The effect of plasma-striking atmosphere: air and air+Ar-gas on the crystallization of Fe-oxide phases was studied using dc thermal arc-plasma processing route. The powders were characterized by x-ray diffraction, vibrating sample magnetometry, transmission electron microscopy, and Mossbauer spectroscopy techniques. At room temperature and O-2 rich atmosphere, arc-evaporated Fe2+ ions oxidize into either gamma-Fe2O3 or Fe3O4 depending upon the combining ratio of Fe with molecular O-2. Fe/O ratio could be adjusted using proper flow rate of Ar gas to crystallize the pure gamma-Fe2O3. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3504597]