In this investigation, we synthesize FeNi alloy magnetic nanoparticles (MNPs) by using both chemical precipitation and combustion methods. The FeNi MNPs prepared by combustion method have a rather high saturation magnetization M{sub}s of ～180 emu/g and a coercivity field H{sub}c of near zero. The functionalized FeNi MNPs which were coated with biocompatible polyethyleneimine (PEI) polymer have also been synthesized. We demonstrated that the PEI coated FeNi MNPs can enter the mammalian cells in vitro and can be used as a magnetic resonance imagine (MRI) contrast agent. The results demonstrated that FeNi MNPs potentially could be applied in the biomedical field. To prepare a higher quality and well controlled Fe-Ni MNPs, we also developed a thermal reflux chemical precipitation method to synthesize FeNi{sub}3 alloy MNPs. The precursor chemicals of Fe(acac){sub}3 and Ni(acac){sub}2 in a molecular ratio of 1:3 reacted in octyl ether solvent at the boiling point of solvent (～300°C) by the thermal reflux process. The 1,2-hexadecandiol and tri-n-octylphosphine oxide (TOPO) were used as reducer and surfactant, respectively. The chemically precipitated FeNi{sub}3 MNPs are well dispensed and have well-controlled particle sizes around 10-20 nm with a very narrow size distribution (±1.2 nm). The highly monodispersive FeNi{sub}3 NPs present good uniformity in particle shape and crystallinity on particle surfaces. The MNPs exhibit well soft magnetism with saturation magnetization of ～61 emu/g and H{sub}c ～ 0. The functionalized magnetic beads with biocompatible polymer coated on MNPs are also generated completed for biomedical applications.
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