Magnetically dead layers at $sp$-impurity-decorated grain boundaries and surfaces in nickel

With the help of ab initio electronic structure calculations, we study segregation of the $sp$ elements from the 13th–16th group and the third, fourth, and fifth period of the Periodic Table (i.e., Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb, and Te) at the $\Sigma$5(210) grain boundary (GB) and (210) free surface (FS) in fcc ferromagnetic nickel, and analyze the geometric configuration and the distribution of magnetic moments at the GB and FS without and with impurities. Whereas there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk nickel (3–7% and 24%, respectively), most of these impurities nearly kill or substantially reduce the magnetic moments at the FS and, when segregating interstitially at the GB (i.e., Si, P, S, Ge, As, and Se), they produce magnetically dead layers at the boundary. We demonstrate that the existence of magnetically dead layers is a common phenomenon at the $sp$-impurity-decorated GB and FS in nickel. It is caused by a strong hybridization of $sp$ states of the impurities with the $d$ states of nickel and a redistribution of electron states in both majority and minority bands.

Figure 1

Supercells used in electronic structure calculations of the $\Sigma$5(210) GB and (210) FS in nickel. The blue numbered spheres indicate Ni atoms in the bulk, the red spheres labeled by “GB” show the GB planes, and the small spheres marked by “int” represent positions of interstitial impurity atoms. The red spheres denoted by “FS” show the surface planes.

Figure 2

Behavior of magnetic moments of nickel atoms as a function of their distance from the GB or FS layer. Interstitially and substitutionally segregated impurities are marked as “int” and “sub,” respectively; for better clarity of the figure, their magnetic moments induced by nickel matrix are given at the position denoted as I, although, in the case of the GB segregation, they lie in the GB plane.

Figure 3

Upper panel: Densities of states of $d$ electrons at Ni atoms lying in the $\Sigma$5(210) GB plane without [red (gray) curve] and with (black curve) interstitially segregated phosphorus for majority (displayed as positive values) and minority (displayed as negative values) states. Lower panel: Densities of states of $d$ electrons at Ni atoms lying at the clean (210) surface [red (gray) curve] and in the first layer under the adsorbed Sb layer (black curve) for majority and minority states. Fermi energy is set to 0 eV in both cases; it is marked by a vertical line.

Figure 4

Total charge density (left) and the spin density (right) for the $\Sigma$5(210) GB with interstitially segregated P (upper panels) and (210) FS with adsorbed Sb (lower panels) in the cut along the $xy$ plane (see Fig. 1) in electrons/Å${}^3$. The arrows indicate the positions of GB, FS, and nickel layers. Nickel atoms are represented by violet spheres in the maps of the charge density (left). Spin density (right) exhibits a dead layer at and around the GB and FS planes. Those nickel atoms which lie in the dead layers have very small magnetic moments and, consequently, are not visible in the spin-density maps. Nickel atoms adjacent to the GB or FS plane bear also small magnetic moments (see text) that are reflected by smaller values of the spin density than in the bulk.