Lattice vacancy migration barriers in Fe-Ni alloys, and why Ni atoms diffuse slowly: An ab initio study

Abstract

The mobility of both Fe and Ni atoms in ferromagnetic Fe${}_x$Ni${}_{1-x}$ alloys ($0.4\le x\le 0.6$) is investigated within the framework of ab initio electronic structure calculations, using the nudged elastic band (NEB) method to accurately quantify energetic barriers to lattice vacancy migration. Both the atomically disordered (A1) fcc phase, as well as the atomically ordered, tetragonal $\mathrm{L}{1}_0$ phase - which is under consideration as a material for a rare-earth-free 'gap' magnet for advanced engineering applications - are investigated. Across an ensemble of NEB calculations performed on supercell configurations spanning a range of compositions and containing disordered, partially ordered, and fully ordered structures, we find that Ni atoms are consistently significantly less mobile than Fe atoms. Crucially, we are able to interpret these findings in terms of the ferromagnetic alloy's underlying spin-polarised electronic structure. Specifically, we report a coupling between the size of local lattice distortions and the magnitude of the local electronic spin polarisation around vacancies. This causes Fe atoms to relax into lattice vacancies, while Ni atoms remain rigidly fixed to their original lattice positions. This effect plays a key role in determining the reduced mobility of Ni atoms compared to that of Fe atoms. These results shed atomic-scale insight into the longstanding experimental observation that Ni exhibits remarkably slow atomic diffusion in Fe-Ni alloys.

v1: 12 pages, 8 figures, 2 tables. v2: 15 pages, 9 figures, 3 tables

References in corpus (10)

• Distribution of Formation and migration energies of point defects in concentrated solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5}, Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2}
• Electronic structure and magnetic properties of L1_0 binary alloys
• Revisiting Néel 60 years on: the magnetic anisotropy of $\mathrm{L}{1}_0$ FeNi (tetrataenite)
• Compositional phase stability in medium-entropy and high-entropy Cantor-Wu alloys from an ab initio all-electron, Landau-type theory and atomistic modelling
• A collinear-spin machine learned interatomic potential for Fe${}_7$Cr${}_2$Ni alloy
• Finite-temperature second-order perturbation analysis of magnetocrystalline anisotropy energy of L10-type ordered alloys
• Magnetic anisotropy of L1${}_0$ FeNi (001), (010), and (111) ultrathin films: A first-principles study
• Integrated ab initio modelling of atomic order and magnetic anisotropy for rare-earth-free magnet design: effects of alloying additions in $\mathrm{L}{1}_0$ FeNi
• BraWl: Simulating the thermodynamics and phase stability of multicomponent alloys using conventional and enhanced sampling techniques
• First Principles Validation of Energy Barriers in Ni${}_{75}$Al${}_{25}$