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Novel ultrasonic-assisted alignment of L10 FePt nanoparticles S. Foglia a,∗ , A. Notargiacomo a , A. Capobianchi b , A.M. Testa b , D. Fiorani b , L. Arrizza c , C. Veroli d a
Istituto di Fotonica e Nanotecnologie, CNR, 00146 Roma, Italy
b
Istituto di Struttura della Materia, CNR, Montelibretti, Roma, Italy
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Università degli Studi dell’Aquila, Centro di Microscopia Elettronica, L’Aquila, Italy
d
Istituto di Studio Materiali Nanostrutturati, CNR, Montelibretti, Roma, Italy
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Article history: Available online xxxx Keywords: Magnetic nanoparticles Assembling
a b s t r a c t Chemically ordered (L10 ) FePt nanoparticles were synthesized by a microemulsion technique and the ordered phase was obtained by heating a mixture of particles and lyophilized NaCl as the separating medium, avoiding coalescence. A chloroform suspension of L10 FePt nanoparticles was evaporated at room temperature, assisted by ultrasonic vibration, producing the arrangement of the particles on the substrate. Parallel lines and flower-like patterns were obtained, starting from a 4 × 10−3 M suspension and using higher dilutions, suggesting that the fast evaporation of chloroform allows the particles deposition on the substrate, following the propagation direction of the ultrasound waves and/or placement at a vibration node, depending on the concentration. © 2008 Elsevier Ltd. All rights reserved.
FePt-based nanostructured materials are promising candidates for future ultrahigh density recording media because of their good chemical stability and high magnetocrystalline anisotropy (∼108 erg/cm3 ) observed in the ordered face-centred tetragonal (fct) L10 phase [1,2]. This large magnetocrystalline anisotropy allows nanometric grains to be thermally stable over typical data storage periods of 10 years. A well-organized magnetic array of such particles should contribute to efforts to design magnetic media capable of increasing densities well beyond 1 Tbit/in2 [3].
∗
Corresponding author. Tel.: +39 0657333343. E-mail address:
[email protected] (S. Foglia).
0749-6036/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2008.11.001 Please cite this article in press as: S. Foglia, et al., Novel ultrasonic-assisted alignment of L10 FePt nanoparticles, Superlattices and Microstructures (2008), doi:10.1016/j.spmi.2008.11.001
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Fig. 1. Summarized results of the characterization of monodispersed L10 FePt nanoparticles: (a) XRD patterns of samples prepared mixing fcc FePt nanoparticles with NaCl in different weight ratios (1%, 2%, 4%) and annealed at 700 ◦ C for four hours; the assignments are the Miller indices (hkl); (b) TEM image of fct FePt particles obtained mixing fcc nanoparticles with lyophilized NaCl in 4 wt% and annealing at 700 ◦ C for four hours; (c) Magnetization vs applied field at 300 K of a four-hour annealed sample prepared mixing FePt nanoparticles with NaCl in 4 wt% (Hc = 4.4 kOe).
In this report the ultrasonic-assisted alignment of L10 FePt nanoparticles on a silicon substrate was achieved by the fast evaporation of a chloroform suspension, producing parallel lines and flower-like patterns. Face-centred cubic (fcc) nanoparticles were chemically prepared in reversed micelle, which can be transformed into fct ones, by heat treatments at temperature above 500 ◦ C, although undesirable coalescing and sintering processes may take place [4]. In order to improve the separation of the particles during heat treatment, the fcc nanoparticles were mixed with NaCl powder, as a separating medium before annealing [5], since it is easy to wash away after the heat treatment due to its high solubility in water and the contamination of the sample is avoided because of its high melting point (>700 ◦ C). The commercial chemical grade salt was lyophilizated before mixing, causing a narrow size distribution of the particles because the salt aqueous solution is uniform and the uniformity is restored in the solid state after a fast freezing at T = −13 ◦ C and under vacuum solvent sublimation (i.e. a lyophilization or freeze drying process). The size distribution of the salt particles was rather sharp with a mean value of 130 nm and the statistical dispersion was measured as the standard deviation and estimated as 30 nm. The homogeneous size of salt particles enhanced the dispersion of FePt nanoparticles in such a matrix. Fcc alloy nanoparticles and lyophilized salt were mixed using different weight percent ratios: 1, 2, 4 and 5% and the mixtures were annealed for 2 and 4 h at 700 ◦ C under hydrogen flow. The nanoparticles were characterized with an X-ray diffractometer (Siefert XRD 3000, Cu Kα radiation), a transmission electron microscope (TEM, Philips CM200 equipped with an OxfordLink ISIS 300 Energy Dispersive Spectroscopy facility), an atomic force microscope (AFM, Tapping Mode configuration, Digital Instruments D3100, Nanoscope IIIa) and a superconducting quantum interference device (SQUID) magnetometer (Hmax = 55 kOe). Fig. 1 represents summarized results of the characterization. The change of the crystal structure was observed by wide angle X-ray diffraction (XRD) measurements. XRD patterns of all the samples that had undergone two hours of annealing showed broad peaks for the diffraction planes (111) at 40◦ , (200) at 47◦ and (220) at 68◦ , indicating that the alloy particles still possessed a chemically disordered fcc structure (not reported). Fig. 1(a) shows XRD patterns of samples annealed for 4 h. Well defined (001), (110), (002) and (202) peaks, identifying the fct phase, appeared when the FePt content reached 4 wt%. The chemical ordering of the samples prepared with an alloy content of 1 and 2 wt% is low, probably because of the imbalance between heat energy and phase transformation energy, and the fctstructure formation could be achieved by prolonging the annealing. No major changes were detected in the spectra of samples with FePt/NaCl = 5 wt%. Diffraction peaks associated with NaCl or any other phases were not found in the XRD patterns, indicating that a minimal contamination by the salt during annealing is present. The whole structural phase transformation from fcc to fct takes place at 700 ◦ C with at least a 4 wt% alloy/NaCl mixture. Please cite this article in press as: S. Foglia, et al., Novel ultrasonic-assisted alignment of L10 FePt nanoparticles, Superlattices and Microstructures (2008), doi:10.1016/j.spmi.2008.11.001
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Fig. 2. AFM images showing the depositions of a 4 × 10−3 M (a), 3 × 10−3 M (b), 2 × 10−3 M (c) and 10−3 M (d) FePt chloroform suspensions on a silicon substrate, driven by ultrasonic vibrations.
Energy dispersive X-ray spectroscopy (EDS) analyses of alloy particles, after heat treatment and salt removal revealed that the average composition of all the samples is Fe/Pt = 53:47, close to the chemical stoichiometry of FePt. The morphology of the annealed nanoparticles was closely monitored by TEM observations. Fcc nanoparticles prepared using the micelle solution show a spherical shape with a mean diameter of 10 nm. The heat treatment of a fcc FePt nanoparticles mixture with salt, reaching 4wt%, produces well-separated fct particles with a mean diameter of 12 nm and sintering processes are not detected, indicating that salt dispersion successfully prevents the aggregation of particles. It was found that the annealed particles are easy to aggregate because of the magnetic attraction. A few monodispersed nanoparticles can be observed depositing on the TEM observation grid by casting a drop of an alcohol suspension of the nanoparticles (Fig. 1(b)). Decreasing the salt content in the mix with the alloy to 5%, the heat treatment produces half of the particles well separated, with a mean diameter of 40 nm, and half as clusters of three particles. The fct phase is formed but sintering has started and a considerable size increase is observed, indicating that the lower the alloy to salt ratio, the less the sintering. Magnetic properties of samples investigated before removing salt, are found to vary according to the weight ratios of NaCl and FePt. The room temperature hysteresis loop of the sample prepared with a FePt content of 4 wt%, annealed at 700 ◦ C for 4 h (Fig. 1(c)) indicates a complete transformation to the high anisotropic fct phase (Hc = 4.4 kOe). The high coercivity of this sample is in accordance with the value expected from a powder of nanoparticles with a high magneto-crystalline anisotropy which is not assembled in an ordered pattern. The virgin curve (dashed curve) shows the characteristics of almost single grain switching, where exchange coupling, even if present, does not lead to a cooperative reversal of grain magnetization [6]. No major changes in magnetic behaviour are detected by increasing the alloy content to 5 wt%. After heat treatment NaCl was removed with de-ionized water and the nanoparticles, obtained by the mixture with salt of 4 wt% (i.e. diameter of 12 nm), were suspended in a chloroform bath at room temperature, followed by ultrasonic pulses. Ultrasonication has been reported to improve the dispersing ability of magnetic nanoparticles and it is possible to suspend particles even using a solvent of density higher than water. Chloroform was selected as a solvent because of its high density (1.48 g/cm3 ) and low boiling point (61.2 ◦ C), which allows a fast removal by evaporation. A drop of the suspension was cast on a silicon substrate, lying on the bottom of a vessel, dipped in an ultrasound bath (50 W, 40 kHz). The ultrasonic force was able to produce suspension jets, during the chloroform evaporation. After solvent evaporation, well separated particles were arranged on a large area (up to 15 × 15 µm2 ), whose pattern depended on the concentration of particles in the solvent: parallel lines and flower-like patterns were obtained, starting from a 4 × 10−3 M suspension and using higher dilutions. These arrangements indicate that the fast evaporation of chloroform allowed particle deposition on the substrate, following the propagation direction of the ultrasound waves and/or placement at a vibration node, depending on the density of particles in the solvent. In addition the suspension droplets spread over the substrate easily and the wetting behaviour provides an equilibrium interface shape driven by the capillarity action phenomenon. Wide agglomerations are Please cite this article in press as: S. Foglia, et al., Novel ultrasonic-assisted alignment of L10 FePt nanoparticles, Superlattices and Microstructures (2008), doi:10.1016/j.spmi.2008.11.001
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not detected, due to the capacity of the ultrasound to overcome the magnetic attraction between the particles. AFM image of the L10 FePt array of nanoparticles obtained by evaporation of a suspension with a 4 × 10−3 M concentration is shown in Fig. 2(a). The nanoparticles were arranged in parallel lines, over very large areas (up to 15 × 15 µm2 ). The particles show a broad size distribution of the widths, compared to TEM investigation results. However the section analysis proved that the average particle thickness was 12 nm, in accordance with TEM results, then the enlargement could disclose coupled particles, both lying on the substrate surface, possibly not even separated. It must also be take in account that the resolution of AFM small-scale measurements at ambient conditions is affected by the tip geometry, which can introduce convolution artefacts, and by the presence of a humidity layer on the top of the substrate [7]. The organization of the particles into linear arrays could be promoted by the propagation of ultrasonic waves. Suspension of particles with different concentrations were tested in order to investigate such a mechanism. The dilution of the suspension to 3 × 10−3 M produced an arrangement formed by lines and circles, as shown in Fig. 2(b), suggesting that the particles could be localized partly at the ultrasound force lines, partly at the vibration nodes. Clusters of particles with a flower-like patterns were obtained by evaporation of a 2 × 10−3 M suspension (Fig. 2(c)), indicating that all the particles were placed only at the nodes of the ultrasound vibration. This organization was detected on large areas, up to 6 × 6 µm2 . The mean radius of a cluster is 90 nm and the mean distances between the clusters is 1, 18 ± 0, 08 µm. Increasing the dilution of the suspension further to 10−3 M, particles organized in well separated –single unit- parallel lines (the calculated mean distance between the lines being 330 nm) were detected in a few large areas of the substrate (Fig. 2(d)), but most of the particles are aggregates (not shown). Using higher dilutions, no particle organization was found on the substrate. Isolated L10 FePt nanoparticles with a grain size of 12 nm have been prepared by taking advantage of the dispersing ability of the ultrasound pulses in overcoming the magnetic attraction force and exploiting the fast evaporation time of the chloroform at ambient conditions. Magnetic properties of individual particles and assemblies need to be further investigated. Acknowledgements This work is supported by the NANOSPIN European Project, contract number NMP4-CT-2004013545. S. Foglia thanks Dr. M. Vasquez Mansilla for his very helpful discussions. References [1] [2] [3] [4] [5] [6] [7]
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