This study reports the formation of ultra-thin cobalt nitride (CoNx) films on a Co/ZnO(002) crystal by low-energy ion sputtering of nitrogen in an ultrahigh vacuum system. The CoNx film formed during ion bombardment in which the nitrogen plasma (N+) results in both sputtering and implantation in the formation process of CoNx, especially for the Co adsorbed layers. Auger electron spectroscopy analysis shows that the composition ratio x as a function of sputtering time was highly related to the N+ ion energy that was varied from 0.5 to 2 key. The composition ratio x of CoNx films is inversely proportional to the ion energy. Low-energy ion sputtering is possible to fabricate ultra-thin CoNx films and to adjust their chemical compositions. (C) 2011 Elsevier B.V. All rights reserved.