Strong magnetic anisotropy of chemically bound Co dimers in a graphene sheet

The magnetism of cobalt atoms and dimers bound by single vacancies in a graphene sheet is investigated by means of relativistic density functional calculations. In both cases, local magnetic moments are formed despite strong chemical binding. While orbital magnetism is suppressed in the Co atoms, magnetic bistability with an anisotropy barrier of about 50 meV is possible in the chemically bound Co dimers. The feasibility of their preparation is demonstrated and a general construction principle for similar (sub-)nanometer size magnets is proposed.

Figure 1

Four isomers (A)–(D) of Co${}_2$-SVG obtained by relaxation of five initial geometries (see Appendix). Large blue spheres: cobalt atoms; small gray spheres: carbon atoms.

Figure 2

Diffusion barrier between Co${}_2$-SVG(AI) and Co${}_2$-SVG(B). The inset shows the considered structure. The height of the Co atom next to the vacancy, $h^{\mathrm{Co}}$, was fixed (abscissa of the figure). Black (open) symbols: space group 156 ($P3m1$), red (full) symbols: space group 1 ($P1$). Relaxation was allowed for the height of the three neighboring carbon atoms, $h^{\mathrm{C}}$ of panel (a), their lateral position, and the distance between the two Co atoms (not shown). Panel (b) displays the total energy relative to the energy of Co${}_2$-SVG(AI). The two minima in the energy curves are related to the two isomers (AI) and (B); the diffusion barrier amounts to $2...2.5$ eV, estimated by extrapolation of the left branch.

Figure 3

Four isomers (A)–(D) of Co${}_2$-SVG obtained by relaxation of five initial geometries; see Fig. 4. Blue spheres denote cobalt atoms and gray spheres denote carbon atoms. Figure 1 shows the same structures in a different view.

Figure 4

Five chosen initial geometries (I)–(V) of Co${}_2$-SVG.