Fe3O4 nanoparticles coated with oleic acid as a surfactant were reduced by hydrogen plasma generated by 13.56 MHz Radio-Frequency (RF). With increasing input RF power to the inductive coil, Fe3O4 nanoparticles could be transformed to almost single phase of Fe3C and finally to α-Fe. The average grain sizes of Fe3C and α-Fe nanoparticles are 19 - 23 nm and 52 -59 nm, respectively. Saturation magnetizations of Fe3C and α-Fe nanoparticles at 5 K were almost same as these of bulk Fe3C and α-Fe. Field cooling procedure from room temperature to 5 K in the magnetic measurement revealed a formation of FeO phase from Fe3O4 by weak reduction. The desired ranges of electron temperature and electron density in order to obtain Fe3C and α-Fe phases were determined by investigating plasma characteristics.
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