Quantum Effects in Single–Molecule Nanomagnets

We review recent results on the spin dynamics in molecular clusters formed by interacting paramagnetic centers. When the spin of the ground state is large and characterized by Ising type magnetic anisotropy the molecule behaves like a nanomagnet exhibiting slow relaxation of the magnetization at low temperature. One of the most interesting examples of single molecule magnet is an octanuclear iron cluster of formula [Fe8O2(OH)12(tacn)6]Br8.9H2O. This cluster has a S = 10 ground state and an easy axis of magnetization with strong rhombic anisotropy. Below 0.4 K the relaxation of the magnetization is temperature independent suggesting a pure tunneling process between the M = +- 10 states. Evidences of the tunneling mechanism also come from the observation of the oscillation of the relaxation rate when the field is applied along the easy axis. Other oscillations due to the topological interference effect (Berry phase) are observed when the field is along the hard axis. In the tunneling process also the magnetic moment of the nuclei plays an important role as suggested by the marked isotope effect observed in 2H and 57Fe enriched samples.