Evolution of Magnetic Anisotropy of an Organometallic Molecule in a Mechanically Controlled Break Junction: The Roles of Connecting Electrodes

Abstract

The precise control of local spin states in magnetic organometallic molecules by means of a mechanically controlled break junction (MCBJ) is a cutting edge frontier of single molecule science. For instance, a fine-tuning of the magnetic anisotropy of a Co(tpy–SH)2 molecule has been achieved experimentally by sandwiching the molecule between two electrodes and gradually enlarging their separation. However, an accurate simulation of such a process has remained rather challenging, which is largely due to the complex structure of the composite junction as well as the static electron correlation originating from the nearly degenerate d-orbitals of the cobalt atom. In this work, by employing an embedding method which combines the high-level complete active space self-consistent field (CASSCF) method and a density functional theory (DFT) method, we carefully simulate the evolution of magnetic anisotropy of the Au–Co(tpy–S)2–Au junction during the mechanical stretching process. It is found that the presence of the connecting electrodes has significant influence on the geometric and electronic structures of the sandwiched molecule, which further affects its magnetic anisotropy. The reasonable agreement between the calculation results and the experimental measurements highlights the potential usefulness of the embedding method for predicting the local magnetic properties of organometallic molecules in MCBJs.