Fast magnetization tunneling in tetranickel and tetracobalt single-molecule magnets

A series of tetra-nuclear cubane complexes was synthesized with nickel(II) or cobalt(II) as the metal centers. The analysis of both of chiMT versus temperature data (chiM is the molar paramagnetic susceptibility) and reduced magnetization measurements confirm the ferromagnetic coupling in the NiII4 cubane molecule to give a S = 4 ground state. In the series of Ni(II) cubane complexes [Ni(hmp)(ROH)X]4, when R is Me or Et, there are significant intermolecular interactions in the lattice causing an exchange bias in the magnetization hysteresis loops and broad peaks in high-frequency electron paramagnetic resonance (HFEPR) spectra. The intermolecular interactions are reduced by introducing bulky aliphatic chains on the ROH ligands. In the case of [Ni(hmp)(dmb)Cl]4 (dmb = 3,3-dimethyl-1-butanol), the aliphatic groups make the NiII 4 cubane complex more insolated to its environment, and still SMM behavior is seen i.e. fast quantum tunneling of magnetization (QTM). The origin of the fast QTM was found to be attributable to the fourth order zero-field splitting (zfs) B44 term. To determine the origin of this big B44 term, HFEPR experiments were carried out on doped sample [Zn 3.91Ni0.09(hmp)4(dmb)4Cl4]. It was found to be a result of both a tilted D term and a significant E term.; Cobalt(II) based cubane complexes were also studied in this thesis. However, owing to strong spin-orbital coupling, it is difficult to determine the spin of the ground state. Both fast QTM in the magnetization hysteresis measurements and nested spin states found in HFERP experiments indicate that entangled states are more likely the situation of this series of CoII 4 complexes.