Chemical ordering of FePt nanoparticles by pulsed laser annealing

An irradiation with the third-harmonic wave of a Nd:yttrium-aluminium-garnet (YAG) laser with a 5 ns pulse width successfully transformed colloid-chemically synthesized FePt nanoparticles (NPs) from disordered face-centred cubic (fcc) into chemically ordered L1(0) (face-centred tetragonal: fct) phase. Granular films and hexane dispersions of fcc-FePt NPs were irradiated by the pulsed laser beam. In the case of the granular film, a coercivity of 1 kOe was achieved after the laser irradiation; meanwhile, the dispersion sample exhibited superparamagnetism, although the crystalline structure was transformed into fct phase. X-ray diffractometry and transmission electron microscopy revealed that coalescence between NPs was fairly well suppressed in the case of the dispersion sample. The superparamagnetism observed in the dispersion sample was due to insignificant coalescence and the resulting small magnetic domain.