Improvement of saturation magnetization of Fe nanoparticles by post-annealing in a hydrogen gas atmosphere

Fe nanoparticles (NPs) were synthesized by the thermal decomposition of Fe(CO)5 and then post-annealing in a hydrogen gas atmosphere to produce highly monodisperse Fe NPs with high saturation magnetization (Ms). The as-synthesized pre-anneal Fe NPs had an expanded α-Fe structure and Ms was only 39% of that for bulk Fe because of the low crystallinity and the inclusion of a surfactant. Post-annealing of the Fe NPs in a hydrogen gas atmosphere at 200 °C improved the crystallinity of the Fe NPs from an amorphous-like structure to a body centered cubic (bcc) structure without any lattice expansion. This result indicates that hydrogen gas plays a significant role in improvement of the crystallinity of Fe NPs. Accompanying the improvement in crystallinity, Ms for the Fe NPs increased from 86 to 190 emu/gnet at 300 K, the values of which include the weight of surfactant. This enhanced Ms is almost the same as that of bulk Fe (218 emu/Fe). It was concluded that the crystallinity has a significant influence on the...