Fe-N (N = 0-15 at%) films have been formed on MgO single-crystal substrates using reactive sputtering by means of ultra-clean sputtering systems. The effect of O atom impurities on the structure and saturation magnetization for α′-Fe-N and (α′ + α″)-Fe16N2 are discussed. The results can be summarized as follows: (1) The preferred grain orientation of α′-Fe-N and α″-Fe16N2 was improved by applying the ultra-clean (UC) process (FWHM: α′(0 0 2) 1.23° → 0.9°, α″(0 0 2) 2.31° → 1.34°). The origin of the improvement in preferred grain orientation was found to be the decrement of O content in the films from 1 to 0.3 at%. (2) At a unit-cell volume of around 25.5 Å3 the value of saturation magnetization, σs has a broad maximum in both processes. There is no strong enhancement of σs for α′-Fe16N2 in promoting the N site ordering. There is no divergence of σs of (α′ + α″)-Fe16N2 between the UC-processed and the normally processed films. The results show that the O impurities of about 1 at% in the film have no effect on the value of σs of Fe-N films.
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