CoPt magnetic nanocrystals in the A1/L10 transformationArnaud Demortière and Christophe Petit Citation: J. Appl. Phys. 109, 084344 (2011); doi: 10.1063/1.3575333 View online: http://dx.doi.org/10.1063/1.3575333 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v109/i8 Published by the American Institute of Physics. Related ArticlesSelf-assembly of single dielectric nanoparticle layers and integration in polymer-based solar cells Appl. Phys. Lett. 101, 063105 (2012) Modification of ultrananocrystalline diamond film microstructure via Fe-coating and annealing for enhancement ofelectron field emission properties J. Appl. Phys. 112, 033708 (2012) Synthesis and properties of phase-change Ge-Sb nanoparticles J. Appl. Phys. 112, 034308 (2012) Formation of monomer to tetramer Ag nanodots in a vanadium oxide nanomesh on Pd(111) J. Appl. Phys. 112, 034902 (2012) Application of well-defined indium tin oxide nanorods as Raman active platforms Appl. Phys. Lett. 101, 053117 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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CoPt magnetic nanocrystals in the A1/L10 transformation

Arnaud Demortierea) and Christophe Petitb)

Laboratoire des Materiaux Mesoscopiques et Nanometriques (LM2N), UMR CNRS 7070, Universite Pierreet Marie Curie (UPMC), 4 place Jussieu 75252 Paris Cedex 05, France

(Received 11 November 2010; accepted 13 March 2011; published online 27 April 2011)

CoPt magnetic nanocrystals have been synthesized in a range of size from 2 to 4 nm using a

colloidal chemical synthetic method. As-synthesized nanocrystals exhibit an equiatomic

composition and a crystalline phase of type alloy CoPt A1 disordered. The morphology and the

magnetic properties were investigated for different crystal size. In order to reach the crystalline

transition toward the L10 ordered phase, a thermal annealing has been carried out on CoPt

nanocrystals of 4 nm. We showed that the use of HOPG substrate both limits the coalescence

effects and promotes the structural ordering. The modification of the crystalline structure selected

area electron diffraction (SAED) and the magnetic properties (superconducting quantum

interference device) has been studied for different heat-treatment conditions. Ferromagnetic

properties at room temperature were achieved for nanocrystals of 16 nm in average size with a

coercivity of 4 kOe. VC 2011 American Institute of Physics. [doi:10.1063/1.3575333]

I. INTRODUCTION

The magnetic alloy nanocrystals (NCs) are promising

materials due to their strong potential in the development of

catalytic,1 biomedical,2 and magnetic3 applications. In this

last domain, bimetallic alloys such as CoPt, FePt, or CoRh

represent a particularly interesting class of materials for the

improvement of the recording on magnetic storage systems.

The nanometer size, the narrow size distribution and the

alloying effects of these materials could lead to areal density

greater than 1 Tbit/in2.4 Among these nanoalloys, L10-

ordered CoPt NCs are good candidates for future high-density

recording media based on the perpendicular recording

(PMR).5 The L10-ordered phase obtained at equiatomic com-

position presents a high magnetocrystalline anisotropy

(4.9� 107 erg/cm3) and a high coercivity (10 kOe), which

enable to exceed the superparamagnetism barrier at a critical

size around of 5 nm.6 Recently, some progresses have been

achieved in the elaboration of these alloy NCs, using pulsed

laser depositions7,8 or thermal decomposition methods,9,10

with even the observation of a partial in situ transformation of

NCs toward the L10 phase without the need of post-stage

annealing. Chemical routes present the advantage to highly

control of the size, shape and composition of NCs. Moreover,

this bottom-up approach yields to low polydispersity of the

nanocrystal size, which can give rise to phenomena of self-

organization and also, the specific morphologies, such as

rods11 and cubes,12,13 can favor the arrangement of magnetic

easy axes. However, CoPt nanocrystals synthesized by colloi-

dal route have an A1-disordered structure. A thermal anneal-

ing is required to transform into ordered structure with a

tetragonal unit cell having a very large anisotropy.14,15 For

supported nanoparticles, the thermal assistances at high tem-

perature (> 300 �C) can lead to coalescence and sintering

effects, which increase the size, modify the shape and destroy

their organization. Indeed, the coalescence driving force and

the change of shape is the minimization of surface energy by

elimination of interfaces and appearance of grain boundaries

and defects. Thus, the elaboration of well-defined nanopar-

ticles in the L10 ordered phase remains very difficult. Further-

more, the nature of the substrate, the positions of crystals on

the support and the organic shell of nanoparticles can affect

the coalescence effects and the ordering process. The study of

the evolution of magnetic properties of CoPt NCs requires a

good knowledge both of magnetic behavior of as-synthesized

NCs and, the crystalline structure and morphology of annealed

nanoparticles.

This paper deals with general study of structural and

magnetic properties of as-synthesized and annealed CoPt

NCs. The size-dependent properties of Co50Pt50 NCs from 2

to 4 nm have been studied. We have focused on both crystal

morphologies and magnetic behaviors. Then, the post-

annealing treatment of 4 nm CoPt NCs have been carried out

from 350 to 500 �C and with two different types of substrate:

amorphous carbon and HOPG. The crystalline ordering and

the coalescence effects have been investigated. The L10

phase has been characterized by both electron diffraction

and high resolution TEM. Finally, magnetic measurements

showed ferromagnetic properties at room temperature for

NCs of 16 nm.

II. EXPERIMENTAL DETAILS

A. Chemical synthesis

Cobalt-platinum nanocrystal samples have been prepared

using a colloidal approach based on the co-precipitation reac-

tion on a liquid-liquid phase transfer method.16 The metallic

salts of CoCl4(TDA)2 and PtCl6(TDA)2 (6.25� 10�3 M) have

been obtained by phase transfer from aqueous solutions of

CoCl42� and PtCl6

2� and organic solutions (toluene) contain-

ing TDAB (tetrakisdecylammonium bromide) (2 g) using as

transfer agent. After discarding of the aqueous phases, both

a)Electronic mail: ademortiere@anl.gov.b)Electronic mail: christophe.petit@upmc.fr.

0021-8979/2011/109(8)/084344/9/$30.00 VC 2011 American Institute of Physics109, 084344-1

JOURNAL OF APPLIED PHYSICS 109, 084344 (2011)

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metallic salt complexes have been stocked in the toluene me-

dium at room temperature. Then, 10 mL of platinum complex

organic solution [PtCl6(TDA)2] and 10 mL of cobalt complex

organic solution [CoCl4(TDA)2] were added to 20 mL of tolu-

ene. After addition of 1 mL of octylamine (ligand), an aque-

ous solution of reductant NaBH4 (378 mg, 10 mL of H2O)

was introduced into the organic mixture with rapid stirring.

The introduction of the reducing solution have been per-

formed by sequence injection of 10 lL every 10 and 20 s. The

total introduction in once of the reducing agent is defined as

the injection at 0 s. The solution is kept under stirring during

16 h after the end of NaBH4 introduction. This method allows

us to control the kinetics of growth and favor the increase of

the nanocrystal size. Thus, NCs obtained at 0, 10, and 20 s

(injection time) exhibit an average diameter respectively of 2,

3, and 4 nm. This method allows an accurate composition con-

trol around Co/Pt equiatomic position with a narrow distribu-

tion of 6 2%.17 Then, the organic phase was evaporated,

ethanol was added in excess and the nanoparticles were pre-

cipitated by centrifugation. This procedure was repeated

twice, and finally, the nanoparticles were dispersed in 4 mL of

toluene.

B. Thermal treatment

Two different types of substrate have been used to per-

form the thermal treatment. Samples of 2D self-assembled

CoPt NCs were prepared by drop deposition method on the

copper grids of TEM coated respectively by amorphous car-

bon and by HOPG (highly ordered pyrolytic graphite). The

concentration of the colloidal solution has been tuned in

order to obtain a monolayer of NCs (1� 10�3 M). TEM

grids have been stuck in the corner of a glass wafer using a

carbon adhesive. Thus, during the annealing, the main part

of the substrate was kept outside of the support. The samples

have been annealed under nitrogen atmosphere at different

temperature from 350 to 500 �C with a temperature rate of

50 �C/min. and a treatment time of 10 min. After the anneal-

ing, the sample was quickly put out to the oven and kept at

room temperature to cooldown.

C. Transmission electron microscopy

A TEM JEOL 1011 (100 kV) was used to analyze as-

synthesized NCs and post-treated NCs. Electron diffraction

(SAED) has been used to study the crystalline structure. The

local structural analyses have been performed by HRTEM,

before and after annealing, using a JEOL 2010 UHR instru-

ment operated at 200 kV (LaB6). To determine the average

size <D> and the standard deviation r, 500 NCs were meas-

ured. A lognormal distribution was used to describe the size

distribution with the polydispersity index defined as the ratio

r/<D>.

D. Magnetic properties

The magnetic measurements were performed with a

superconducting quantum interference device magnetometer

SQUID. The samples have been prepared on HOPG sub-

strates in the same deposition conditions that TEM samples

to obtain a film of CoPt NCs (monolayer). The thermal treat-

ments have been carried out in the similar condition that for

TEM grids. A magnetic field of 75 G was applied during the

zero field cooling (zfc) and field cooling (fc) measurements.

To obtain the zfc curves, the samples were cooled down

under zero field up to 2.5 K. Then, a magnetic field of 75 G

was applied and the zfc curve was measured to increase the

temperature from 2.5 to 300 K. The fc curves were measured

with the same magnetic field in decreasing the temperature

from 300 to 2.5 K. Magnetic hysteresis loops were recorded

at 3 K with a magnetic field applied parallel to the monolayer

film.

III. COPT NCs IN THE A1 DISORDERED PHASE

A. Structure and morphology of as-synthesized NCs

The colloidal synthetic route used in this study enables

us to prepare samples of Co50Pt50 NCs in three different

sizes: 2, 3, and 4 nm. The control of the average size was

performed by tune of the growth kinetics, which is induced

by increase of the injection time of the reducing agent.16

Figures 1(a) and 1(b) show high-resolution TEM images of

two characteristic single CoPt NCs of 2 nm with two differ-

ent morphologies. HRTEM observations reveal well faceted

and crystallized nanoparticles. The first image exhibits a dec-

ahedral shape corresponding to a fivefold symmetry polyhe-

dron with the {111} facets, which is confirmed by the

Fourier transform [Fig. 1(a)]. This morphology can be con-

sidered as a packing of five similar crystalline blocks of face

centered cubic (fcc) structure separated by an imperfect

twin.18 The second image shows a different morphology that

seems to be a truncated octahedron (monocrystal) which is

an equilibrium shape of the fcc crystal. These shapes are the

two main morphologies observed for this sample of 2 nm

with around of 20% of decahedron and 80% of truncated oc-

tahedron (estimation on 300 particles). This variety of shape

is in agreement with many works on the stable shape of NCs

at very small size.19,20 Figures 1(c) and 1(d) show HRTEM

images of NCs of 3 and 4 nm in size, respectively. In these

two samples, the majority of morphologies is identified as

being a truncated octahedron (or cuboctahedron). The

FIG. 1. (Color online) High-resolution TEM images of CoPt NCs of 2 nm

(a, b), 3 nm (c), and 4 nm (d).

084344-2 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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Fourier transform exhibits the (111) and (200) reflections

corresponding to the A1-disordered phase, respectively at

2.1 and 1.9 A. From 3D graphic representation, the position

and the crystalline facets, {111} (blue) and {100} (white),

can be roughly estimate. Furthermore, in both cases, contrary

to the first sample, the decahedral shape was not observed.

These simple observations seem to show that stable mor-

phologies dependent on the size of NCs.

The classic framework used to understand the morphol-

ogy of crystals has been developed from the theory of

Gibbs�Curie�Wulff.21 This one indicates that the most sta-

ble morphology for fcc NCs (monometallic) is either the

cuboctahedron or the truncated octahedron. Favre et al.22

have shown in a simple calculation of surface tension of

cobalt and platinum (fcc) that the truncated octahedron could

be the most stable shape at 2 nm for CoPt NCs in vacuum.

Here, we show clearly that the evolution of the morphology

is function of the size of NCs. Indeed, the truncated octahe-

dral shape is present for all sizes whereas the decahedral

shape is only observed for the smallest size. Gruner et al.have studied the multiply twinned morphologies of bimetal-

lic FePt and CoPt NCs both in the A1 disordered and L10 or-

dered phase.23 Their ab initio calculations based on the

geometric relaxations on the Born�Oppenheimer surface,

demonstrated that for the smallest cluster the most favorable

morphology is the icosahedron. On the same way, Ferrando

et al. have demonstrated using ab initio simulations in vac-

uum that the size of Pt and CoPt clusters affects theirs mor-

phologies.24,25 However, they have shown that the

decahedral shape is not stable for the larger size of CoPt

nanoalloys.25 Recently, experimental and simulation investi-

gations have shown the effects of temperature on the struc-

ture and morphology of CoPt NCs with the growth at room

temperature of the nonequilibrium icosahedral morphol-

ogy.26 In our case, we observe that the decahedral shape is

stable at 2 nm but not at 4 nm, which is consistent with struc-

tural calculations. It should be noticed that the decahedral

shape and its evolution with size was never observed experi-

mentally for CoPt nanoalloys obtained by colloidal

chemistry.

However, contrary to the theoretical prediction for the

disordered CoPt NCs,27 we did not observe any icosahedral

shape in our samples. In the other side, the growth mecha-

nism should be taking into account in the understanding of

the final stable morphology.28 The syntheses of different

samples have been performed by control of kinetics condi-

tions that can drastically affect the stability of nuclei and

lead to a modification of the shape. Furthermore, it is worth

noticing that this type of synthesis in liquid systems system-

atically implies coating ligands, which interact with the

surfaces of the nuclei during the growth process. This is not

considered in the calculation mentioned above where the

surfaces of the cluster are free. Nevertheless as amine-coat-

ing ligand used here are weakly bound to the NCs29 their

effect on the equilibrium shape is low. Thus our results are

in good agreement, for the largest size, with the theoretical

works on “naked” cluster in vacuum. The relationship

between the size and the morphology of NCs has been

clearly observed but remains hard to understand and control

in the case of colloidal systems, because that depends of sev-

eral crucial parameters such as the kinetics growth or ligand

effects.30

B. Magnetic properties of as-synthesis CoPt NCs

SQUID measurements have been performed on these

three previous samples of CoPt NCs. As showed in Fig. 2,

the three zfc curves exhibit a maximum, usually defined as

being the TB blocking temperature. From the TB values

showed in Table I, the total magnetic anisotropy energy KTVcan be estimated by the relation KV ¼ kBTB lnðs=s0Þ

FIG. 2. Zero field cooled (zfc)/field cooled (fc) curves measured (circles)

and calculated (full lines) of CoPt NCs of 2 nm (a), 3 nm (b), and 4 nm (c).

Inset: hysteresis loops measured at 5 K.

084344-3 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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� 28kBTB, where V is the particle volume, K is the magnetic

anisotropy energy per volume unit, s measurement time (100

s) and s0¼ 10�10 s. As expected for the A1 phase, whatever

the size, CoPt NCs shows a superparamagnetic behavior at

room temperature. The ferromagnetic behavior is observed

below the blocking temperature, which decreases with the

decrease of the nanocrystal size. However, the magnetocrys-

talline anisotropy K increases drastically up to 30.7� 105

erg.cm�3 (Table I). This behavior is usually explained by

surface effects, which are extremely important at 2 nm,

where 67% of atoms are located at the surface. The number

of atoms by nanocrystal was calculated to take into account

the TEM size, the lattice parameter of 3.8 A (A1-Co50Pt50)

and the truncated octahedron as morphology. The high influ-

ence of surface atoms, which have coordination weaker than

the atoms of the core, induces an important modification of

the magnetic anisotropy energy.31

In addition to the estimation obtained from TB values,

the magneto-crystalline anisotropy can be quantitatively

estimated from a theoretical model developed by Gittleman

et al.32 This model has been used to fit the experimental

zfc/fc curves and so analyze the data with more details.

Here, we assume that CoPt nanoparticles have a uniaxial an-

isotropy, a random orientation and a system of noninterac-

tion magnetic particles. Indeed, a simple calculation of the

dipolar interaction energy between two particles, i.e.,

Edd=kB ¼ l0=4pkBð Þ m2=d3ð Þ (d being the center-center dis-

tance), has shown that in our systems the magnetic interac-

tions are very weak and can be assumed as negligible: 0.8

K (2 nm), 2 K (3 nm), and 4 K (4 nm). The magnetic sus-

ceptibilities can be defined as the sum of the superparamag-

netic and blocked contributions, as shown in the following

expressions:31

vzfcðTÞ ¼ M2SðTÞ

3kBT

ðVbðTÞ

Vmin

qðVÞV2dV þM2SðTÞ3K

ðVmax

VbðTÞqðVÞVdV;

(1)

vfcðTÞ ¼ M2SðTÞ

3kBT

ðVbðTÞ

Vmin

qðVÞV2dV þM2SðTÞ3K

lnsmes

s0

� �

�ðVmax

VbðTÞqðVÞVdV; (2)

where Vb(T)¼ (kBT/K) ln(smes/s0) is the blocking volume

with smes¼ 100 s and s0¼ 10�10 s, Vmin and Vmax are the

minimum and maximum volumes of the distribution, Ms(T)

saturation magnetization, q(V) is the volume distribution

defined as being a log-normal function. The standard devia-

tion of q(V) is given by r of the diameter distribution

obtained by TEM measurements. We have assumed that the

energy anisotropy K was not dependent to the temperature.

Then, we have approximated that the spin wave low Ms(T)

was approximated as being constant with the temperature,

which is acceptable in a range of temperature from 3 to 50

K. The fit has been performed for the three zfc/fc curves, as

showed in Fig. 2. These fits allow us to extract the effective

anisotropy constants, see in Table I. These values are lower

and in disagreement with those estimated from the TB values.

However, fit curves have been calculated with the size distri-

bution, the anisotropy value obtained should be more realis-

tic than in the case of calculation from only a value of TB. As

showed in Fig. 2, the fit curves for samples of 3 and 4 nm

match well enough with the experimental zfc/fc curves,

whereas for the sample 2 nm the mismatch is important. For

a size of 2 nm, the influence of surface effect with 67% of

atoms on the surface and the presence of decahedron struc-

ture impact strongly the atom coordination and easy axis. In

fact, the model used here does not take into account these

different aspects, which influence significantly the global

magnetocrystalline anisotropy. We can see clearly the limit

of this simple model. A more complex model is needed in

these nanoscale systems such as the model of Preisach–Neel

that is based on a nonlinear Stoner–Wohlfarth energy barrier

hypothesis.33 Insets of the Fig. 2 show the hysteresis loop of

three samples measured at 3 K from �2T to 2T. This con-

firms the ferromagnetic behavior at 3 K. These measure-

ments exhibit an usual increase of the Hc coercivity and

the Mr remanence with the raise of the nanoparticle size.

Moreover, we can see that the aimantation is nonsaturation.

This can be explained by the proximity of the measure tem-

perature (3 K) with the blocking temperature, show that all

nanoparticles are not blocked and then present a superpara-

magnetic behavior, especially for the smallest CoPt NCs.

IV. COPT NANOCRYSTALS IN L10 ORDERED PHASE

A thermal annealing is required in order to reach the

L10 ordered phase. As only CoPt NCs larger or equal to

<D>¼ 4 nm are expected to be ferromagnetic at room tem-

perature, we have focused on the transition at this size. The

annealing was performed at different temperatures from 350

to 500 �C for 10 min and, under an inert flowing nitrogen

atmosphere, the NCs being deposited on two different types

of TEM grid. It has been previously shown for colloidal

CoPt NCs that these annealing conditions with a high heating

rate minimize the coalescence effects.34 Figures 3(a) and

3(d) show TEM images of the cobalt-platinum NCs. The size

distribution presents an average diameter of 4 nm with a pol-

ydispersity of 9% [Fig. 3(c)]. NCs deposited on amorphous

carbon (A-Carbon) exhibit a tendency to self-assemble in a

local 2D hexagonal [Fig. 3(b)] but also many vacant zones.

Figure 3(d) shows NCs deposited on HOPG with large and

homogeneous 2D self-assembly. It is worth noticing that the

substrate nature influent the self-assembly process of NCs, as

TABLE I. Average diameter, polydispersity, blocking temperature, anisot-

ropy constant, atom numbers, percentage of surface atoms, and magnetic

moment.

<DTEM> (nm) 2 3 4

Polydispersity (r/D) 15% 11% 9%

TB Blocking temperature (K) 3.3 5.6 9.0

K (from TB) (105 erg.cm�3) 30.7 15.4 10.5

K (from fit) (105 erg.cm�3) 10.3 7.2 5.2

N atoms (% surface) 305 (67%) 1030 (50%) 2442 (33%)

m moment/NP (lB) 285 637 1176

084344-4 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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already observed in the case of cobalt or silver NCs.35,36 The

HOPG substrate has a high crystalline surface that favors the

assembly of NCs on the surface compared to A-Carbon. Our

approach here is that the nature of the substrate and the lay-

out of NCs on the substrate can impact the diffusion and coa-

lescence effect during the thermal treatment.

A. Thermal treatment: A1/L10 structural transition andcoalescence effects

The structural ordering, distortion of the self-assembly

and agglomeration of NCs have been observed by TEM and

electron diffraction (SAED). Figures 4 and 5 show TEM

images and SEAD patterns of CoPt NCs annealed at 350,

400, 450, and 500 �C, and respectively deposited on the

TEM grids of ACarbon and HOPG. These analyses show

first that the thermal treatment leads to coalescence and sin-

tering effects, which induces a modification of the size and

shape, as well as a distortion of the monolayer of NCs. The

evolution of nanocrystal size with the annealing temperature

is presented on the Table II. Below 400 �C, TEM images ex-

hibit a low and quasisimilar increase of nanoparticle size for

both substrates. From 450 �C, a strong raise of the average

size is observed, which drastically differs between HOPG

and ACarbon substrates [Figs. 4(c) and 5(c)]. The coales-

cence effects appear weaker by using the HOPG substrate,

FIG. 3. TEM images of CoPt NCs (4 nm) on TEM grid of amorphous car-

bon (a) and HOPG (b).

FIG. 4. (Color online) TEM images and SAED patterns of CoPt NCs on

amorpheous cabone substrate after annealing treatment at 350 �C (a,e),

400 �C (b,f), 450 �C (c,g), 500 �C (d,h).

084344-5 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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where the final size reach 16.4 nm against 34.6 nm for ACar-

bon. Moreover, for ACarbon, the CoPt sample becomes

almost a heterogeneous film whereas for HOPG, CoPt NCs

keep separate and form well-faceted crystals.

First, we can assume that the annealing destroys or

desorbs the capping molecules surrounding the NCs. Then,

the diffusion of the supported NCs onto the substrate occurs

during the annealing process, inducing the agglomeration of

crystals. Here, the strong difference between HOPC and

ACarbon demonstrates clearly that the substrate plays a key

role during the thermal treatment. This can be related to the

diffusion rate of NCs onto the surface according to the nature

of substrate. From the solid state diffusion, the coalescence

process can be described by the sc characteristic coalescence

time, defined as sc ¼ 3kBTNð Þ= 64prSDðTÞ½ �, where T is the

temperature, N atoms in the particle pair undergoing coales-

cence, rS surface tension, and D(T) is the solid state diffu-

sion coefficient.37 Thus, the diffusion is a function of

temperature, diffused material and support type. Here, as the

diffusion coefficient of CoPt NCs on the surface depends on

the nature of the substrate, the coalescence effects change

with the use of HOPG (ordered surface) or ACarbon (amor-

phous surface).

The layout of NCs onto the substrate before the anneal-

ing has also to take into account. The level of ordering and

the capacity of 2D monolayer are a crucial parameter. Indeed,

as showed recently, the periodic assembly in 2D monolayers

confers to metallic NCs an “immunity” to the physical stress

blocking the coalescence process.38 In our case, the Fig. 3(b)

shows that monolayers deposited on HOPG substrate present

a high level of organization, which can induce a higher resist-

ance during the annealing process. Thus, these observations

show that a suitable choice can drastically reduced the coales-

cence effects. Here, though the monolayers are strongly

modified at high temperature, the use of HOPG allows limit-

ing the coalescence. This leads to the formation of individual

well-faceted CoPt NCs with an average size of 16.4 nm and a

polydispersity of 24%. This process appears without chang-

ing the composition. For both substrates, EDX composition

analysis shows that the annealed samples have kept an equia-

tomic composition after the thermal treatment and the

agglomeration of NCs.

The structure of annealed CoPt NCs has been character-

ized by electron diffraction (SAED), as shown in Figs. 4

and 5. For the annealed samples at Ta¼ 350 and 400 �C, the

SAED patterns exhibit diffraction rings indexed as corre-

sponding to a A1 disordered CoPt structure (fcc), (111),

(200), (220), (311) d-spacings. At these annealing condi-

tions, the onset of the ordering transition is not reached. For

both substrates, SAED patterns show the appearance of a

new structure from Ta¼ 450 �C. The indexation of different

diffraction rings corresponds to a L10 ordered CoPt structure

(tetragonal), as indicated in Figs. 4(g) and 4(h) and 5(g) and

5(h). From the SAED patterns, a and c lattices parameter

have been calculated from the positions of (110) and (200)

rings, respectively. Then, the ordering parameter can be cal-

culated as following: S ¼ ð1� c=aÞ=ð1� 0:973Þ,39 where cand a are the unit cell parameters obtained by electron dif-

fraction, that allows to quantify the degree of tetragonal

ordering. For the samples annealed at 450 �C, we obtained

S¼ 0.47 and S¼ 0.54 for ACarbon and HOPG substrate. For

FIG. 5. (Color online) TEM images and SAED patterns of CoPt NCs on

HOPG substrate after annealing treatment at 350 �C (a,e), 400 �C (b,f),

450 �C (c,g), 500 �C (d,h).

TABLE II. Average size and polydispersity at different annealing tempera-

tures for the samples deposited on ACarbon and HOPG.

A Carbon HOPG

Ta (�C) Size (nm) Polydis. (%) Size (nm) Polydis.(%)

350 5.3 7 4.6 8

400 7.1 10 7.2 13

450 17.3 18 12.7 19

500 34.6 26 16.4 24

084344-6 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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Ta¼ 500 �C, we obtained S¼ 0.58 and S¼ 0.65, respec-

tively. It is worth noting that this data set is only indicative

values to evaluate the degree of ordering because the estima-

tion of c and a lattice parameter from SAED patterns

remains difficult. Nevertheless, these results show a differ-

ence of the ordering parameter between both used substrates.

The HOPG seems to favor further the ordering compared to

the A-Carbon, keeping in the same time the average size

smaller due to the limited coalescence process. These two

effects are probably related; new investigations are in pro-

gress understand this relationship. For both cases, the onset

of the A1/L10 transition appear from Ta¼ 450 �C and con-

tinue at Ta¼ 500 �C. However, at 500 �C the transition is not

complete, the maximum reached being S¼ 0.65. The order-

ing process is a kinetic process that includes atomic jumps

both on the surface and in the bulk of the NCs. As the coales-

cence effect, the ordering process can be defined by the so

characteristic ordering time, so ¼ k2=Ddiff , where k is the

mean diffusion length and Ddiff is the diffusion coefficient.

From this model, the A1/L10 transition temperature has been

estimated at 550 �C.40 Here, a transition has been observed at

Ta¼ 450 �C. This gap can be explained by the small size of

NCs (4 nm) where the surface is major. Indeed, Buffat et al.shown that the decrease of the nanoparticle size can drasti-

cally modify the thermodynamical conditions, as example

the fusion temperature.41

Figure 6 exhibits high resolution TEM images of CoPt

nanocrystals after a annealing at 500 �C on HOPG substrate.

The insets of Figs. 6(a) and 6(c) give the power spectrum of

each image where the indexed reflections corresponds to

d-spacing (001), (002), and (001), (200), respectively. These

data are characteristic of the tetragonal L10 structure, which

is consistent with the previous structural analyses. Further-

more, no clear relationship between the nanocrystal size and

the ordered structure has been found in our study for the

annealed NCs. Figure 6(b) presents an annealed nanocrystal

of hexagonal shape and lattice fringes with an interdistance

of 1.9 A corresponding to d-spacing (200). Finally, Fig. 6(d)

exhibits a nanocrystal with a stacking fault, which is induced

by the recrystallization after or during the annealing.

The coalescence and sintering effects induce re-crystal-

lization of annealed NCs and favor other stable morpholo-

gies. As shown in a recent work of the Penuelas et al., the

coalescence effects can strongly influence the morphology of

CoPt NCs.42 However, these effects occur on the surface of

the substrate and, thus the epitaxial effects can appear and,

orient the crystalline growth and the obtained nanoparticle

shape. The epitaxial effects have not been clearly observed

on all samples. However, it has been recently observed a

quasiepitaxial growth process on HOPG for similar metallic

NCs during a mild annealing process.43 From HRTEM

images, NCs seem to have a low thickness normal to the sub-

strate, thus forming kinds of platelets, which could be an in-

dication of “oriented growth” during the ordering process.

B. Evolution of magnetic properties after theannealing process

Magnetic measurements have been performed using a

SQUID magnetometer for the samples annealed at various

temperatures and deposited on HOPG. The zero field cooling

(zfc) and field cooling fc curves of CoPt NCs samples

annealed from 300 to 500 �C are plotted in Fig. 7. The block-

ing temperature TB raises with the increase of the annealing

temperature, as following: 17, 23, 38, 58, and above 290 K

(see Fig. 7). This TB evolution is related to both increase of

nanocrystal size due to the coalescence effect and increase of

the Ka magnetocrystalline anisotropy due to the structural

change from A1 to L10. First, the increase of the average

size is accompanied by an increase of the size distribution

(see Table II), which induces a broadening of the zfc curve

and raises the Tirr irreversible temperature (e.g., the tempera-

ture for which zfc and fc curves join each other). Second,

these behaviors are also due to both an increase to the Ka ani-

sotropy and to an enlargement of the distribution of Ka,

which contribute also to the raise of Tirr. For Ta¼ 500 �C, the

drastic shift of TB toward the highest temperature is in agree-

ment with the crystalline ordering of CoPt NCs. Here, we

obtained CoPt NCs of around 16 nm (24% in polydispersity)

that are ferromagnetic at temperature very close to the room

temperature (above 290 K). Furthermore, Fig. 8 show the

plot of the inverse of the susceptibility, a linear fit has been

performed on the paramagnetic part. From the Curie�Weiss

law, we can extract of this fit the Tw Weiss constant value

(intersection of the dotted line on Fig. 8 with the abscissa

axis), which gives an indication of magnetic interactions in

the sample. Tw is equal to 128, 90, and 55 K for Ta¼ 350,

400, and 450 �C, respectively. First, these positive tempera-

tures indicate the presence of ferromagneticlike dipolar inter-

actions. Then, the decrease of Tw with the annealing

temperature can be explained by the increase of contacts and

agglomerations between NCs that leads to a decrease of

dipolar interactions.FIG. 6. HRTEM images of CoPt NCs after annealing at 500 �C on HOPG

substrate.

084344-7 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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Figure 9 shows the broadening of the hysteresis loops of

CoPt NCs measured at 3 K in the ferromagnetic regime

according to the increase of the annealing temperature from

350 to 500 �C. The values of the coercivity and the reduced

remanence Mr/Ms, where Mr is the remanent magnetization

and Ms the saturation magnetization, against the different Ta

temperatures are plotted in Fig. 10. The coercivity weakly

increases from 1.6 to 2.4 kOe for annealing temperature up

to 450 �C and then, Hc shifts at 4 kOe for Ta¼ 500 �C. This

enhancement of Hc is due first to the increase of average size

and then A1/L10 phase transition. Conversely to the struc-

tural properties presented above, the transition appears to be

smoother. The low value of the coercivity obtained at

Ta¼ 500 �C, i.e., 4 kOe, comes from both the no-complete

crystalline transition and also the presence of polycrystalline

structure which leads to reduce the Ka magnetocrystalline

anisotropy.44 Futhermore, multiple c axes in individual nano-

particles can lead to an important decrease of the magneto-

crystalline anisotropy.45 On another hand, from Ta¼ 450 �C,

the ratio Mr/Ms of the remanence and saturation magnetiza-

tion drastically rises up to 0.68 and reaches 0.72 for

Ta¼ 500 �C. This behavior of Mr/Ms can suggest an

enhancement of the Ka anisotropy of CoPt nanocrystals.46

The Mr/Ms values � 0.5 is a typical value for random mag-

netic nanoparticles with an uniaxial anisotropy, as in the

case of as-synthesized NPs.47 However, for magnetic NPs

having a pure cubic anisotropy, the Mr/Ms has to equal to

0.8.48 The values of Mr/Ms obtained around of 0.7 for the

annealing temperature of 450 and 500 �C can be explain by

the presence of a mixture of both uniaxial and cubic anisot-

ropy in the samples, as previously assumed with S ordering

parameter. However, it should be noted that Mr/Ms value

can be influenced by interparticle exchange coupling, which

can occur here as the coating agent disappear during the

annealing process.

FIG. 7. (Color online) zfc/fc curves of CoPt NCs at different annealing tem-

perature. 300, 350, 400, and 450 �C. (Inset) 500 �C.

FIG. 8. (Color online) Plot of inverse of the magnetic susceptibility vs the

temperature.

FIG. 9. (Color online) Hysteresis loops at 3 K of CoPt NCs at different

annealing temperature. 350, 400, 450, and 500 �C.

FIG. 10. (Color online) Coercivity Hc and reduced remanence Mr/Ms in

function of the anneal temperature for the CoPt NCs.

084344-8 A. Demortiere and C. Petit J. Appl. Phys. 109, 084344 (2011)

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V. CONCLUSIONS

In summary, we have synthesized A1 CoPt NCs of 2, 3,

and, 4 nm of diameter using biphasic chemical method. The

study of the size and the morphology of these NPs has been

carried out by TEM and HR TEM. The size-dependence of

the morphology of NCs has been clearly observed, in partic-

ular at 2 nm with the decahedral shape. In particular, our

results confirm the theoretical calculations showing the pres-

ence of decahedral shape for 2 nm in size CoPt nanoalloys,

which disappears for the largest size. The investigation of

the magnetic properties of these small CoPt NPs has been

shown that the interparticle dipolar interactions are very

weak and the influence of surface effect is significant for

NCs of D � 4 nm. The annealed NPs have been studied in

term of evolution of size, shape, and structure. We demon-

strated that the HOPG substrate is able to limit the coales-

cence effects during the thermal treatment and also favor the

ordering of nanoalloys. The L10 ordering structure has been

observed from an annealing temperature of 450 �C. The esti-

mation of the ordering parameter (S¼ 0.65) suggests that the

crystalline transition is no complete, even for Ta¼ 500 �C.

We obtained the CoPt NPs that are ferromagnetic at room

temperature with an average size of 16 nm. The evolution of

different magnetic parameters has been studied according

the annealing temperature. The annealed NPs present inter-

ested magnetic behaviors, but both the complete A1/L10

transformation was not reached and the coalescence effects

are reduced but remain significant. Others investigations on

annealing method are needed to avoid the lost of homogene-

ity of the size and shape during the A1/L10 transformation in

these monodisperse CoPt NCs.

ACKNOWLEDGMENTS

The authors would like to thank Pierre Bonville and Eric

Vincent, DRECAM/SPEC CEA, Saclay, France, for the use

of their SQUID equipment and fruitful discussions, as well as

Patricia Beaunier, LRS/UPMC, Paris, France, for her help in

the High Resolution TEM. The authors gratefully acknowl-

edge Bernard Barbara, Louis Neel, Grenoble, France, for stim-

ulating discussions.

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