Monodispersed 4.1 nm FePt nanoparticles with narrow size distribution were successfully synthesized by the chemical polyol process with co-reduction of Fe(acac)3 and Pt(acac)2 in the presence of 1,2 hexadecanediol as a reducing agent and oleic acid and oleyl amine as a surfactant. To achieve L10 ordered structure, annealing at high temperature is required to realize phase transformation from face center cubic (fcc) to face center tetragonal (fct) phase. In this situation, FePt nanoparticles joining together and their size become larger than 20 nm. This is due to thermal decomposition of organic surfactant (oleic acid and oley amnie) at temperature around of 350 oC. In the present work, we could to prevent sintering of FePt nanoparticles during the annealing process at temperatures of 650 and 750 oC by using the core/shell structure. In this case, MnO nanoparticles were used successfully as the shell around of each FePt core particles to protect them from sintering. As results, coercivity, Hc, of FePt and FePt/MnO nanoparticles after annealing at 650 oC is equal with 5 kOe and 2 kOe, respectively. With increasing annealing temperature to 750 oC, the coercivity of these nanoparticle increases dramatically as value of 10 kOe and 5 kOe, respectively.

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