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

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.