Amorphous to crystalline transformation of ultrafine $\mathrm{Fe_{62}B_{38}}$ particles

$Fe_{62}B_{38}$ particles, with sizes of 10–200 nm, have been produced by chemical reduction of Fe(II) ions in aqueous solution by KBH$_4$. Electron and X-ray diffraction, X-ray absorption fine structure and Mössbauer spectroscopy results reveal that the particles are amorphous. The Mössbauer parameters and the estimates of temperatures for crystallization and for ferromagnetic to paramagnetic transition suggest that the amorphous structures are similar for the particles and ribbons or films produced by liquid-squench or sputtering. The increase of the magnetic hyperfine field with annealing temperature, as deduced from $^{57}Fe$ Mössbauer spectroscopy, is attributed to atomic rearrangement in the amorphous phase. Crystalline Fe2B and α-Fe are the products when the particles are annealed in Ar and H2 at around 715 K. It is proposed that iron and boron partly separate during the initial stages of crystallization and subsequently form Fe$_2$B. Passivation of the particles makes them oxidatively stable, even when heated in air at temperatures up to 650 K