Study on Composition Distribution and Ferromagnetism of Monodisperse FePt Nanoparticles
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Monodisperse FePt nanoparticles with size of 4.5 and 6.0 nm were prepared by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in benzylether. The crystallography structure, size, and composition of the FePt nanoparticles were examined by X-ray diffraction and transmission electron microscopy. Energy dispersive X-ray spectrometry measurements of individual particles indicate a broad compositional distribution in both the 4.5 and 6 nm FePt nanoparticles. The effects of compositional distribution on the phase-transition and magnetic properties of the FePt nanoparticles were investigated.
KeywordsFePt Nanoparticles Compositional distribution Magnetic properties
Chemical synthesized FePt nanoparticles (NPs) have attracted much research interest because of their potential application in ultrahigh density magnetic recording media, bio-medical application and catalyst [1, 2, 3]. FePt has two phases, one is fcc and the other is fct. Fcc is the commonly obtained format, soft magnet and can transform to hard-magnetic fct phase at high temperatures. Since IBM group demonstrated a hot organometallic synthesis for the monodisperse fcc FePt NPs in 2000, the method was subsequently developed by many groups aiming at achieving FePt nanoparticles with controlled size, shape, self-assemblies as well as functional composite structures [4, 5, 6, 7, 8, 9, 10]. Although many progresses were made in this area, there have been relatively limited investigations on the formation mechanism of the FePt NPs. For instance, a wide atomic composition distribution was reported in the 3.1 nm FePt NPs by Yu et al. . Such a broad compositional distribution was recently observed in the 2.9 nm FePt NPs by Thompson et al.  and 4.6 nm FePt NPs by Bagaria et al. . However, the composition distribution of bigger (larger than 5 nm) FePt NPs is not clear yet. It’s well known that the particle-to-particle compositional variation would significantly affect the phase transition as well as magnetic performance of the FePt system. To understand the mechanism of the formation of FePt NPs and their magnetic properties, an insight into the atomic composition of the FePt NPs is of significance.
In this study, we focus on the atomic composition and magnetic properties of FePt nanoparticles prepared by organic solution method. Monodisperse FePt NPs were synthesized via simultaneous reduction of platinum acetylacetonate and thermo-decomposition of iron pentacarbonyl in benzylether using oleyl amine and oleic acid as surfactants. The size and composition of the nanoparticles were tuned by changing the dose of Fe(CO)5 and surfactants (oleyl amine and oleic acid) in the reactions. After systematically examined the isolated FePt nanoparticles using TEM-EDX, we observed an inhomogeneous compositional distribution in both the 4.5 and 6.0 nm equiatomic FePt NPs. It was also found that the 6 nm Fe50 Pt50 nanoparticles have wider composition distribution than that of the 4.5 nm Fe48Pt52NPs. This is attributed to the wider size distribution of Pt nuclei in the formation of bigger FePt nanoparticles. The effects of composition variation on the magnetic properties of the FePt nanoparticles are discussed.
The FePt nanoparticles were prepared by slightly modify the synthesis procedure reported by Chen . In the synthesis of 4.5 nm FePt NPs, 0.5 mmol platinum acetylacetonate (Pt(acac)2) was added to a flask and mixed with 20 ml benzyl ether under a nitrogen atmosphere. After the solution was stirred for 15 min at room temperature, the flask was heated up to 100°C and then 1 mmol oleic acid, 1 mmol oleylamine and 2 mmol Fe(CO)5 were added. The heat rate was keeping at 5°C/min during the synthesis. The solution was then directly heated to reflux temperature for 30 min before cooling to room temperature. Similar procedure was employed for the synthesis of 6 nm FePt NPs. In the reaction, the dose of Fe(CO)5 was 2.5 and 2 mmol olcic acid, 2 mmol oleylamine were used while the other conditions were kept unchanged.
After the prepared black solution was cooled to the room temperature, 20 ml ethanol was added into the solution, the black products were then precipitated by mild centrifugation (5,000 rpm). The yellow–brown supernatant was discarded. The precipitate were redispersed in 10 ml hexane and precipitated again with 20 ml ethanol by centrifugation. Further purification of the product was performed by dispersing the product into hexane, precipitating it out with ethanol, and centrifuging. Finally, the purified nanoparticles were dispersed in mixture of 10 ml hexane.
The size and morphology of the nanoparticles are characterized using transmission electron microscopy (TEM, Tecnai 20 ST, FEG). The individual particle composition measurements by energy dispersive X-ray spectrometry (EDS) were performed on the FEI Tecnai STEM with a spot size of ~2 nm. The crystallography structure of the FePt nanoparticle powder was characterized by X-ray diffraction (XRD, Bruker D5) with CuKα radiation. The magnetic properties of the FePt nanoparticles were measured by a vibrating sample magnetometer (VSM) at 300 K.
Results and Discussion
The HADDF image of the 6.0 nm NPs is also presented in Fig. 3. Atomic composition between Fe20Pt80 and Fe77Pt23 were obtained by measuring fifteen isolated FePt NPs in this system. Though the analysis is not enough to give a histogram of the compositional distribution, an even wider composition variation is observed for the 6 nm FePt NPs. These results may be understood by the reaction mechanism of the FePt NPs. It has been demonstrated that the organometallic reaction involves the formation of a Pt (or Pt rich) seed cluster followed by further growth of Fe atoms onto the seed [7, 12, 17]. The composition variation of the FePt NPs can be caused by different size of Pt nuclei and un-homogenous supply of Fe source onto the Pt nuclei . Specially, in the formation of bigger FePt NPs, more surfactants were used to lower nucleation rates of the Pt precursor and this may give rise to larger difference in the size of the Pt nuclei. Smaller Pt seeds have great surface specific area and allow more Fe atoms grow on them. Likewise, less Fe atoms would grow onto the bigger Pt seeds to form a Pt rich FePt nanoparticle. Consequently, a wide composition distribution is formed due to the separation of nucleation and growth of FePt NPs in reaction.
It should be noted that the formation mechanism of FexPt1−x depends on experimental parameters such as the choice of precursors for Fe and Pt, surfactants, solvents and flux time. Hence, the composition distribution of the FePt NPs is sensitive to the fabrication parameters. Recent literatures have shown that in the core–shell type FePt NPs, the inorganic shell could protect the inner FePt core from sintering during the annealing process and coercivity over 1T was obtained for the well-separated FePt NPs [22, 23]. These results suggest that the developed chemical synthesis allows precisely control over the composition variation as well as the uniformity of the FePt composited nanoparticles. In order to apply the FePt nanoparticles for various applications, especially for future ultrahigh-density data storage applications, direct access to the compositional distribution of the FePt NPs are also vital in future research.
In summary, we conducted a study on the composition distribution of FePt nanoparticles prepared by the oganometallic method, in which platinum acetylacetonate and Fe pentacarbonyl was used as precursors and oleyl amine and oleic acid as surfactants. We evidenced by STEM-EDX analysis that both the as-prepared 4.5 and 6.0 nm FePt nanoparticles have a wide compositional distribution, which is caused by the separation of nucleation and growth of particles in reaction. However, such wide composition distribution has limited influence on the ferromagnetism of sintering FePt nanoparticles through a “composition–compensation” process in thermal annealing.
This work is supported in part by the National Nature Science Foundation of China under Grant No. 50801023 and 50801022, NSF, STD and ED of Hubei Province (No. 2007ABC005, No. 2009CDA035, No. 2008BAB010 and No. Z20091001).
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