First-principles study of a single-molecule magnetMn12monolayer on the Au(111) surface

Abstract

The electronic structure of a monolayer of single-molecule magnets ${\mathrm{Mn}}_{12}$ on a Au(111) surface is studied using spin-polarized density-functional theory. The ${\mathrm{Mn}}_{12}$ molecules are oriented such that the magnetic easy axis is normal to the surface, and the terminating ligands in the ${\mathrm{Mn}}_{12}$ are replaced by thiol groups (-SH) where the H atoms are lost upon adsorption onto the surface. This sulfur-terminated ${\mathrm{Mn}}_{12}$ molecule has a total magnetic moment of $18{\ensuremath{\mu}}_{B}$ in the ground state, in contrast to $20{\ensuremath{\mu}}_{B}$ for the standard ${\mathrm{Mn}}_{12}$. The ${\mathrm{Mn}}_{12}$ molecular orbitals broaden due to the interaction of the molecule with the gold surface and the broadening is of the order of $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is an order of magnitude less than the single-electron charging energy of the molecule so the molecule is weakly bonded to the surface. Only electrons with majority spin can be transferred from the surface to the sulfur-terminated ${\mathrm{Mn}}_{12}$ since the gold Fermi level is well above the majority lowest unoccupied molecular orbital (LUMO) but below the minority LUMO. The amount of the charge transfer is calculated to be 1.23 electrons from a one-dimensional charge density difference between the sulfur-terminated ${\mathrm{Mn}}_{12}$ on the gold surface and the isolated sulfur-terminated ${\mathrm{Mn}}_{12}$, dominated by the tail in the electronic distribution of the gold surface. A calculation of a level shift upon charging provides 0.28 electrons being transferred. The majority of the charge transfer occurs at the sulfur, carbon, and oxygen atoms close to the surface. The total magnetic moment also changes from $18{\ensuremath{\mu}}_{B}$ to $20{\ensuremath{\mu}}_{B}$, which is due to rearrangements of the magnetic moments on the sulfur atoms and Mn atoms upon adsorption onto the surface. The magnetic anisotropy barrier is computed including spin-orbit interaction self-consistently in density-functional theory. The barrier for the ${\mathrm{Mn}}_{12}$ on the gold surface decreases by $6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in comparison to that for an isolated ${\mathrm{Mn}}_{12}$ molecule.