Origin of Magnetic Anisotropy in Nickelocene Molecular Magnet and Resilience of Its Magnetic Behavior

The robustness of nickelocene's (NiCp2, Cp = cyclopentadienyl) magnetic anisotropy and addressability of its spin states make this molecular magnet attractive as a spin sensor. However, microscopic understanding of its magnetic anisotropy is still lacking, especially when NiCp2 is deposited on a surface to make quantum sensing devices. Quantum chemical calculations of such molecule/solid-state systems are limited to density functional theory (DFT) or DFT+U (Hubbard correction to DFT). We investigate the magnetic behavior of NiCp2 using the spin-flip variant of the equation-of-motion coupled-cluster (EOM SF-CC) method and use the EOM-SF-CC results to benchmark SF-TD-DFT. Our first-principle calculations agree well with experimentally derived magnetic anisotropy and susceptibility values. The calculations show that magnetic anisotropy in NiCp2 originates from a large spin-orbit coupling (SOC) between the triplet ground state and the third singlet state, whereas the coupling with lower singlet excited states is negligible. We also considered a set of six ring-substituted NiCp2 derivatives and a model system of the NiCp2/MgO(001) adsorption complex, for which we used SF-TD-DFT method. To gain insight into the electronic structure of these systems, we analyze spinless transition density matrices and their natural transition orbitals (NTOs). The NTO analysis of SOCs explains how spin states and magnetic properties are retained upon modification of the NiCp2 coordination environment and upon its adsorption on a surface. Such resilience of the NiCp2 magnetic behavior supports using NiCp2 as a spin-probe molecule by functionalization of the tip of a scanning tunneling microscope.

Automated summary

The robustness of the magnetic anisotropy and addressability of the spin states in nickelocene (NiCp2, Cp = cyclopentadienyl) make it an attractive spin sensor. However, the microscopic understanding of its magnetic anisotropy is lacking, especially when NiCp2 is used in quantum sensing devices on a surface. In this study, we use the spin-flip variant of EOM-CC method to investigate the magnetic behavior of NiCp2 and compare the results with SF-TD-DFT. Our calculations agree well with experimental data and reveal that the magnetic anisotropy in NiCp2 arises from a large spin-orbit coupling between the triplet ground state and the third singlet state. We also analyze the electronic structure of ring-substituted NiCp2 derivatives and NiCp2/MgO(001) adsorption complex using SF-TD-DFT, which provides insights into the retention of spin states and magnetic properties upon modification of the coordination environment and adsorption on a surface.