Magnetic nanoparticles (NPs) have attracted attention for their potential use electronic devices and nano -bioengineering applications. Fe NPs made from an Fe(CO)x-Oleylamine (OlAm) reacted precursor by thermal decomposition show high saturation magnetization (σs) (∼140 emu/gnet at 300 K). However, the σs never reaches the bulk value (218 emu/g at 300 K) of iron. This is due to the unique crystalline structure of the Fe NPs. Fe NPs covered with OlAm (OlAm-Fe NP) have an expanded α(b.c.c.) structure and ultra-fine grains. We improved the σs of Fe NPs via phase control by adjusting the adsorption ability of a surfactant such as trioctylamine (TOcAm) or tribenzylamine (TBeAm). These surfactants have weaker adsorption ability than that of OlAm. Rietveld analysis revealed that the crystalline structure of Fe NPs covered with TOcAm or TBeAm was drastically changed into a rather α-Fe phase, in contrast to the expanded α phase observed for the OlAm-Fe NPs. TOcAm-Fe NPs had 10- 80 vol.% of the α-Fe phase, and TBeAm-Fe NPs had 80- 99 vol.% of the α-Fe phase, which indicates an α-Fe single phase. A σs of 199 emu/gnet for the TBeAm-Fe NPs was obtained at 300 K. This remarkable improvement is due to the increase volume fraction of the α-Fe phase. These results suggest that α-Fe phase formation using weak adsorption surfactant was highly effective for improving the σs.
