Origin of anomalous diffusion in iron mononitride thin films

Abstract

In this work we aim to resolve the counterintuitive diffusion behavior of Fe and N atoms in an iron mononitride (FeN) thin film. It was observed that in spite of their larger size, Fe atoms tend to diffuse more rapidly than smaller N atoms. This only happens in the N-rich region of the Fe-N phase diagram; in N-poor regions, the N diffusion coefficient is orders of magnitude larger than that of Fe. Detailed self-diffusion measurements performed in FeN thin films reveal that the diffusion mechanism of Fe and N is different: Fe atoms diffuse through a complex process which, in addition to volume diffusion, is predominantly controlled by fast grain boundary diffusion. On the other hand, N atoms diffuse through a classical volume diffusion process. Observed results are explained in terms of stronger Fe-N (than Fe-Fe) bonds generally predicted theoretically for mononitride compositions of transition metals.