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Temperature dependence and uniaxial magnetocrystalline anisotropy properties of the chemically synthesized 4 nm L1 0 -Fe 55 Pt 45 nanoparticle assembly by a modified polyol route are reported. As-prepared nanoparticles are superparamagnetic presenting fcc structure, and annealing at 550°C converts the assembly into ferromagnetic nanocrystals with large coercivity ͑H C Ͼ 1 T͒ in an L1 0 phase. Magnetic measurements showed an increasing in the ferromagnetically ordered fraction of the nanoparticles with the annealing temperature increases, and the remanence ratio, S = M R / M S Х 0.76, suggests an ͑111͒ textured film. A monotonic increase of the blocking temperature T B , the uniaxial magnetocrystalline anisotropy constant K U , and the coercivity H C with increasing annealing temperature was observed. Magnetic parameters indicate an enhancement in the magnetic properties due to the improved Fe 55 Pt 45 phase stabilizing, and the room-temperature stability parameter of 67, which indicates that the magnetization should be stable for more than ten years, makes this material suitable for ultrahigh-density magnetic recording application.

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