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 Fe(2+) ions oxidize into either gamma-Fe(2)O(3) or Fe(3)O(4) 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-Fe(2)O(3). (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3504597]