Origin of the fast magnetization tunneling in tetranuclear nickel single-molecule magnets

The single-molecule magnet behavior of Ni4 complexes is characterized by magnetization hysteresis loops. The origin of the fast magnetization tunneling is the result of transverse quartic zero-field interactions ( the B44 term in the spin Hamiltonian) as indicated by high-field EPR experiments. The presence of ligand dynamics that become static at low temperatures and are the origin of the splitting of EPR signals observed at low temperature is confirmed by heat capacity and low temperature X-ray crystallography experiments.