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

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 equation-of-motion coupled-cluster (EOM-CC) framework. 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. 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.