Unravelling the origins of sluggish atomic diffusion in Fe-Ni alloys: Ab initio calculations, atomistic simulations, and a theoretical analysis

For more than half a century, it has been consistently experimentally reported that Ni atoms exhibit remarkably slow atomic diffusion when compared to Fe atoms in the Fe-Ni binary system and related alloys. Here, we report the results of a detailed computational investigation which reveal that this unexpected behaviour has its origins in the alloy’s spin-polarised electronic structure, i.e. its magnetism. Our approach utilises ab initio electronic structure calculations, a newly-developed machine-learned interatomic potential, a range of atomistic simulation techniques, and a detailed theoretical analysis. Taken in combination, our results conclusively demonstrate that Fe atoms, with their reduced d-electron count and sizeable magnetic moments, are able to lower their energy by relaxing substantially into adjacent lattice vacancies, while Ni atoms remain rigidly fixed to their original lattice positions [1]. In turn, this leads to substantially reduced lattice vacancy migration barriers for Fe-vacancy interchanges as compared to Ni-vacancy interchanges. As well as providing a clear mechanistic explanation for longstanding experimental observations, this work also has implications for formation of the atomically-ordered L1₀ phase of FeNi, a potential rare-earth-free ‘gap’ magnet.

[1] arXiv:2508.19124.