First-principles study of a single-molecule magnet Mn_{12} monolayer on the Au(111) surface

TL;DR

This study uses spin-polarized density-functional theory to analyze the electronic and magnetic properties of a sulfur-terminated Mn12 single-molecule magnet on a gold surface, revealing weak bonding, charge transfer, and reduced magnetic anisotropy.

Contribution

It provides the first detailed first-principles analysis of Mn12 molecules on Au(111), including electronic structure, charge transfer, and magnetic anisotropy changes upon surface adsorption.

Findings

Mn12 monolayer has a total magnetic moment of 18 μB on Au(111).

Charge transfer of approximately 1.23 electrons occurs from gold to the molecule.

Magnetic anisotropy barrier decreases by 6 K when adsorbed on gold.

Abstract

The electronic structure of a monolayer of single-molecule magnets Mn$_{12}$ on a Au(111) surface is studied using spin-polarized density-functional theory. The Mn$_{12}$ molecules are oriented such that the magnetic easy axis is normal to the surface, and the terminating ligands in the Mn$_{12}$ are replaced by thiol groups (-SH) where the H atoms are lost upon adsorption onto the surface. This sulfur-terminated Mn$_{12}$ molecule has a total magnetic moment of 18 $\mu_B$ in the ground state, in contrast to 20$\mu_B$ for the standard Mn$_{12}$. The 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 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 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, dominated by the tail in the electronic distribution of the gold surface. A calculation of level shift upon charging provides 0.28 electrons being transferred. The majority of the charge transfer occurs at the S, C, and O atoms close to the surface. The total magnetic moment also changes from 18 $\mu_B$ to 20 $\mu_B$, due to rearrangements of the magnetic moments on the S 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 Mn$_{12}$ on the gold surface decreases by 6 K in comparison to that for an isolated Mn$_{12}$ molecule.