The origin of anomalous diffusion in iron mononitride thin films

TL;DR

This study investigates the unusual diffusion behavior of Fe and N in FeN thin films, revealing different mechanisms and bond strengths that influence their diffusion rates in various phase regions.

Contribution

It provides a detailed analysis of Fe and N diffusion mechanisms in FeN thin films, highlighting the role of grain boundary diffusion and bond strength differences.

Findings

Fe diffuses faster than N in N-rich regions despite larger atomic size.

Fe diffuses mainly via grain boundary diffusion, N via volume diffusion.

Stronger Fe-N bonds explain the diffusion behavior.

Abstract

We have studied the origin of a counter intuitive diffusion behavior of Fe and N atoms in a iron mononitride (FeN) thin film. It was observed that in-spite of a larger atomic size, Fe tend to diffuse more rapidly than smaller N atoms. This only happens in the N-rich region of Fe-N phase diagram, in the N-poor regions, N diffusion coefficient is orders of magnitude larger than 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 a volume diffusion, pre-dominantly controlled by a fast grain boundary diffusion. On the other hand N atoms diffuse through a classical volume-type diffusion process. Observed results have been explained in terms of stronger Fe-N (than Fe-Fe) bonds generally predicted theoretically for mononitride compositions of transition metals.