Direct Chemical Synthesis of L1 0 FePt Nanostructures Irene Zafiropoulou, Eamonn Devlin, Nikos Boukos, Dimitrios Niarchos, Dimitrios Petridis, and Vassilios Tzitzios* Institute of Materials Science, NCSR “Demokritos”, Aghia ParaskeVi, 15310 Athens, Greece ReceiVed February 2, 2007 ReVised Manuscript ReceiVed March 6, 2007 Bimetallic FePt and CoPt nanoparticles in the ordered face- centered tetragonal phase (fct) combine chemical stability, high magnetocrystalline anisotropy (K u ≈ 7 × 10 6 J/m 3 ), and high coercivity values. These unique properties allow reduction of the particles size below 10 nm with simultaneous stabilization of their magnetization against thermal fluctua- tions and demagnetizing effects; such properties are necessary for ultrahigh-density magnetic storage applications. 1-4 Nowadays, a plethora of chemical routes for the synthesis of ultrafine, monodispersed FePt nanoparticles in the sub- 10 nm range size have been developed. 5-13 The as-made nanoparticles have a disordered face-centered cubic (fcc) structure and reveal superparamagnetic behavior. A thermal treatment (annealing) of the as-made nanoparticles at tem- peratures above 550 °C is required in order to obtain the mag- netically interesting face-centered tetragonal (fct) phase. 1 However, annealing at these temperatures induces complete decomposition of the protective organic layer surrounding the surface of each particle and consequently leads to an undesir- able aggregation and sintering. As a result, the nanoparticles lose their solubility and most importantly their size and shape homogeneity, which is essential for self-assembly. Recently, different methods have been used to attempt to lower the FePt phase-transition temperature. Introduction of a third metal, such as Ag, 15 Au, 16 and Sb, 17 into FePt chemically synthesized nanoparticles has lowered the fcc to fct transformation temperature to approximately 400 °C. Recently, Howard et al. 18 reported the direct synthesis of fct FePt nanoparticles using Collman’s reagent, Na 2 Fe(CO) 4 , as reducing agent for the Pt 2+ . The reaction occurs in hydrocarbon solvents at 330 °C in the presence of surfactants under an inert atmosphere. The as-made FePt nanoparticles are partially ordered with an average particle size of 6-8 nm and reveal coercivities of ∼1300 and 3100 Oe at room temperature and 10 K, respectively. Kang et al. 19 synthesized partially ordered FePt nanoparticles by thermal decomposi- tion of Fe(CO) 5 and reduction of Pt(acac) 2 in hexadecylamine at 360 °C in the presence of 1-adamantanecarboxylic acid. The particles exhibit coercive fields of 500 and 800 Oe in the perpendicular and parallel directions, respectively. A similar procedure has been reported by Jia et al. 20 for FePtAu nanoparticles of different compositions, with coercivities varying from a few hundred to a few thousand Oe. In this work, the direct synthesis of hard magnetic L1 0 FePt and FePtAu nanoparticles, without any postannealing step is reported. The nanoparticles synthesized present considerable ordering and a high coercive field of 5.8 kOe. In a typical synthesis route, 2 mL of oleylamine (Fluka), 2 mL of oleic acid (Fluka), 0.26 mmol Fe(acac) 3 (Aldrich), 0.25 mmol Pt(acac) 2 (Alfa Aesar), and 0.077 mmol AuCl 3 (Alfa Aesar) are added to 20 mL of liquid paraffin (Fluka) to synthesize a material with Fe 44 Pt 43 Au 13 composition, which has been proved to present the lowest transformation temperature. 16 Liquid paraffin is used as a high-boiling-point solvent, whereas oleylamine plays the role of mild reducing agent and surfactant, along with oleic acid. The mixture is heated to 200 °C under a N 2 atmosphere for 20 min; subsequently, the temperature is raised to reflux conditions (390-400 °C) for 3 h. The synthesis procedure was also performed under a 4% H 2 -96% Ar atmosphere. To control the morphology of the nanoparticles synthesized, instead of oleic acid, which leads to the formation of worm-like nanostructures, we used PVP (polyvinyl pyridine, Fluka, MW ) 1500, 30 mg) and pluronic tri-block copolymer (F-127, Sigma, 30 mg). The resulting black precipitant is magneti- cally separated and washed several times with a hexane- ethanol mixture. The same procedure was followed for the synthesis of bimetallic FePt nanoparticles, without Au addition. * Corresponding author. E-mail: tzitzios@ims.demokritos.gr. (1) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (2) Hyeon, T. Chem. Commun. 2003, 927-934. (3) Tzitzios, V.; Niarchos, D.; Gjoka, M.; Boukos, N.; Petridis, D. J. Am. Chem. Soc. 2005, 127, 13756-13757. (4) Sun, S. AdV. Mater. 2006, 18, 393-403. (5) Sun, S.; Anders, S.; Thomson, T.; Baglin, J. E. E.; Toney, M. F.; Hamann, H. F.; Murray, C. B.; Terris. B. D. J. Phys. Chem. B 2003, 107, 5419-5425. 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