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Effect of magnetism on surface segregation in FeNi alloys

Modelling the segregation of the various chemical species in the vicinity of crystallographic defects in FeNi alloys is essential because it affects the macroscopic properties of these materials, which are widely used in technological applications. We present here a theoretical study of surface segregation, within a mean-field approach based on the tight-binding Ising model grounded on density functional theory calculations. The most important result is that, although FeNi presents none of the driving forces (i.e. surface energy, size mismatch) which generally favour surface enrichment in the same element in the whole range of concentrations, there exists a wide temperature range in which Ni is found to segregate at the surface irrespective of the concentration. This is due to a complex interplay between magnetic and ordering/phase separation effects.