First-principles study of magnetism at grain boundaries in iron and nickel

The geometric and magnetic structures of fully relaxed symmetrical tilt Sigma 5(310) grain boundaries (GBs) in iron and Sigma 5(210) GBs in nickel have been investigated using density-functional theory. We found for both GBs an enhancement of the local magnetic moments of atoms in the GB plane (2.55 mu(B) for iron and 0.67 mu(B) for nickel) which is correlated with the larger local atomic volume compared to the bulk. At larger distances from the GB the variation of the local magnetic moments follows the changes in the exchange splitting in the spin-polarized local density of states imposed by the local variations in the atomic geometry. When Si and Sri impurity atoms in interstitial or substitutional positions appear at the Sigma 5 (310) GB in iron, the local magnetic moments of iron atoms are reduced for silicon and almost unchanged for tin. We also calculated the segregation enthalpies of both impurities and confirmed the experimental fact that silicon is a substitutional and tin an interstitial segregant; the calculated values of segregation enthalpy are in good agreement with experiment.