Synthesis, Self-assembly and Magnetic Properties of FePtCu Nanoparticle Arrays
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Synthesis, Self-assembly and Magnetic Properties of FePtCu Nanoparticle Arrays Xiangcheng Sun, S. S. Kang, J. W. Harrell and David E. Nikles Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama, 35487-0209 Z. R. Dai, J. Li and Z. L. Wang School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 ABSTRACT FePtCu nanoparticles with varying composition were synthesized by chemical solutionphase reduction of platinum and copper reagents and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers. As prepared the particles had fcc structure with an average diameter of 3.5 nm and were superparamagnetic. The particles were well dispersed in hydrocarbon solvents and could be self-assembled into two or three dimensions particles arrays with a variety of close-packing arrangements. Heat-treatment of the self-assembled films at temperatures above 550°C transformed the particles from the fcc to the L10 phase, giving in-plane coercivities as high as 9000 Oe. Xray diffraction revealed that the Cu remained in the annealed FePtCu films and the presence of an extra peak, indicated a second phase was present. Consistent with one or more phases, the magnetic hysteresis curves could be decomposed into a hard component (Hc > 5,000 Oe) and a soft component (Hc < 2,000 Oe). Unlike our earlier results for Ag in FePt, adding Cu to FePt did not lower the temperature required for phase transformation from the fcc to the fct L10 phase. INTRODUCTION The important report by S. Sun et al [1] on the synthesis, self-assembly and magnetic properties of nanometer-sized FePt particles caused an explosion of interest in the use of these particles in granular magnetic recording media. The FePt nanoparticles were prepared by the polyol reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in dioctyl ether in the presence of oleyl amine and oleic acid. As made the FePt particles with particle size from 3 to 5 nm showed fcc structure and were superparamagnetic. The particles could be dispersed in hydrocarbon solvents using oleic acid and oleyl amine as surfactants. When the dispersions were cast onto solid substrates and the solvent evaporated, the particles self-assembled into close-packed arrays. After heat treatment at temperatures above 550°C the particles transformed to the L10 phase, having high magnetocrystalline anisotropy [2], giving films with high coercivity. However, there was considerable particle coalescence and loss of particle positional order when the films were heated to temperatures above 550°C [3]. Particle coalescence leads to increased switching volumes and increases the distribution of particle volumes, which defeats the
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objective of making small monodisperse nanoparticles. It would be highly desirable if the temperature required for the fcc to fct (tetragonal phase) transformation were lower, at least below temperatures where the particles coalesce
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