Magnetic interactions among defects on the boron-doped Si(111)-√3 ×√3 surface

We study the possible magnetism on the Si(111)-√3 ×√3 surface, which is stabilized for the highly boron-doped samples, using first-principles calculations based on the density-functional theory. When the silicon adatom on top of the boron atom is removed to form a defect structure, three silicon dangling bonds are exposed with half-filled doubly degenerate energy levels in the bandgap, stabilizing the local magnetic moment of 2 μ_B. When there are many such defect structures adjacent each other, they are found to align antiferromagnetically as expected for the exchange interaction among the half-filled orbitals. In the meanwhile, we demonstrate that the ferromagnetism can be stabilized by adjusting the number of electrons, suggesting the possibility towards spintronic applications of this unique silicon surface structure.