Abstract

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)<inf>3</inf> 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 FePt<inf>3</inf>/Fe<inf>2</inf>O<inf>3</inf> into FePt by annealing, characteristics of hysteresis also suggest the co-existence of other minor phases. When either Fe(tmhd)<inf>3</inf> or Fe(hfac)<inf>3</inf> 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)<inf>3</inf> and Fe(dbm)<inf>3</inf>.