Monodispersed ferromagnetic FePt nanoparticles, partially ordered in the L10 phase, were directly prepared without further annealing by high temperature synthesis (z300 C) involving poly(N-vinyl-2- pyrrolidone) and Triton X-100 as protective agent and reaction solvent respectively. Depending on the synthesis conditions, nanoparticles with average sizes ranging from 5 to 7 nm and coercive fields reaching 0.1 T at 300 K were obtained, but they invariably aggregate by magnetic dipolar interaction. By increasing the solvent viscosity (using PEG 600), 5 nm superparamagnetic nanoparticles are embedded in an amorphous matrix derived from solvent condensation/decomposition, thus avoiding aggregation. Nanoparticles are then completely converted to the hard tetragonal L10 phase, preserving the original size, by annealing in a vacuum at higher temperatures that, at the same time, transform the matrix into amorphous carbon. Annealing at 650 C for 3 h leads to coercive fields of about 0.25 T at RT and 1.3 T at 5 K (without reaching the saturation magnetization) and to a peculiar squeezing of the hysteresis loops. Subsequent treatments at higher temperatures induce a further shrinking of the loop and a reduction of the coercive field. The possible explanation takes into account that, by raising the annealing temperature, an increasing number of nanoparticles becomes free to rotate inside the matrix, aligning like ‘‘nano-compasses’’ with the applied magnetic field. However a fraction of nanoparticles remains still locked to the matrix, generating a superimposed magnetically hard contribution.
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