Preparation of Fe-Pt alloy particles by pulsed las...

Chemical Physics Letters 428 (2006) 426–429
www.elsevier.com/locate/cplett
Preparation of Fe–Pt alloy particles by pulsed laser ablation in
liquid medium
Yoshie Ishikawa, Kenji Kawaguchi, Yoshiki Shimizu, Takeshi Sasaki, Naoto Koshizaki *
Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
Received 29 May 2006; in final form 14 July 2006
Available online 1 August 2006
Abstract
Fe–Pt alloy particles were prepared by pulsed laser ablation in degassed hexane and deionized water. The atomic composition of the
obtained binary metal particles significantly depended on the liquid media. Fe 23 Pt 77 close to FePt 3 and Fe 38 Pt 62 which contributed to the
L1 0 phase formation were produced in hexane, however only Fe 26 Pt 74 close to FePt 3 was produced in water. The absence of oxygen
atoms in the molecule of the liquid medium was important because oxidation of iron species led to deviation from stoichiometric alloying.
As-prepared particles produced in hexane had a random phase that was changed to the L1 0 phase by heat treatment at temperatures
higher than 773 K.
Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction
In recent years pulsed laser ablation in liquid (PLAL)
has become an attractive method to prepare colloidal solution
containing nanoparticles [1–5]. Because PLAL does
not need a vacuum system and has a high collection yield,
it is superior to the laser ablation in gas phase. Stable colloid
formation of noble metals such as Au, Pt and Ag, and
various metal oxides has been demonstrated, using a simple
metal plate as a target [1–5]. However, studies on laser
ablation of an alloy target in a liquid medium are very few.
FePt nanoparticles have attracted much attention since
they are an important candidate for high-density recording
media, due to the high magnetic anisotropy of the ordered
FePt L1 0 phase and good chemical stability [6–10]. Some
chemical synthesis methods have been employed for FePt
nanoparticle fabrication. The polyol process is based on
the reduction of Pt(acac) 2 (acac: acetylacetonate) and Fe(acac)
3 by a polyalcohol such as ethylene glycol and 1,2-hexadecanediol,
or a combination of polyol reduction of
Pt(acac) 2 and thermal decomposition of Fe(CO) 5 [6–8].
* Corresponding author. Fax: +81 29 861 6355.
E-mail address: koshizaki.naoto@aist.go.jp (N. Koshizaki).
The reverse micelle method to reduce aqueous solution of
Pt and Fe salts in micelles is also used for FePt nanoparticle
fabrication [9,10]. Although these methods are effective
for homogeneous nanoparticle preparation, many byproduct
are concomitantly formed. PLAL does not necessarily
need chemical reagents in solution for nanoparticle preparation;
therefore purification is not necessary. In this study,
we attempted to prepare Fe–Pt nanoparticles using FePt
binary metal as a target and investigated the influence of
the liquid medium on the composition of prepared binary
metal particles.
2. Experimental
Platinum iron particles were produced by laser ablation
of a Fe 50 Pt 50 disordered binary metal plate as a target in
30 cm 3 of deionized water (>18 MX) or hexane (Wako
Pure Chemical Industries, Ltd., 96.0%). The target of the
FePt plate was prepared by electron beam melting in a vacuum
and fixed on the wall of a stainless-steel cell with a
quartz window. After degassing of oxygen dissolved in
water or hexane by Ar bubbling for 3 h, the cell was closed.
The target was then ablated with the second harmonic
(532 nm) of a Nd: YAG laser operated at 10 Hz with a
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.07.076

Y. Ishikawa et al. / Chemical Physics Letters 428 (2006) 426–429 427
maximum output of 100 mJ/pulse through the quartz window
for 30 min. The laser beam was focused onto the target
plate with a beam 1 mm in diameter, using a lens with a
focal length of 50 mm.
The obtained colloidal suspensions after ablation were
centrifuged at 50000 rpm for sedimentation. The morphology
of the collected particles was observed using a field
emission scanning electron microscope (SEM; Hitachi
S4800) and a transmission electron microscope (TEM;
JEOL JEM 2000FXII). X-ray powder diffraction (XRD)
analysis of the collected particles was performed using a
powder diffractometer (Rigaku RAD-C system) with Cu
Ka radiation. The local composition ratio of Fe to Pt of
particles was estimated with energy dispersive X-ray spectroscopy
(EDS). The obtained particles were also annealed
under an Ar atmosphere for conversion into an ordered
L1 0 structure. The magnetic properties of the particles were
measured with a superconducting quantum interference
device (SQUID; Quantum Design, MPMS) at 5 and 300 K.
3. Results and discussion
Only the peaks from the Fe–Pt binary metal were
observed by XRD measurement of the particles obtained
by pulsed laser ablation of the FePt target in hexane and
water. Fig. 1 presents (200) peaks of Fe–Pt binary metal
from the products obtained, and (200) peak positions of
random FePt and FePt 3 are also demonstrated [11]. The
particles prepared in hexane had two peaks, whereas those
prepared in water had only one peak. The atomic fraction
of the alloy could be roughly estimated from the lattice
parameter of the binary solid solution using Vegard’s
law. The relationship between the Fe atomic percent x
and the lattice parameter a of random phase Fe–Pt alloy
is expressed as follows [11]:
a ðÅÞ ¼3:9224 0:0024 x ð0 6 x 6 50Þ
The particles prepared in hexane contained Fe 23 Pt 77 and
Fe 38 Pt 62 , while the particles prepared in water contained
only Fe 26 Pt 74 . Thus, crystallized particles were always Ptrich.
Fig. 1. X-ray powder diffraction spectra of Fe–Pt alloy particles prepared
by laser ablation in (a) hexane and (b) water.
Figs. 2 and 3 depict SEM and TEM images of Fe–Pt particles
prepared in hexane. The SEM image revealed that a
large droplet-like particle 200 nm in diameter was surrounded
by a continuous structure. The TEM image confirmed
that this structure was composed of fine particles
smaller than 50 nm. According to EDS measurement, the
average Fe/Pt atomic ratio of particles 100–400 nm in diameter
(Fig. 2) was 0.3, and that of the area with a large
amount of particles 10–50 nm in diameter (Fig. 3a) was
0.6, indicating Pt-rich particle formation. Thus, crystalline
particles observed by XRD probably corresponded to the
comparatively larger particles in Figs. 2 and 3a. In contrast,
particles in the area depicted in Fig. 3b were mostly smaller
than 10 nm and Fe-rich (1.6 in Fe/Pt atomic ratio). Since an
Fe-rich phase was not detected in the XRD spectrum, these
smaller particles were possibly amorphous. Similarly in
water, the Fe/Pt atomic ratio of particles was 0.5 for larger
particles 30 to 300 nm in diameter, and 4.8 for those smaller
than 10 nm, probably in an amorphous state.
Large droplet particles may be formed by the ejection of
molten liquid from the target surface. A deviation of droplet
particle composition from the target composition occurs
due to preferential vaporization of elements caused by the
difference in volatility [12]. The Fe species, which have
higher vapor pressure than Pt, readily vaporizes, resulting
in Pt-rich droplet particle formation. In contrast, small particles
10–40 nm in diameter were still Pt-rich with relatively
high Fe content, compared to the droplets. These small particles
were probably formed by nucleation of condensed
vapor, ion, and/or cluster in the plume containing much
Fe vapor evaporated from the molten target [12]. However,
Fe-rich particles were detected not by XRD but by EDS
from the particles smaller than 10 nm. Certain Fe species
contained in these smaller particles were possibly oxidized,
because high oxygen content was also detected in these particles
by EDS measurement. A trace amount of dissolved
oxygen in hexane was probably unavoidable in our experiment
system. Small Pt-rich particles were formed in hexane
because the Fe species might be consumed by this oxidation
process. The effect of oxidation is especially pronounced in
the case of water because water can generate more active
oxygen species by water-molecule decomposition. Thus,
the absence of oxygen atoms in the molecule of the liquid
medium was important for an inhibition of Fe atom consumption
by oxidation. Crystallite sizes of Fe 23 Pt 77 and
Fe 38 Pt 62 in hexane and Fe 26 Pt 74 in water were estimated to
be 12, 16, and 14 nm by extracting the effect of line broadening
due to the small crystallite size from the measured XRD
line broadening containing the lattice strain effect [13]. The
crystallite sizes of Fe 38 Pt 62 in hexane were comparable as
determined by XRD and TEM measurements. In contrast,
the Fe 23 Pt 77 in hexane and Fe 26 Pt 74 in water were mainly
composed of droplets a few hundred nanometers in size,
though crystallite sizes determined by XRD measurement
were around 10 nm. This was probably due to the quenching
of droplets by the liquid medium, resulting in the formation
of amorphous phase or small crystallites.

428 Y. Ishikawa et al. / Chemical Physics Letters 428 (2006) 426–429
Fig. 2. SEM image of particles prepared in hexane.
Fig. 3. TEM images of particles prepared in hexane from the area of
aggregated particles with diameters less than (a) 40 nm and (b) 10 nm.
Fig. 4 illustrates the XRD pattern change with the heat
treatment of the Fe–Pt binary metal particles prepared by
laser ablation in hexane and water. No ordered phase
was observed from as-prepared particles in both cases. In
the case of hexane (Fig. 4a), a minor L1 2 phase (ordered
FePt 3 ) peak and a major ordered L1 0 phase (ordered FePt)
peak were observed in the particles heat-treated at 773 K.
The L1 0 phases appeared in a lower 2h angle than in Ref.
[14] due to deviation from the Fe 50 Pt 50 atomic ratio
[11,14]. Fe 23 Pt 77 and Fe 38 Pt 62 in the as-prepared particles
in hexane contributed the formation of L1 2 and L1 0 phases,
respectively. In contrast, only the L1 2 peak was detected by
annealing at 773 K for the particles prepared in water
(Fig. 4b). This result also corresponded with the Fe 26 Pt 74
formation close to FePt 3 observed in the as-prepared parti-
Fig. 4. X-ray powder diffraction spectra of as-prepared and heat-treated
Fe–Pt alloy particles prepared by laser ablation in (a) hexane and (b)
water.

Y. Ishikawa et al. / Chemical Physics Letters 428 (2006) 426–429 429
for stoichiometric alloying. The L1 0 phase was obtained
by heat treatment (above 773 K) of particles obtained by
laser ablation in hexane. The particles containing L1 0
phase exhibited low coercitivity compared with that of
bulk, due to a significant composition deviation from
Fe 50 Pt 50 .
Acknowledgement
Fig. 5. Hysteresis loops of the Fe–Pt alloy particles prepared in hexane for
as-prepared particles and those heat-treated at 873 K.
cles in water (Fig. 1b). However, heat treatment at 873 K
brought L1 2 phase disappearance and additional phase formation
close to L1 0 phase in the case of hexane. The additional
phase formation close to L1 0 phase was also
observed in the case of water after heat treatment at
873 K. These changes were possibly caused by the reduction
of Fe oxide species with consumption of L1 2 phase
[15]. Thus, the ablation in hexane was more effective than
that in water for preparing L1 0 phase, because the particles
obtained in hexane after heat treatment dominantly consisted
of L1 0 phase. This was probably due to the existence
of particles with Fe 38 Pt 62 composition, which was closer to
the 1:1 composition favorable for L1 0 phase formation.
Fig. 5 depicts the magnetic hysteresis loops of the particles
prepared in hexane for as-prepared particles and particles
that were heat-treated at 873 K, which was measured
at 5 K. The as-prepared particles showed no coercitivity,
while that of particles after heat treatment was 3500 Oe
at 5 K and 300 Oe at 300 K. This value was obviously
smaller than that of bulk and particles previously reported
in many papers [6–8,16–18], due to a significant composition
deviation from Fe 50 Pt 50 [6].
4. Conclusion
A mixture of Fe 23 Pt 77 and Fe 38 Pt 62 was obtained by
laser ablation of a FePt target in degassed hexane, and
Fe 26 Pt 74 was obtained in degassed deionized water. The
absence of oxygen in the liquid medium was important
The present work was supported by the Research Fellowships
of the Japan Society for the Promotion of Science
for Young Scientists. This study was partially supported by
the Industrial Technology Research Grant Program ’05
from the New Energy and Industrial Technology Development
Organization (NEDO) of Japan.
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