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.

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

“…As a result of local geometric change around the Fe core, the electronic and magnetic properties of the molecule vary significantly during the tip control process. The axial MAE (the out-of-plane component of the magnetic anisotropy tensor) of the tip/Fe-OEP-Cl/Pb(111) junction is calculated by employing the embedding approach as follows Here, D mol CASSCF is the axial MAE of an isolated Fe-OEP-Cl molecule (referred to as the reference molecule) calculated by the CASSCF method in conjunction with the N -electron valence perturbation theory (NEVPT2). − D env is the direct contribution of the surrounding environment (the STM tip and the Pb substrate) to the MAE of the single-molecule junction, which is evaluated by Here, D jun DFT and D mol DFT are the axial MAEs of the whole junction and the reference molecule computed by the same DFT method, respectively. Usually, D env is a small correction to D mol CASSCF , with the latter capturing the multireference character of the Fe 3d electrons.…”

“…Experimentally, the control of the local magnetic moment of a single molecule has been realized by chemical doping − (e.g., attachment/detachment of small atoms or molecules), or by applying external means, such as electric or magnetic field, light, temperature, and mechanical stress. ,− In particular, the scanning tunneling microscope (STM) has been widely employed to control the local spin state of an adsorbed molecule. ,− By adjusting the position of the atomically sharp tip, the single-molecule junction transits from a tunneling regime (tip far away from the molecule) to the contact regime (tip close to the molecule), − so that the molecular structure ,,, and spintronic properties could be tuned continuously and precisely. ,− …”

“…(3) The static correlation or multireference character of the d electrons on the TM core must be accurately accounted for . To overcome the above challenges, we have proposed an embedding approach, ,, with which the local spin state of the magnetic molecule is determined at the level of complete active space self-consistent field (CASSCF), while the influence of environment is characterized by a density functional theory (DFT) method. By employing the embedding approach, we have simulated the STM-tip control of the Fe-OEP/Pb(111) junction. , The calculation results accurately reproduce the measured evolution of MAE with a remarkably small discrepancy of less than 0.3 meV, and thus accentuate the crucial influence of the surrounding environment. , In this work, we will simulate and scrutinize the spin control process of an Fe-OEP-Cl/Pb(111) junction, and particularly, elucidate the unique role of the single-atomic Cl ligand in the evolution of MAE.…”

“…5,31−34 In particular, the scanning tunneling microscope (STM) has been widely employed to control the local spin state of an adsorbed molecule. 19,35−38 By adjusting the position of the atomically sharp tip, the single-molecule junction transits from a tunneling regime (tip far away from the molecule) to the contact regime (tip close to the molecule), 36−38 so that the molecular structure 7,17,34,39 and spintronic properties could be tuned continuously and precisely. 37,40−43 Among the few experimental realizations of the STM-tip control of magnetic molecules, Heinrich et al have measured the evolution of MAE of an iron octaethylporphyrin chloride (Fe-OEP-Cl) molecule adsorbed on a superconducting Pb(111) substrate with a high precision of sub-meV.…”

“…(3) The static correlation or multireference character of the d electrons on the TM core must be accurately accounted for. 45 To overcome the above challenges, we have proposed an embedding approach, 31,34,46 with which the local spin state of the magnetic molecule is determined at the level of complete active space self-consistent field (CASSCF), while the influence of environment is characterized by a density functional theory (DFT) method. By employing the embedding approach, we have simulated the STM-tip control of the Fe-OEP/Pb(111) junction.…”