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

More documents

Recommendations

Info

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.
Page 1: Chemical Physics Letters 428 (2006)

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

Create successful ePaper yourself

Delete template?

Save as template?