Magnetic properties of nanocrystallized Fe–Pt–B melt-spun ribbons

L10 FePt nanoparticles have been prepared by etching grain boundaries of heat-treated melt-spun Fe–19Pt–25B (at.%) alloy ribbons. It is revealed that an L10 FePt nanocrystalline phase is directly formed from the Fe–Pt–B amorphous ribbons by long-time heat treatment at low temperatures in the vicinity of 723 K. With increasing heat treatment time, dimensions of the nanocrystallized FePt grains increase, accompanied by a change from soft ferromagnetic to hard ferromagnetic. The ribbon crystallized at 723 K for 1.8 ks consists of only an FePt L10 phase and its coercivity is as low as 0.381 kA/m. However, it increases to 372 kA/m with increasing grain size of precipitated L10 phase to about 30 nm by heat treatment for 86.4 ks, while the saturation magnetic flux density remains constant at about 0.4 T. Etching boundaries in heat-treated ribbons has been performed to obtain ferromagnetic L10 FePt nanoparticles and several particles were observed by means of transmission electron microscopy.