Theoretical Investigations on the Molecular Magnetic Behavior of Actinide Molecules [AnPc2]0/- (An = U, Cf): Prediction of the High Magnetic Blocking Barrier and Magnetic Blocking Temperature in [CfPc2].

Abstract

Searching for single-molecule magnets (SMM) with large effective blocking barriers, long relaxation times, and high magnetic blocking temperatures is vitally important not only for the fundamental research of magnetism at the molecular level but also for the realization of new-generation magnetic memory unit. Actinides (An) atoms possess extremely strong spin-orbit coupling (SOC) due to their 5f orbitals, and their ground multiplets are largely split into several sublevels because of the strong interplay between the SOC of An atoms and the crystal field (CF) formed by ligand atoms. Compared to TM-based SMMs, more dispersed energy level widths of An-based SMMs will give a larger total zero field splitting (ZFS) and thus provide a necessary condition to derive a higher U_eff. In combination of the density functional theory (DFT) as well as the CF model Hamiltonian and ab initio calculation, we have investigated the structural stability and electronic structures as well as the magnetodynamic behavior of [AnPc₂]^0/- (An = U, Cf) molecules. We find that An atoms can strongly interact with its ligand N atoms in forming An-N ionic bonds, and 5f electrons are more localized in the Cf atom than in the U atom, giving U⁴⁺(5f²) and Cf³⁺(5f⁹) valence states. Although the UPc₂ molecule has a modest value of U_eff = 514 cm^-1, it is not a good SMM due to the easy occurrence of quantum tunneling of magnetization (QTM). Based on the consistent results of CF Hamiltonian and ab initio calculations on the [CfPc₂]^- molecule, we propose that almost prohibited QTM within the Kramers doublets (KDs) as well as very low transition probabilities between different states via hindered spin-flip transitions would result in a high U_eff = 1401 cm^-1. The estimated high magnetic blocking temperature (T_B) of 58 K renders [CfPc₂]^- an excellent SMM candidate, implying that magnetic hysteresis could be observed in future experiments.