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

Abstract

The robustness of nickelocene's (NiCp 2 , 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 NiCp 2 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 NiCp 2 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 NiCp 2 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 NiCp 2 derivatives and a model system of the NiCp 2 /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 NiCp 2 coordination environment and upon its adsorption on a surface. Such resilience of the NiCp 2 magnetic behavior supports using NiCp 2 as a spin-probe molecule by functionalization of the tip of a scanning tunneling microscope.

The impact of spin-vibrational coupling on magnetic relaxation of a Co(ii) single-molecule magnet

“…52,53 In recent times, natural transition orbitals have also been used to analyse the orbital excitations and provide finer molecular orbital interpretation. 81 Nevertheless, the following d-orbital splitting analysis complemented with CASSCF orbitals satisfactorily predicts the uni-axial magnetic anisotropy exhibited by complex 1 and 3. Regarding complex 1, the d-orbital splitting pattern elucidated through CASSCF+NEVPT2 calculations conveys that d xy and d x 2 Ày 2 orbitals are the most stabilized and nearly degenerate orbitals.…”

MQM 2022: The 10th Triennial Conference on Molecular Quantum Mechanics

“…Chem. C , Alessio and co-workers in the Krylov group at the University of Southern California utilized the spin-flip EOM-CC (EOM-SF-CC) method to investigate the magnetic anisotropy of nickelocene in order to benchmark corresponding spin-flip TD-DFT (SF-TD-DFT) calculations . They found good agreement with experimentally derived values and that the principal contributions to the anisotropy arise from interactions between the triplet ground state and the third singlet state.…”

Coupled-cluster treatment of complex open-shell systems: the case of single-molecule magnets

“…As the focus in molecular magnetism turns toward transition metals coordinated by increasingly large ligands, often binding with various environments, there is a growing demand for quantum chemical methods that are both efficient and reliable in characterizing these systems. 17,18 Describing open-shell electronic states is typically more challenging compared to closedshell electronic states, and standard single-reference methods, such as density functional theory (DFT), are often qualitatively incorrect. Traditionally, multi-reference methods, e.g., complete active space perturbation theory (CASPT2) 19 and n-electron valence-state perturbation theory (NEVPT2), 20 are employed to approximate the exact multi-configurational wave functions of these systems, extracting magnetic properties through phenomenological Page 2 of 30 Physical Chemistry Chemical Physics Physical Chemistry Chemical Physics Accepted Manuscript spin Hamiltonians.…”

“…The performance of the computational approach has been tested against experiments and NEVPT2 calculations for a set of Fe(III), Fe(II) and Ni(II) molecular magnets, illustrating its reliability in computing magnetic exchange interactions, energy barriers, and magnetizations and susceptibilities. 18,25,26 However, the computational cost of EOM-CC methods scales steeply with the system size, rendering their application to large molecular magnets impractical.…”