Magnetic phase transition of annealed FePt based nanoparticles synthesized by using Fe(β-diketonate)3

• We report annealing effects on FePt nanoparticles prepared by the novel Fe sources.
• XANES and VSM results indicated the transition of magnetic phase after annealing.
• Particle agglomeration could be prevented by self-assembly into a large monolayer.
• Particles with a heterodimer shape could be transformed to spheroid after annealing.

FePt nanoparticle is under development as ultrahigh density recording material because it retains ferromagnetic properties even if its dimension is reduced below 10 nm. However, the heat treatment, required to transform the superparamagnetism of as-synthesized particle into ferromagnetic phase, brings about undesirable increases in particle size. This work demonstrates that the magnetic phase transition without excessive agglomeration can be obtained by annealing long-range monolayer assembly of FePt based nanoparticles. The magnetic properties are dependent on the type of Fe(β-diketonate)3 used in the synthesis due to the variations in elemental compositions and mixed phases in annealed nanoparticles. Whereas X-ray characterizations confirm the conversion of FePt3/Fe2O3 into FePt by annealing, characteristics of hysteresis also suggest the co-existence of other minor phases. When either Fe(tmhd)3 or Fe(hfac)3 is employed as starting materials, the Fe fraction is much less than that of Pt resulting in narrow hysteresis loop with low coercivity. By contrast, the coercivity is enhanced with the Fe:Pt ratio closer to the unity in the case of Fe(acac)3 and Fe(dbm)3.

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