Understanding the Sub-meV Precision-Tuning of Magnetic Anisotropy of Single-Molecule Junction

Abstract

Fine-tuning the magnetic anisotropy energy (MAE) of a magnetic molecule with a high precision of sub-meV has been realized experimentally by manipulating the tip of a scanning tunneling microscope (STM). Understanding the mechanisms behind the observed evolution of spin excitation energy is essentially important for potential spintronic applications. In particular, it is crucial to unveil the influence of the surrounding environment on the molecular spin state. To this end, we carry out the first-principles simulation on the STM-tip control of an iron octaethylporphyrin chloride (Fe-OEP-Cl) molecule adsorbed on the Pb(111) substrate. By carefully taking into account the atomic structures of the tip and the substrate as well as the multireference feature of the Fe 3d electrons, the experimentally measured evolution of spin excitation energy, including a continuous increase, followed by a sudden drop in the MAE, is accurately reproduced by our simulation with a maximal discrepancy of less than 0.3 meV. Based on a comprehensive analysis of the change in geometric and electronic structures of the whole single-molecule junction, the exotic evolution of MAE is attributed to the variation of the ligand confinement effect and the resulting charge transfer. The unique role of the single-atomic Cl ligand is clarified by comparing the evolution under the STM-tip control with that of the tip/Fe-OEP/Pb(111) junction. The theoretical insights provided by this work would be valuable for the on-demand design of the mechanically controlled magnetic nanojunctions.