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Synthesis of monodispersed fcc and fct FePt/FePd nanoparticles bymicrowave irradiation{
H. Loc Nguyen,a Luciano E. M. Howard,a Sean R. Giblin,bd Brian K. Tanner,b Ian Terry,b
Andrew K. Hughes,a Ian M. Ross,c Arnaud Serres,b Hannah Burckstummera and John S. O. Evans*a
Received 19th August 2005, Accepted 5th October 2005First published as an Advance Article on the web 24th October 2005
DOI: 10.1039/b511850f
A simple microwave heating method has been used for the stoichiometrically controlled synthesis
of FePt and FePd nanoparticles using Na2Fe(CO)4 and Pt(acac)2/Pd(acac)2 as the main reactants.
By varying the solvents and surfactants, the microwave assisted reactions have shown a significant
advantage for the rapid production of monodisperse fcc FePt nanoparticle metal alloys which can
be converted to the fct phase at low temperatures (364 uC). Microwave reactions at high pressure
(closed system) have led to the direct formation of a mixture of fcc and fct phase FePt
nanoparticles. Room temperature structural and magnetic properties of materials have been
characterized by X-ray diffraction, HRTEM and magnetic measurements. The onset of ordering
has been investigated by in situ high temperature X-ray diffraction studies.
Introduction
The preparation of nanoscale magnetic materials is an
extremely active research area due to their potential uses in
magnetic recording devices, biomedical applications, magne-
tooptical systems and in numerous other areas.1 Of the many
nanoparticle alloys that have been studied for future genera-
tion magnetic storage applications, self-assembled Ll0 FePt
nanoparticle arrays are promising candidates owing to their
large uniaxial magnetocrystalline anisotropy [K u $ 7 6
107 erg cm23] and good chemical stability.2 Calculations
indicate that particles as small as 2.8 nm have a sufficient
anisotropy energy K uV (V is the magnetic grain volume) to be
exploited for permanent data storage, leading to significant
advances in hard disk drive areal densities over materials
currently used.3
Many approaches to the preparation of metal nanoparticles
have been reported4 including chemical reduction,5 UV
photolysis,6 thermal decomposition,7 metal vapour decom-
position,8 electrochemical synthesis9 and sonochemical decom-
position.10 Chemical routes11 appear to offer the best route to
monodisperse FePt nanoparticles.2,7a In a typical preparation
simultaneous decomposition of iron pentacarbonyl and reduc-
tion of platinum acetylacetonate by polyol reducing agents
or co-reduction of iron and platinum salts in the presence
of surfactants leads to formation of face centered cubic (fcc)
FePt alloys.
To obtain self-assembled Ll0 FePt nanoparticle super-
lattices, which are required for storage applications, the
as-synthesized nanoparticles typically have to be annealed at
high temperature to transform the material from the fcc Fe/Pt
disordered phase to the face centered tetragonal (fct) Fe/Pt
ordered phase, the so called Ll0 structure (Fig. 1). During the
annealing process, however, agglomeration of the particles
can lead to a dramatic increase in both particle size and
size dispersion.8c,12 This hinders applications as high-density
recording materials. Different methods have been attempted to
lower the FePt phase transition temperature (T t) and particle
sintering or to establish a direct route to fct nanoparticleformation. Introduction of a third metal into FePt alloys,13
although reported at lower T t, has resulted in particles which
retain the problems of agglomeration or decomposition on
further annealing at higher temperature. Partially ordered fct
FePt nanoparticles have recently been obtained by chemical
routes including the simultaneous reduction of Fe(II)/Pt(II)
salts and from Fe(CO)5/Pt(acac)2 using conventional heating
methods.14 These preliminary results generally show a low
ordering ratio, small room temperature (RT) coercivity of fct
particles and frequently relatively broad particle size disper-
sion. Recently a multistep process involving coating particles
with an inert silica coating during annealing followed by its
subsequent removal in base has been described. This process
aDepartment of Chemistry, University Science Laboratories, Universityof Durham, South Road, Durham, UK DH1 3LE.E-mail: [email protected] bDepartment of Physics, University Science Laboratories, University of Durham, South Road, Durham, UK DH1 3LE cDepartment of Electronic and Electrical Engineering, University of Sheffield, Mappin Building, Mappin Street, Sheffield, UK S1 3JDd Present address: ISIS facility, Rutherford Appleton Laboratory,Chilton, Didcot, OXON.{ Electronic supplementary information (ESI) available: Magneticresults of sample 1, 2, 4, HRTEM and SAED results of sample 1 of Table 1. See DOI: 10.1039/b511850f
Fig. 1 Schematic representation of the FePt phase transformation
from the fcc to fct structure.
PAPER www.rsc.org/materials | Journal of Materials Chemistry
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can apparently lead to particle ordering without sintering.15
Disordered fcc particles have also been annealed to the fct
phase with minimal sintering in a NaCl matrix.16
In a recent communication, we have presented a straight-
forward stoichiometrically controlled synthesis of FePt nano-
particles using Collman’s reagent, Na2Fe(CO)4, as a reducing
agent for platinum acetylacetonate, Pt(acac)2.17 An advantage
of the method is that the electrons required to reduce Pt(II) arelocated on the Fe source rather than on an additional species
(the reaction can be schematically written as Fe22 + Pt2+ A
FePt). This process assures the ideal 1 : 1 stoichiometry is
achieved which is important since the magnetically important
fct phase only forms over an Fe12xPt range of x y 0.4–0.6;
other workers have shown that conventional procedures lead
to individual particles with a range of stoichiometries.18
Further, the reduction step that is key to nanoparticle alloy
formation requires the simultaneous presence of Fe and Pt
ions to occur, leading to the product alloy being intimately
mixed on an atomic scale. Using this route we have shown
that it is possible to produce fcc FePt nanoparticles which can
be converted to the fct structure at low temperatures withminimal agglomeration without the presence of a third metal.
By varying the surfactants and temperature regimes it was also
possible to synthesise FePt nanoparticles with the important
fct structure directly in solution, without any post-synthesis
heat treatment.17
In order to improve the FePt nanoparticle preparation
reported in our preliminary work, an alternative method of
energy supply has been investigated for heating reactions more
efficiently. Microwave dielectric heating has recently attracted
the attention of chemists for, inter alia, organic reactions,19
molecular sieve preparation,20 and syntheses of inorganic
complexes21 as it can lead to much higher heating rates
than those achieved by conventional heating. The rapid anduniform heating provided by microwaves has potential benefits
for nanoparticle synthesis. Microwave irradiation was recently
reported to be a successful synthetic method for single metal
nanoparticles such as Pt, Ir, Rh, Pd, Au, Ru22 and Ag.23
Application of microwave dielectric heating for binary Pt–Ru
nanoparticles was also reported, in which a uniform size of
2–3 nm was obtained in the presence of a polymer as a
protective layer for the particles.24 We are only aware of one
previous publication on the synthesis of FePt by microwave
methods in which platinum(II) chloride and iron(II) acetate
were reduced in ethylene glycol; to achieve a 1 : 1 stoichiometry
excess Fe was used.25 In this work the as-prepared super-
paramagnetic material was described as being amorphous (nopeaks were present in its X-ray diffraction pattern) and
crystalline FePt was only formed on heating to 600 uC. Little
characterisation of the as-prepared material was given and
the annealed material was reported as having a bimodal size
distribution. Selected area electron diffraction (SAED) pat-
terns reported did not show the ordering peaks one would
expect for fct FePt and certain ordering peaks (e.g., the 001,
112 and 113 peaks based on a pseudo-cubic cell setting)
appeared to be missing from the X-ray data presented.
In this work we have investigated the use of microwave
irradiation as an energy source for the preparation of FePt
using the Fe22/Pt2+ methodology. We show that monodisperse
crystalline fcc particles of controlled size (and thus suitable for
self-assembly) can be readily produced. The reaction has been
performed using a variety of solvents and surfactants leading
to control over particle size. Under certain conditions it is also
possible to prepare fct particles directly. We have also extended
this chemistry to the microwave synthesis of FePd nano-
particles. The structure and properties of key materials have
been characterized by X-ray diffraction (XRD), transmissionelectron microscopy (TEM) and magnetic measurements.
Experimental
Materials and instruments
Platinum acetylacetonate [Pt(acac)2] was purchased from
STREM, Pd(acac)2, disodium tetracarbonylferrate-dioxane
complex [Na2Fe(CO)4?1.5C4H8O2], dioctyl ether, oleylamine
and oleic acid from Aldrich, n-nonadecane from Lancaster.
Octyl ether was degassed for 15 min before each use. All
chemicals were weighed, placed into reaction flasks and
sealed in a N2 filled glove box before transferring to microwave
apparatus. Other AR (analytical reagents) grade organicsolvents used for purification (e.g., hexane and absolute
ethanol) were used as purchased. The microwave-assisted
reactions were carried out in a CEM 300W Discover Focus
Synthesis Microwave with a 2.45 GHz working frequency.
Reactions under ambient pressure were performed in 100 ml
thick glass walled vessels connected to a condenser under Ar.
Reactions at elevated pressure and temperatures were
performed in 10 ml sealed vials.
Nanoparticle preparation in an open vessel
In a typical reaction a mixture of Pt/Pd acetylacetonate
(0.3 mmol), disodium tetracarbonylferrate (0.3 mmol) with
appropriate amounts of surfactants and solvents (Table 1) was
placed in a 100 ml thick walled glass vessel connected to a
condenser and Ar input. The mixture was sonicated at 60 uC
for 1 h before transferring into the microwave apparatus.
Reaction was typically carried out using control parameters of
300 W max power, 250 uC max temperature and 250 psi max
pressure. A high reaction temperature (215 uC) was obtained
using octyl ether as solvent whilst lower temperature (130–
150 uC) was achieved for reactions carried out in nonadecane.
After reaction the dark product mixture was allowed to cool to
room temperature before adding 100 ml of absolute ethanol to
precipitate dark particles. The product was separated by
centrifugation then dispersed in hexane (20 ml) in the presence
of appropriate surfactants and precipitated by adding ethanol
(40 ml). After centrifugation, the material was washed one
more time in a similar solvent mixture, dried in air at room
temperature and stored under N2.
Nanoparticle preparation in a closed vessel
FePt nanoparticles were synthesized from a mixture of plati-
num acetylacetonate (0.17 mmol), disodium tetracarbonyl-
ferrate (0.17 mmol) with appropriate amounts of surfactants
and solvents for specific reactions (Table 1). Rapid heating
(1 to 5 min) to the desired reaction temperature could be
readily achieved. Safety warning: when using sealed systems
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there is a potential hazard due to rapid build up of high
pressure in the reaction vessel and the release of CO during the
reaction. The control system of the microwave reactor used is
designed to remove power if a rapid pressure build up is
encountered. To minimise such risks, sealed-system reactions
were typically heated to the desired temperature in 2 stages,
each taking y1 min. Despite these precautions rapid heating
that could lead to explosions was experienced on occasions;
we believe this is related to the formation of large particles
which provide a self-accelerating heating mechanism.We therefore advise caution and the use of appropriate
containment/shielding methods when such reactions are
attempted. At the end of reactions the pressure was released
and the dark mixture was allowed to cool to room temperature
before adding absolute ethanol to precipitate dark particles.
After centrifugation, the black product was dispersed in
hexane (5 ml) in the presence of the surfactants used during
the synthesis, precipitated by adding ethanol (10 ml) and
centrifuged. The materials were washed one more time, dried
in air at room temperature and stored under N2.
Characterisation methods
XRD data used to confirm sample purity and particle size were
collected on a Bruker D8 Advance diffractometer equipped
with a Cu tube and a Sol-X energy dispersive detector. The
sample was mounted on a zero background (511) silicon wafer.
Data were typically collected from 10–90u 2h (step size 5 0.02u
and time per step 5 10 s) at room temperature. A variable
divergence slit giving a constant area of sample illumination
was used. In situ variable temperature X-ray diffraction data
were collected using a Bruker AXS D8 Advance diffractometer
equipped with a Cu tube, a Ge(111) incident beam mono-
chromator (l 5 1.5406 A) and a Vantec-1 PSD. High tem-
perature measurements were performed using an Anton Parr
HTK1200 high temperature furnace. Temperature calibrationwas determined using an external Al2O3 –Si mixture of
standards.26 The powdered sample was mounted on an
amorphous silica disc. Variable temperature XRD data
were collected over a temperature range of 297–924–296 K.
Measurements (48 in total) were recorded over 48 h (every
25 K, 60 min each, a 0.2 K s21 heating/cooling rate between
temperatures, a 2h range of 5–130u and a step time of 0.33 s).
Data were rebinned onto a step size of 0.05u for Rietveld
analysis. A slow flow of 5% H2 –95% Ar gas was passed over
the sample for the experiment’s duration. XRD derived
particle sizes quoted throughout the paper were obtained
from Rietveld refinements of data sets. Peak shapes were fitted
by convolution of a Scherrer-type broadening term of form
(l/size)cosh and a strain term of the form strain 6 tanh with an
instrumental resolution function derived from a highly crystal-
line CeO2 standard recorded under equivalent conditions.
Samples for transmission electron microscopy (TEM)
analysis were prepared as dilute dispersions in hexane with a
small amount of surfactants. A drop of particle dispersion was
allowed to evaporate slowly on an amorphous carbon film
supported on a standard 3 mm copper grid (200 mesh, Agar
Scientific). High resolution TEM (HRTEM) was performed ina JEOL 2010F field-emission gun (FEG) TEM operating at
200 kV. This instrument is capable of forming sub-nanometre
analytical electron probes facilitating high spatial resolution
compositional analysis via an Oxford Instruments LINK/ISIS
X-ray energy-dispersive spectrometer (EDS) (Si/Li detector,
1024 channels, 20 keV range). EDS spectra were acquired
from single nanoparticles and also regions of the specimen
containing clusters of y300 particles using a 30 s preset live
time acquisition. Quantification of the data was performed
using the Cliff–Lorimer thin section technique assuming an
average material density of 14.6 g cm23 and a specimen
thickness equal to the average projected diameter of the
particle(s) being studied.Magnetic studies were carried out using a Quantum Design
SQUID magnetometer. Magnetization curves as a function
of applied field were measured with fields up to 50 kOe at
temperatures of 10 K and 290 K. Zero-field cooling/field
cooling (ZFC/FC) experiments were made at 100 Oe with
temperatures ranging from 2 to 300 K on samples mounted in
low background gelatin capsules. Data are presented per g
of sample owing to the difficulty in accurately assessing the
percentage of surfactant molecules in an individual sample.
Result and discussion
Initial reactions (Table 1) were performed using a 1 : 1 molarratio of Fe and Pt sources and a molar equivalent of oleyl
amine and oleic acid surfactants. Nonadecane was chosen as
solvent since experiments using conventional heating have
shown that it can lead to low particle agglomeration during
subsequent heat treatment.17 Heating these reagents to 150–
170 uC in an open system for 40 min led to the formation of a
black suspension. The XRD of this material (Fig. 2a) is typical
of a chemically disordered fcc structure possessing broad peaks
at 41, 47, 68 and 82u 2h which are indexed as the (111), (200),
(220) and (311) reflections respectively. The fcc structure of
the particles was also verified by electron diffraction where
d -spacings calculated from radii of pattern rings were
Table 1 Reaction conditions and XRD particle sizes of fcc FePt nanoparticles synthesized by microwave heating
Reaction System Temperature/uCHeatingtime/min
Holdtime/min Solvent Surfactants (ratio used)
Particlesize/nm
1 FePt/open 150 30 10 Nonadecane Oleyl amine–oleic acid (1 : 1)a 2.66(7)b
2 FePt/open 215 10 30 Octyl ether Oleyl amine–oleic acid (1 : 1) 2.19(5)3 FePt/closed 150 + 250 1.5 + 1 2 + 2 Octyl ether Oleyl amine–oleic acid (1 : 1) 3.26( 7)4 FePt/closed 280 5 55 Octyl ether Oleyl amine–oleic acid (1 : 1) 3.16(6)5 FePd/open 282 12 80 Octyl ether Oleic acid (3) 7.20(2)a Molecular mole ratio of surfactants in comparison with molecular mole ratio of main reactants. b Estimated standard uncertainties inparentheses, see text for definition of particle size.
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consistent with XRD results (see electronic supplementary
information (ESI){). Rietveld refinement of the data performed
using Topas Academic27 suggested an average diameter of
particles of 2.66(7) nm and cell parameter of 3.8744(9) A,
similar to fcc FePt nanoparticles synthesised under similar
conditions by normal heating.17 We note that the temperature
at which this reaction was performed is significantly lower
than required using conventional methods (150 uC vs 330 uC).
Further attempts using different conditions of temperature,
time, solvent and vessel (open or closed) system have also led
to production of fcc FePt nanoparticles as shown in Table 1.
FePd nanoparticles could also be prepared by a similar route.
Fig. 2b shows the XRD pattern of y7.2 nm FePd particles
prepared in dioctyl ether with a 1 : 3 molar ratio of metal–oleic
acid surfactant at 282 uC. The refined cell parameter was
3.8972(4) A.
Fig. 3 shows a TEM image of FePt particles produced
by this route. These can be seen to be essentially spherical
and of uniform size. Using image analysis software,28 size
measurement of 188 randomly selected particles shows that the
FePt particles have a narrow size distribution. Fitting with a
log-normal distribution leads to a measured mean diameter of
2.58 nm and a dispersion s of 5% (Fig. 4). The size is consistent
with that indicated by XRD. The HRTEM images (Fig. 3
insert) of individual particles demonstrated that they were
single crystals with lattice fringes consistent with the (200) and
(220) d -spacing of y
1.9 A˚
andy
1.3 A˚
respectively. EDSanalysis of clusters containing y300 particles gave an overall
average Fe : Pt stoichiometry of 52(3) : 48(3). Analysis of
individual particles revealed that a range of compositions are
present with some particles either Fe or Pt rich. Averaging a
large number of individual particles gave a stoichiometry of
48(7) : 52(7). Yu and co-workers have reported that individual
particles prepared by the conventional polyol synthetic route
can have a wide stoichiometry range with a significant
proportion of particles being either Fe- or Pt-rich. In fact,
they report that only 29% of individual particles lie in the
0.4 , x , 0.6 FexPt12x range that would allow the transition
to the L10 phase to occur.18 We note that unlike Yu et al.
we find that 75% of the individual particles lie within thisrange. This suggests that significantly better control over
individual particle stoichiometry is achieved with our
Fe22/Pt2+ synthetic route.
The fcc particles prepared by this route can be converted to
the magnetically important fct L10 phase by annealing under a
Fig. 2 Rietveld fits of X-ray patterns of (a) as-prepared fcc FePt
samples (observed and calculated patterns for FePt with differencebelow) and (b) fcc FePd.
Fig. 3 HRTEM image of monodispersed fcc FePt particles, insert
shows nanoparticles with a fringe spacing consistent with the (200)
plane of fcc structure.
Fig. 4 Particle diameter histogram of fcc FePt nanoparticles; the line
plotted corresponds to the fit using a log-normal distribution.
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flow of 5% H2 in Ar. Fig. 5 shows a narrow 2h range of a series
of diffraction experiments recorded at increasing temperatures;
each pattern was recorded over y1 h with rapid heating
between data collections. Two different phenomena can be
observed from these data. Firstly from a temperature of
around 637 K (364 uC) extra peaks appear at 2h values of y24
and 33u. These peaks can be indexed as the (001) and (110)
reflections of the fct phase (using a pseudo-cubic cell of a y 3.85, c y 3.71 A) and provide direct evidence of the
ordering phase transition. The recorded ordering temperature
is consistent with previous experiments on annealing FePt
nanoparticles synthesized using this synthetic method and
conventional heating, and is significantly lower than the
600 uC required to order previously reported materials.25 It
is also clear from Fig. 5 that peaks sharpen on heating which
is evidence of particle growth.
Quantitative information on both these processes has been
obtained by Rietveld refinement using the Topas Academic
software suite. It is difficult to extract reliable quantitative
information on the early stages of ordering for materials such
as this as much of the information is contained in the relativelybroad superlattice peaks. Due to correlations with the back-
ground (which itself has significant slowly varying contribu-
tions due to the sample mounting and furnace environment for
variable temperature experiments), it is extremely hard to
estimate their intensity correctly. We have therefore adopted a
strategy in which a sixth order background polynomial was
fitted to each of the 48 data sets in an initial round of
Rietveld refinements using a fully disordered model in which
2h ranges corresponding to ordering peaks were excluded.
We believe that this produces the ‘‘least biased’’ estimate of the
background at each temperature that can be achieved. These
background polynomials were then used as fixed functions
in a separate round of Rietveld refinements in which six
parameters (scale factor, a and c cell parameters, overallatomic displacement parameter, particle size and order
parameter) were refined at each temperature. The sample
height was found to vary smoothly with temperature and was
introduced to each refinement as a fixed though temperature-
dependent parameter. To allow refinement through the
fccA fct phase transition a psuedo-cubic cell setting was used
throughout in space group P 4/mmm (Fe at 1a and 1c Wyckoff
sites and Pt at 2e), and the fractional occupancy (frac) of Fe
on Pt sites and Pt on Fe allowed to refine. The order parameter
for the phase transition is thus given by 1–2frac, and ideally
varies from 0 (fcc) to 1.0 (fct).
Fig. 6a shows the temperature dependence of the unit cell
size. Below the fccA
fct ordering temperature individual valuesof a and c are ill-defined by Rietveld refinement (the material is
cubic and peaks are broad so psuedo-cubic tetragonal values
show considerable scatter) so we choose to plot (volume)1/3 on
warming which provides an average measure of cell parameter
over the whole temperature range. A significant reduction in
this parameter is seen from around 450 K. A reduction in
volume is expected for the fccA fct transition and is well
known in, for example, AuCu binary alloys. We note that the
reduction in cell volume occurs before significant ordering
peaks are visible in Fig. 5, and before significant particle
growth. Cell volume is therefore perhaps the most accessible
indication that particle ordering is beginning to occur.
On cooling the material retains the fct structure as expected.The overall cell volume reveals a positive thermal expansion
coefficient throughout, though the c-axis shows a small thermal
contraction (aa 5 +2.0 6 1025, ac 5 25.2 6 1026 K21 from
925–295 K) with the c/a ratio varying from 0.9464(2) at
925 K to 0.9615(3) at 295 K. The c-axis also shows a marked
contraction just above 700 K which is presumably associated
with the Curie temperature which is around 723 K.29 Cell
parameters on cooling of a 5 3.857(7), c 5 3.708(1) A at
296 K compare to literature values of a 5 3.855, c 5 3.71130
or a 5 3.85 and c 5 3.71 A.31 A recent neutron scattering
measurement on large single crystals of bulk FePt shows a
similar temperature dependence of the c cell parameter.32
The order parameter and particle size dependence ontemperature are shown in Figs. 6b and 6c. Below around
450 K the order parameter is approximately constant. Early in
the ordering process precise values of the order parameter are
hard to derive and the low temperature values plotted of y0.2
on warming are probably not significantly different from zero;
the increase in R-factor on forcing the order parameter to be
exactly 0.0 for these refinements is , 0.15% for temperatures
below 558 K. A significant rise in order parameter can be seen
above y500 K, a temperature slightly higher than that at
which the cell volume decrease occurs. On cooling the material
retains its fct structure with the room temperature order
parameter refining to 1.01(3). The indication of perfect Fe/Pt
Fig. 5 25 diffraction patterns recorded on annealing as-prepared fcc
FePt as a function of temperature from 297 to 924 K. Indication of
ordering is clearly visible at 637 K (364 uC) and above. Data collected
on cooling are not shown but derived quantities are included in Fig. 6.
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order provides further support for the stoichiometric nature of
the particles. The mean particle size also increases significantly
from y500 K indicating particle sintering as the protective
surfactants burn off. The particle size remains, as expected,
essentially unchanged on cooling.
Fig. 7a shows the magnetization of sample 1 as a function of
increasing temperature after cooling in a small residual field of
,1 Oe (‘‘zero’’ field cooling) and in a 100 Oe field. Clear
evidence for superparamagnetic behaviour with a blocking
temperature of T y 17 K is seen. The sharp rise of the ZFC
data indicates that the particle size dispersion is low, in support
of the TEM conclusions. The magnetization vs field loops
(Fig. 7b) of the as-synthesized sample showed small coercivity
values at both 10 and 290 K, confirming the superparamag-
netic properties of the particles. The data also indicate the
presence of a very minor component which saturates at low
field. Hysteresis loops of an annealed sample (see Fig. S4 in the
ESI{) gave coercivities of 14.7 and 10.6 kOe at 10 and 290 K
respectively.
We have recently reported the direct preparation of orderedfct FePt nanoparticles using the Fe22/Pt2+ route with conven-
tional heating at 389 uC in tetracosane. This has prompted
us to attempt the preparation of ordered fct particles using
microwave irradiation. A closed microwave system was chosen
to access the high temperatures at which fcc FePt particles
might be transformed directly to the ordered phase in solution.
It proved, however, difficult to control heating rates of such
reactions and pressure build-up in reaction vessels sometimes
led to reaction vessel bursting. Fig. 8 shows a diffraction
pattern of an FePt sample prepared with octyl ether and an
oleyl amine surfactant (2 : 1 surfactant to metal ratio) at 280 uC
under microwave irradiation for 10 min. Superlattice peaks are
Fig. 6 (a) Temperature dependence of the (cell volume)1/3 on
warming and cooling (closed/open triangles respectively) and a (open
circles) and c (open squares) cell parameters on cooling; (b) order
parameter on warming (closed symbols) and cooling (open symbols);
(c) particle size on warming (closed symbols) and cooling (open
symbols). Error bars show ¡1 standard uncertainty as derived by
Rietveld refinement, and are probably an underestimate of the true
uncertainty on parameters. Where error bars are not shown they aresmaller than the size of the plotted symbol.
Fig. 7 (a) ZFC and FC magnetisation curves as function of tem-
perature from 5 to 290 K at a field 100 Oe; (b) Magnetic hysteresisloops measured at 10 and 290 K of as-prepared fcc FePt particles as
prepared in reaction 1 of Table 1.
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clearly present at y24 (001) and 33u (110) 2h indicating FePt
particles in the fct phase have formed. The overlap of sharp
diffraction peaks on broader peaks at y40.5 (111) and 47u
(200) 2h respectively suggests the coexistence of fcc and fct
FePt structural phases. Rietveld refinement confirms the
existence of the ordered FePt structure giving a and c cell
lattice parameters of 3.8463(2) and 3.7214(3) A, an order
parameter of 0.90(1), and an estimated particle size of
24 nm. The cell parameter of the cubic component refines to
3.872(2) A. These results suggest that rapid heating had
simultaneously caused a phase transformation from the fcc to
fct phase and decomposition of surfactants leading to rapidparticle size increase. Hysteresis loops measured at 10 K and
290 K (Fig. S5 in the ESI{) also suggest two phase behaviour
with a kink at low field. The measured coercivities were 7.0 kOe
at 290 K and 9.0 kOe at 10 K.
Conclusions
The general synthetic route presented here provides a
straightforward and stoichiometrically controlled synthesis of
FePt nanoparticles. Using microwaves for reaction heating
shows significant advantages for production of monodispersed
fcc FePt nanoparticle alloys which can be conveniently
converted into the ordered fct phase on annealing at lowtemperature (364 uC). Reactions can be performed very rapidly
(6 minutes or less) and at temperatures lower than using
conventional heating. The Fe22/Pt2+ route allows good
control over both the overall stoichiometry and the stoichio-
metry of individual particles. High temperature reactions
in the microwave led to the direct formation of a mixture
of fcc and fct FePt nanoparticles. The fct nanoparticles
were shown to have a particle size of y24 nm and strong
coercivity indicating ferromagnetic behavior. Further exten-
sions of FePt nanoparticles synthesis by microwave heating
with varying solvent, surfactants and metals have been
investigated.
Acknowledgements
The authors thank Vivian Thompson for TEM images,
Prof Todd Marder and Dr Patrick Steel for access to
microwave facilities EPRSC and ONE-NE, via the Durham
Nanotechnology Innovation Centre and Seagate Technology
for financial support.
References
1 (a) K. J. Klabunde, Nanoscale Materials in Chemistry, John Wiley& Sons, New York, 2001; (b) D. L. Peng, T. Hihara and
K. Sumiyama, J. Magn. Magn. Mater., 2004, 277, 201; (c)V. F. Puntes, K. M. Krisknan and A. P. Alivisatos, Science,2001, 291, 2115.
2 (a) S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser,Science, 2000, 287, 1989; (b) D. Weller and A. Moser, IEEE Trans.Magn., 1999, 35, 4423.
3 M. Plumer, J. Van Ek and D. Weller, The Physics of Ultra-High-Density Magnetic Recording , Springer, New York, 2001.
4 (a) C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem.Rev., 2005, 105, 1025; (b) B. L. Cushing, V. L. Kolesnichenko andC. L. O’Connor, Chem. Rev., 2004, 104, 3893.
5 (a) S. Sun, S. Anders, T. Thomson, J. E. E. Baglin, M. F. Toney,H. F. Hamann, C. B. Murray and B. D. Terris, J. Phys. Chem. B ,2003, 107, 5419; (b) T. Iwaki, Y. Kakihara, T. Toda, M. Abdullahand K. Okuyama, J. Appl. Phys., 2003, 94, 6870; (c) B. Jeyadevan,A. Hobo, K. Urakawa, C. N. Chinnasamy, K. Shinoda andK. Tohji, J. Appl. Phys., 2003, 93, 7574; (d ) B. M. Leonard,N. S. P. Bhuvanesh and R. E. Schaak, J. Am. Chem. Soc., 2005,127, 7326.
6 K. Torigoe and K. Esumi, Langmuir, 1993, 9, 1664.7 (a) S. Sun, E. E. Fullerton, D. Weller and C. B. Murray,
IEEE Trans. Magn., 2001, 37, 1239; (b) B. Stahl, N. S. Gajbhiye,G. Wilde, D. Kramer, J. Ellrich, M. Ghafari, H. Hahn, H. Gleiter,J. Weißmuller, R. Wurschum and P. Schlossmacher, Adv. Mater.,2002, 14, 24; (c) Y. Hou, H. Kondoh, T. Kogure and T. Ohta,Chem. Mater., 2004, 16, 5149; (d ) M. Chen and D. E. Nikles,J. Appl. Phys., 2002, 91, 8477.
8 (a) K. Kakizaki, Y. Yamada, Y. Kuboki, H. Suda, K. Shibata andN. Hiratsuka, J. Magn. Magn. Mater., 2004, 272–276, 2200; (b)P. T. L. Minh, N. P. Thuy and N. T. N. Chan, J. Magn. Magn.Mater., 2004, 277, 187; (c) D . L . P en g, T . H ih ara a ndK. Sumiyama, J. Magn. Magn. Mater., 2004, 277, 201.
9 M. T. Reetz and W. Helbig, J. Am. Chem. Soc., 1994, 116, 7401.
Fig. 8 Rietveld fit of X-ray diffraction data of as-prepared FePt samples containing a mixture of fcc and fct particles. (observed and calculated
patterns for FePt, and difference below).
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10 K. S. Suslick, M. Fang and T. Hyeon, J. Am. Chem. Soc., 1996,118, 11960.
11 T. Hyeon, Chem. Commun., 2003, 8, 927.12 (a) T. Thomson, B. D. Terris, M. F. Toney, S. Raoux,
J. E. E. Baglin, S. L. Lee and S. Sun, J. Appl. Phys., 2004, 95,6738; (b) H. Zeng, S. Sun, T. S. Vedantam, J. P. Liu, Z.-R. Dai andZ.-L. Wang, Appl. Phys. Lett., 2002, 80, 2583.
13 (a) S. Wang, S. S. Kang, D. E. Nikles, J. W. Harrell and X. W. Wu,J. Magn. Magn. Mater., 2003, 266, 4 9 ; (b) Y. K. Takahashi,M. Ohnuma and K. Hono, J. Magn. Magn. Mater., 2002, 246,
259; (c) C. L. Platt, K. W. Wierman, E. B. Svedberg, R. VanDe Veerdonk, J. K. Howard, A. G. Roy and D. E. Laughlin,J. Appl. Phys., 2002, 92, 6104.
14 (a) B. Jeyadevan, K. Urakawa, A. Hobo, N. Chinnasamy,K. Shinoda, K. Tohji, D. D. J. Djayaprawira, M. Tsunoda andM. Takahashi, Jpn. J. Appl. Phys., 2003, 42, L350; (b) S. Kang,Z. Jia, S. Shi, D. E. Nikles and J. W. Harrell, J. Appl. Phys.,2005, 97, 10J318; (c) S. Kang, Z. Jia, S. Shi, D. E. Nikles andJ. W. Harrell, Appl. Phys. Lett., 2005, 86, 62503; (d ) M. Takahashi,T. Ogawa, D. Hasegawa and B. Jeyadevan, J. Appl. Phys., 2005,97, 10J307.
15 S. Yamamoto, Y. Morimoto, T. Ono and M. Takano, Appl. Phys.Lett., 2005, 87, 32503.
16 K. Elkins, D. Li, N. Poudyal, V. Nandwana, Z. Jin, K. Chen andJ. P. Liu, J. Phys. D: Appl. Phys., 2005, 38, 2306.
17 L. E. M. Howard, H. L. Nguyen, S. R. Giblin, B. K. Tanner,
I. Terry, A. K. Hughes and J. S. O. Evans, J. Am. Chem. Soc.,2005, 127, 10140.18 A. C. C. Yu, M. Mizuno, Y. Sasaki and H. Kondo, Appl. Phys.
Lett., 2004, 85, 6242.19 (a) R. N. Gedye, W. Rank and K. C. Westaway, Can. J. Chem.,
1991, 69, 706; (b) M. Larhed and A. Hallberg, J. Org. Chem., 1996,
61, 9582; (c) D. R. Baghurst and D. M. P. Mingos, J. Organomet.Chem., 1990, 384, C57.
20 C. -G. Wu and T. Bein, Chem. Commun., 1996, 8, 925.21 (a) A. R. Barron and C. C. Landry, Science, 1993, 260, 1653; (b)
A. G. Whittaker and D. M. P. Mingos, J. Chem. Soc., DaltonTrans., 2002, 3967; (c) A. G. Whittaker and D. M. P. Mingos,J. Chem. Soc., Dalton Trans., 2000, 1521; (d ) A. G. Whittaker,Chem. Mater., 2005, 17, 3426.
22 (a) W. Tu and H. Liu, J. Mater. Chem., 2000, 10, 2207; (b) W. Yu,
W. Tu and H. Liu, Langmuir, 1999, 15, 6.23 K. Patel, S. Kapoor, D. P. Dave and T. Mukherjee, J. Chem. Sci.,2005, 117, 53.
24 F. Bensebaa, N. Patrito, Y. Le. Page, P. L’Ecuyer and D. Wang,J. Mater. Chem., 2004, 14, 3378.
25 R. Harpeness and A. Gedanken, J. Mater. Chem., 2005, 15, 698.26 (a) K. G. Lyon, G. L. Salinger, C. A. Swenson and G. K. White,
J. Appl. Phys., 1977, 48, 865; (b) Y. Okada and Y. Tokumaru,J. Appl. Phys., 1984, 56, 314; (c) D. Taylor, Br. Ceram. Trans. J.,1984, 83, 92.
27 Topas Academic: http://pws.prserv.net/Alan.Coelho/.28 Image Tool: http://ddsdx.uthscsa.edu/dig/itdesc.html/.29 T. S. Vedantam, J. P. Liu, H. Zeng and S. Sun, J. Appl. Phys.,
2003, 93, 7184.30 (a) S. Saita and S. Maenosono, Chem. Mater., 2005, 17, 3705; (b)
T. J. Klemmer, N. Shukla, C. Liu, X. W. Wu, E. B. Svedberg,
O. Mryasov, R. W. Chantrell, D. Weller, M. Tanase andD. E. Laughlin, Appl. Phys. Lett., 2002, 81, 2220.31 Joint Committee for Powder Diffraction Standards, International
Centre for Diffraction Data, Newton Square, PA.32 Y. Tsunoda and H. Kobayashi, J. Magn. Magn. Mater., 2004,
272-276, 776.
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