical methods have been proved to be very
effective for the preparation of nanostructures with various
2. Experimental
Ã
The bimetallic CoxPt1ꢀx nanoparticles were synthesized by
simultaneous reduction of Pt(acac)2 and decomposition of cobalt
Corresponding author. Tel.: +30 2310998148; fax: +30 2310998003.
E-mail address: orestis.kalogirou@physics.auth.gr (O. Kalogirou).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.05.014

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precursors in long-chain amines. Additional surfactants such as
1-adamantanecarboxylic acid (ACA) or trioctylphosphine-oxide
(TOPO) were occasionally used so as to control the growth of the
particles and hinder aggregation. Reaction temperatures varied
between 260 and 350 1C. The experimental details for each one of
the samples are shown in Table 1.
annealing were prepared by drop-casting from the colloidal
dispersions onto Si substrates. Certain samples were annealed in
powder form. The post-stage heating conditions included Ar flow,
a heating rate of ꢂ20 1C/min and a hold at 700 1C for 1 h.
Magnetization vs. field (M–H) plots were obtained applying a
superconducting quantum interference device (Quantum Design
MPMS-5 SQUID) and a vibrating sample magnetometer (1.2H/CF/
HT Oxford Instruments VSM) between 10 and 300 K.
In a typical procedure (e.g. sample S1 of Table 1), 177 mg
Co2(CO)8 (0.5 mmol) and 197 mg Pt(acac)2 (0.5 mmol) were
inserted in
a three-neck flask containing 30 ml oleylamine.
The mixture was degassed with Ar flow for 20 min under stirring.
Afterwards it was heated to 135 1C and remained at this
temperature for 1 h. The solution had started to get a darker
colour at 90–95 1C, compared to the initial blue-green colour due
to the precursors. Then the mixture was further heated up to
350 1C at a rate of 4 1C minꢀ1 and refluxed for 1 h. After cooling, the
solid product was isolated by centrifugation with acetone and
washed twice with a mixture of hexane and excess acetone.
Finally the particles were stored into hexane.
In order to acquire transmission electron microscopy (TEM)
images, drops of the colloidal solutions were placed on carbon-
coated copper grids. A 100 kV JEM 100CX microscope was used to
examine the morphological properties of the particles. Informa-
tion regarding the structural properties of the prepared nanopar-
ticles was received by X-ray powder diffraction (XRD) with a
3. Results and discussion
3.1. Morphology and structure
Previous works have already reported the successful isolation
of several different CoxPt1ꢀx nanostructures, via various synthetic
pathways. We managed to extend the possibility of fabricating
nanostructures with controlled morphologies and tuned physical
properties, by investigating the role of miscellaneous surfactants,
solvents and reducing agents. Fig. 1 shows TEM images for S1 and
S2 samples, which were produced using dicobalt octacarbonyl as
the source of Co. In Fig. 1a, we notice the formation of ‘nanowires’
in S1 with length that can reach microscale values and a thickness
of about 3–4 nm. Oleylamine, an organic surfactant which is
widely used in the preparation of nanoparticles by chemical
methods, seems to favor the one-dimensional growth under the
current synthetic conditions. It is likely that oleylamine self-
organizes into an elongated reverse-micelle-like structure within
Philips PW 1820 diffractometer using CuK radiation. Elemental
a
analysis was carried out by a 20 kV JEOL 840A scanning electron
microscope (SEM) equipped with an OXFORD ISIS 300 energy
dispersive X-ray spectroscopy (EDX) analyzer. Specimens for
Table 1
Starting reagents and reaction temperatures for the synthesis of CoxPt1ꢀx nanostructures with tuned morphologiesa.
Sample code
Co precursor
Pt precursor
Surfactants
Solvent
Reductant
Temperature
(1C)
S1 (Nanowires)
Co2(CO)8 (0.5 mmol)
Pt(acac)2
OAm (30 ml)
350
260
280
300
260
(0.5 mmol)
Pt(acac)2
S2 (Flower-like structures) Co2(CO)8 (0.3 mmol)
OAm, DOE (15 ml, 15 ml)
HDA, DPE (9 g, 6 ml)
TOAm (30 ml)
(0.45 mmol)
Pt(acac)2
S3 (Bimodal size
distribution)
Co(ac)2.4H2O
(0.6 mmol)
ACA (1.2 mmol)
HDOL
(0.2 mmol)
Pt(acac)2
(1.2 mmol)
S4 (Spherical particles)
Co(ac)2.4H2O
(0.75 mmol)
Co(ac)2.4H2O
(0.6 mmol)
ACA, TOPO (1.25 mmol,
0.25 mmol)
(0.5 mmol)
Pt(acac)2
S5 (Irregular shape)
ACA (1.36 mmol)
HDA, OAm, DPE (5.4 g, 15 ml,
9 ml)
HDOL
(0.25 mmol)
(1.2 mmol)
a
OAm ¼ oleylamine, TOAm ¼ trioctylamine, TOPO ¼ trioctylphosphine-oxide, ACA ¼ 1-adamantanecarboxylic acid, DOE ¼ dioctyl ether, HDOL ¼ 1,2-hexadecanediol,
HDA ¼ hexadecylamine, DPE ¼ diphenyl ether. HDA, OAm and TOAm are in some cases categorized as solvents, but they act at the same time as surfactants. When HDOL is
not used, the amines are also the reducing agents. Sample S4 was produced with a synthetic protocol that is described in Ref. [24].
Fig. 1. TEM images for the samples prepared by applying Co2(CO)8 as cobalt precursor [S1 and S2 samples, images (a) and (b), respectively].

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which the CoPt nuclei are formed. In a similar synthetic approach,
Wang et al. [15] have reported that FePt nanowires can
be produced by simultaneous decomposition of Fe(CO)5 and
Pt(acac)2 in pure oleylamine, without the use of any other
reagents. Besides, using the same reactants as in Ref. [15],
Shukla et al. reported the fabrication of Fe50Pt50 nanowires
which were 40–200 nm long and had a diameter of 1–2 nm [14].
It has to be noted that in our case, the nanowires were obtained
only by following a well-defined experimental procedure, which
involves the employment of a standard heating rate (4 1C minꢀ1),
hold at 135 1C for 1 h, intermediate stirring (500–700 rpm) and 1 h
aging at reflux temperature (350 1C). Even slight deviations from
the above procedure leads to the formation of more aggregated
and/or spherical nanostructures rather than nanowires. However,
by replacing the half amount of oleylamine with dioctylether,
which is a rather ‘inert’ organic solvent with a linear molecular
structure, ‘flower-like’ structures with an average size of ꢂ30 nm
were received, together with a minority of smaller ꢂ4–5 nm
particles (sample S2, Fig. 1b). The flower-like structures were
probably formed via oriented attachment of several smaller
nanocrystals during synthesis. These structures retain their
morphology even after repeating washing cycles, implying that
they are not ‘loose’ formations and certainly not a product
of random aggregation. Similar shapes have been reported for
nanoparticles such as Fe3O4 [16] and Pt [17], and the mechanism
of ‘oriented aggregation’ is often proposed for the development
of such morphologies, especially when bulky or long-chain
amines are used as surfactants. It seems that the role of
octylether in the ‘rarefaction’ of oleylamine is important for the
observation of this specific 3D morphology in S2. For instance,
when certain amounts of oleylamine were replaced by another
‘inert’ solvent, 1-octadecene, the result was simply the formation
of one-dimensional structures, but with a lower aspect ratio
(nanorods instead of nanowires) [15].
The samples S3–S5 were produced using cobalt acetate
tetrahydrate as Co-precursor, while keeping the same precursor
(platinum acetylacetonate) for Pt. TEM images for these samples
are depicted in Fig. 2. Concerning S3 sample (Fig. 2a), we notice a
bimodal particle distribution. There are small particles with an
average diameter at 3.1 nm and bigger ꢂ27 nm nanostructures. It
has to be noted that Co(ac)2 ꢃ 4H2O is a relatively stable precursor,
being decomposed at temperatures over ꢂ235 1C. For this reason,
the insertion of both precursors at room temperature results in a
somewhat ‘slow’ nucleation that is consistent with an increment
in final particle size. The formation of the larger ꢂ27 nm
structures is probably due to oriented attachment of the initially
formed smaller nanocrystals. The role of ACA as a surfactant
toward the production of 3D morphologies has been shown also in
other cases [16]. The bulky adamantyl end groups of ACA
molecule, due to their steric effects, probably protect a number
of free surface sites from being coordinated by ACA molecules, and
thus, increase the surface energy. The surface energy is not stable
in all the surface area, so the growth is favored in specific
crystallographic directions. Besides, the combination of ACA and
HDA has been reported to lead to 3D agglomerates of controlled
size, also in the case of NixPt1ꢀx nanostructures [18].
Fig. 2b illustrates the morphology of sample S4. This sample is
composed of monodisperse spherical particles, with an average
size of ꢂ3 nm. The shape of S4 particles can be attributed mainly
to the use of trioctylamine as the reaction solvent. Being a highly
bulky tertiary amine, trioctylamine has much different steric
effects compared to single-chain amines such as oleylamine or
hexadecylamine. Kim et al. [19] have observed that the sole use of
TOA during the preparation of GaP nanocrystals resulted in
spherical particles, while hexadecylamine induced one-dimensional
growth (nanorods). By varying the initial HDA/TOA ratio, they
were able to produce both spherical particles and nanorods.
Trioctylamine is a weak-bonding ligand and cannot always
prevent the aggregation of nanoparticles, despite its significantly
bulky structure, if not assisted by surfactants of different type.
Consequently, the use of surfactant mixture ACA/TOPO in S4
intended to provide a rigid ligand shell for the limitation of any
possible aggregation among the particles. Regarding the sample
S5 (Fig. 2c), the particles possess irregular shapes probably due to
the application of multiple surfactants. However, some tendency
to form multiangular morphologies is noticed. The average size of
S5 particles was measured at ꢂ18 nm.
The XRD spectra of the produced samples are presented in
Fig. 3. The arrows in XRD figures denote the ‘bulk’ reflections of
Co–Pt alloys, as well as pure Co and Pt elements and CoO. The
values for these reflections were derived from the JCPDS database.
For sample S1 the peaks at 40.41 and 47.01 indicate that the
nanowires are composed of CoPt3 alloy (Fig. 3a). For sample S2
(Fig. 3b) the main peak at ꢂ40.21 implies the existence of a
CoxPt1ꢀx alloy with xo0.25, as this peak is located at a somewhat
lower 2
(40.51). The annealing of sample S2 caused a shift of the prevalent
peaks toward higher 2 values, implying a partial structural
y value compared to the expected for the CoPt3 alloy
y
transformation to the fct-CoPt phase. Regarding S3 sample
(Fig. 3c), the basic CoPt3 peak at 40.41 for the as-produced alloy,
slides to 41.21 after annealing, revealing also in this case a partial
transformation to the compositionally ordered tetragonal CoPt
structure. Elemental analysis for S3 showed that the Co/Pt ratio in
the particles was close to 2/1. In agreement to this result, a weak
peak at 44.11 is observed indicating the existence of some portion
of fcc-Co in the as-prepared sample. Surprisingly, the annealing
conditions, though inert, led to some oxidation of free Co, as the
peak at 42.31 indicates the presence of a minority of CoO particles
in the annealed material. It is possible that the oxygen which was
needed for the partial oxidation of the particles originated from
the decomposing surfactant molecules during annealing process.
The XRD measurement of sample S4 is not shown, due to the bad
Fig. 2. TEM images for the samples synthesized by employing Co(CH3COO)2 ꢃ 4H2O as cobalt source [S3-S5 samples, images (a)–(c), correspondingly].

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Fig. 3. XRD patterns for S1, S2, S3 and S5 samples [(a), (b), (c) and (d) respectively].
quality of this pattern, reflected by high noise and amorphous
background. The small size of S4 particles as well as the high
amount of organic surfactants in this sample probably
deteriorated the clarity of this measurement. However, a Co/Pt
ratio of 56/44, as educed by elemental analysis (SEM–EDS), gives
the basic information concerning the composition of the sample
under discussion. Sample S5 appears to have
a significant
percentage of cobalt, also considering the large initial Co–Pt
ratio (refer to Table 1). The peaks at ꢂ41.21 and 48.01 imply the
presence of an alloy much richer in Co than CoPt3 (Fig. 3d). In the
annealed sample the main peaks are shifted to 42.41 and 49.41,
respectively, corresponding rather to a Co-rich alloy (e.g. Co3Pt),
taking into account Vegard’s law about bimetallic alloys. In this
particular sample (Fig. 3d), the post-stage heating treatment
resulted in a significant increase of the crystalline grain size, from
ꢂ8 nm for the ‘as-prepared’ particles to ꢂ21 nm for the sintered
product, as calculated using Scherrer’s formula.
Fig. 4. VSM loops for sample S1 (nanowires) at 80 and 300 K.
3.2. Magnetic study
for temperatures 80 and 300 K. We notice coercivity (Hc) of about
ꢂ95 Oe at room temperature which is raised up to ꢂ1.1 kOe at
80 K. This value for Hc is higher compared to coercivity values
Concerning the magnetic properties of the as-produced CoPt3
nanowires, Fig. 4 shows M–H plots for sample S1 obtained by VSM

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measured at 80 K for Co–Pt alloy nanowires prepared by
decomposition of Co2(CO)8 and Pt(acac)2 in ethylenediamine
[11]. In principle, the large coercivity of S1 at low temperatures
is due to the fact that ferromagnetic nanowires exhibit shape
anisotropy, as two-dimensional systems. The saturation magneti-
zation of S1, measured at ꢂ10 emu/g, is similar to corresponding
Ms values for other as-synthesized fcc-CoPt3 nanostructures
[20,21].
The ‘flower-like’ structures of S2 show ferromagnetism at
room temperature, probably due to their relatively large average
size (ꢂ30 nm). The ꢂ150 Oe coercivity at room temperature was
increased up to approximately 1 kOe after annealing, as measured
by VSM in both cases (Fig. 5). The annealed nanomaterial displays
a significantly higher magnetization (Msꢂ31 emu/g compared to
ꢂ10 emu/g for the ‘as-prepared’ S2), also explained by the partial
structural transformation (revealed by XRD measurement) and
the size increment that was caused by the post-stage thermal
treatment. The Ms value for S2 after annealing is in agreement
with respective literature values, where also a dramatic increase is
reported for the saturation magnetization of the post-stage heat
treated CoPt nanoalloys [12,22].
Fig. 6 shows SQUID magnetic measurements for sample S3. For
the as-prepared particles (Fig. 6a), a coercivity of ꢂ400 Oe was
received from room-temperature hysteresis loops, indicating a
clear ferromagnetic character. The coercivity was increased up to
ꢂ1.4 kOe at 10 K. These coercivity values imply a sufficient Co–Pt
alloying, support the XRD results about the nature of the particles
and argue against a sum of separate Co and Pt elemental phases.
Similar values for the coercivity of the as-prepared alloy (525 Oe
at room temperature, 1.2 kOe at 5 K) were observed for the
3D CoPt nanostructures synthesized by Tzitzios et al [12]. The
saturation magnetization of the as-prepared S3 was measured at
ꢂ43 emu/g at 10 K. Regarding the annealed S3 particles (Fig. 6b), a
room-temperature coercivity of 3.3 kOe was extracted from the
hysteresis loops. Thus, the partial transformation to the highly
ordered fct phase shown by XRD measurements is endorsed by
the SQUID results, through the observation of such a notable
increase in coercivity. The remanence-to-saturation ratio Mr/Ms
was ꢂ0.7 at room temperature, reaching 0.8 at 10 K. Concerning
the low-temperature (10 K) coercivity for the annealed S3
particles, we notice a value of 6.7 kOe, compared to 1.4 kOe at
10 K for the as-prepared particles. In the low-temperature
measurement (10 K), the loop for the annealed S3 possesses
a somewhat ‘constricted’ shape, which is attributed to the
composition of this sample. In particular, this kind of hysteresis
behavior reveals the co-existence of two magnetically different
phases in the material. The peculiar bending noticed in Fig. 6b
indicates the presence of a ‘hard’ magnetic phase along with a
‘soft’ one. It is reasonable to assume that most of the particles
of the annealed S3 sample show ‘hard’ magnetic characteristics,
thus resulting in the observed large coercivity, but there is also
a minority of particles that despite the annealing maintain ‘soft’
magnetic character which however cannot seriously influence
the overall magnetic behavior of this sample. The ‘soft’ magnetic
contribution could be attributed to a portion of oxidized particles
(shown by XRD analysis, Fig. 3c) as well as to a part of the alloy
particles which did not transform to the fct-CoPt phase, even after
annealing.
Sample S4 consists of quite small particles (davꢂ3 nm). This
size, for instance, is far below the superparamagnetic limit of
CoPt3 phase (Dcrꢂ9 nm) [23]. Magnetization presents low values
(o5 emu/g), even at low temperatures, without being saturated
even at the highest applied magnetic field. The non-zero value for
Fig. 5. Room-temperature M–H plots for sample S2 before and after annealing.
Fig. 6. M–H plots for the as-prepared S3 particles [powder, image (a)], and the annealed S3 particles [drop-casted on Si substrate, image (b)].

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employment of amines as high-boiling-point solvents, surfactants
and reducing agents simultaneously, facilitated the production of
CoxPt1ꢀx nanostructures with sufficient yield in Co–Pt alloy in the
majority of the samples. The highly bulky trioctylamine resulted
mainly in spherical particles, while oleylamine induced one-
dimensional growth and favored the formation of ‘in-situ’ self-
assembled flower-like nanostructures or nanowires. Additional
surfactants such as ACA and TOPO assisted also the control of the
morphology of the particles, by influencing their shape and
improving the degree of separation among each other. Depending
on the composition of the samples, annealing at 700 1C in inert gas
was proved to provoke the transformation to more ‘ordered’
phases (fct-CoPt), with ‘harder’ magnetic response, as shown
by a remarkable increase in coercivity values, even at room
temperature.
Acknowledgements
Fig. 7. Room-temperature M–H plots for sample S5 with and without annealing
treatment.
This work was funded by the General Secretariat for Research &
Technology of Greece (Code: PENED03667) and the European
Union Marie-Curie RTN ‘Syntorbmag’(Contract no.: MRTN-CT-
2004-0055667).
coercivity (o100 Oe in all temperatures), could be attributed
to a slight ferromagnetic contribution due to the existence of a
minority of equiatomic CoPt particles, as the superparamagnetic
limit of the CoPt-phase is at ꢂ2.8 nm. In any case, it has to be
noted that in general, the size decrease in CoxPt1ꢀx nanoparticles
is associated with a deterioration of the overall magnetic behavior
of the material. As an example, Shevchenko et al. recorded
the reduction of Ms and Hc values in CoPt3 nanoparticles, by
decreasing their size from 9.4 to 6.6 and further to 3.8 nm [21].
Further details concerning the magnetic behavior of such small
and larger spherical CoxPt1ꢀx particles can be found in our
previous relevant work [24].
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4. Conclusions
The current work was focused on the tailoring of the
morphology of CoxPt1ꢀx nanostructures, which had also variable
compositions, depending on the synthetic parameters. The