Segregation of sp -impurities at grain boundaries and surfaces: comparison of fcc cobalt and nickel

We perform systematic ab initio investigations of the segregation of 12 non-magnetic sp -impurities (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Σ5(210) grain boundary (GB) and (210) free surface (FS) in fcc ferromagnetic cobalt and analyse their effect on structural, magnetic and mechanical properties; the results are compared with those obtained previously for nickel. It turns out that there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk cobalt (4.7% and 17%, respectively). However, segregated sp -impurities sharply reduce this magnetization. As shown previously, in nickel most of the above impurities nearly destroy or substantially reduce the magnetic moments at the FS and, when segregated interstitially (i.e. Si, P, S, Ge, As, and Se), also at the GB, so that they provide atomically thin, magnetically dead layers, which may be very desirable in spintronics. The reduction of magnetic moments at the Σ5(210) GB in fcc ferromagnetic cobalt is, in absolute values, very similar to that in nickel. However, as the magnetic moment in bulk cobalt is higher, we do not observe magnetically dead layers here. Further, we find the preferred segregation sites at the Σ5(210) GB for the sp -impurities studied, and their segregation enthalpies and strengthening/embrittling energies with their decomposition into their chemical and mechanical components. It turns out that interstitially segregated Si is a GB cohesion enhancer, and interstitially segregated P, S, Ge, As, and Se and substitutionally segregated Al, Ga, In, Sn, Sb and Te are GB embrittlers in fcc cobalt. As there is essentially no experimental information on GB segregation in cobalt, most of the present results are theoretical predictions which may motivate future experimental work. (paper)