Quantum confinement on nanoislands as a tool for tailoring exchange interaction: An ab initio study

We explore the influence of the quantum confinement of surface electrons by nanoscale paramagnetic islands on the exchange interaction between single adatoms adsorbed on top of the structure. We demonstrate that it is possible to enhance, reduce, or even reverse the exchange coupling at various adatom-adatom separations by deliberate choice of the island’s size.

Figure 1

(Color online) System setup used in our calculations to demonstrate the possibility of tailoring the exchange coupling between single adatoms adsorbed on islands by adjusting the island’s size. The separation of adatoms remains constant.

Figure 2

(Color) LDOS distribution on islands with (a) $R=20$, (b) 36, and (c) $52\text{Å}$.

Figure 3

(Color) A map of the exchange interaction energy of two Co adatoms adsorbed on a hexagonal Cu island of $20\text{Å}$ in radius, given that one of the adatoms is residing in the island’s center.

Figure 4

(Color online) A scheme of principal arrangements of Co adatoms on an island chosen for investigation: (a) horizontal (h), (b) horizontal symmetric (Hs), (c) vertical (V), and (d) vertical symmetric (Vs).

Figure 5

(Color online) (a) Exchange coupling energies of two Co adatoms aligned according to configuration H (see Fig. 4) at separations of $7.6\text{Å}$ (black squares), $10.2\text{Å}$ [red (dark gray) circles], $12.8\text{Å}$ [blue (gray) triangles pointing up], and $17.9\text{Å}$ [green (light gray) triangles pointing down] as a function of the island’s size $R_{island}$. (b) Exchange coupling energies of two Co adatoms aligned according to configuration Hs (see Fig. 4) at separations of $7.6\text{Å}$ (black squares), $12.8\text{Å}$ [red (dark gray) circles], $17.9\text{Å}$ [blue (gray) triangles pointing up], $28.2\text{Å}$ [green (light gray) triangles pointing down], and $38.2\text{Å}$ [cyan (light gray) diamonds] as a function of the island’s size. In both cases corresponding exchange energy values on a clean surface are given as a last point of each curve (denoted on the horizontal axis as $\infty$). Connecting lines are only there to guide the eyes.

Figure 6

(Color online) (a) Exchange coupling energies of two Co adatoms aligned according to configuration V (see Fig. 4) at separations of $4.4\text{Å}$ (black squares), $8.9\text{Å}$ [red (dark gray) circles], and $13.3\text{Å}$ [blue (gray) triangles pointing up] as a function of the island’s size $R_{island}$. (b) Exchange coupling energies of two Co adatoms aligned according to configuration Vs (see Fig. 4) at separations of $4.4\text{Å}$ (black squares), $8.9\text{Å}$ [red (dark gray) circles], $13.3\text{Å}$ [blue (gray) triangles pointing up], and $28.2\text{Å}$ [green (light gray) triangles pointing down] as a function of the island’s size. In both cases corresponding exchange energy values on a clean surface are given as a last point of each curve (denoted on the horizontal axis as $\infty$). Connecting lines are meant as a guidance for the eyes only.