Mesoscopic Spin Tunneling in Molecular Crystals
  The phenomenon of Quantum Tunneling of Mesoscopic Spins is reviewed in the light of the behavior of the archetype of these systems: the molecular complex Mn12-ac. Most observations can be understood in the framework of the reduced Hilbert space dimension 2S+1=21. Due to the large spin S=10, the energy barrier preventing spin rotation is large, and as a consequence, quantum relaxation is very slow. The application of a magnetic field of a few Tesla below 1 K allows to observe tunneling (i) between the states m=-10 and m=10-n with n=8 to 11 if the field is longitudinal, or (ii) between the two ground-states m~-10 and m~10 if the field is transverse. The crossover temperature between ground-state and thermally assisted tunneling in a longitudinal field extrapolates in zero field at ~1.7 K. The observation of square root relaxation at short-times/low-temperatures and of exponential relaxation at long-times/high-temperatures, as observed previously above 1.5 K, confirms the important role of the spin bath dynamics which is out of equilibrium in the first regime and at equilibrium in the second one. In a second part of this paper a new molecule, so-called V15, with resultant spin S=1/2 is investigated. Contrary to high spin molecules, the energy barrier of low spin molecules is small or null, and the splitting between the symmetrical and anti-symmetrical states is sufficiently large to allow spin-phonon transitions during spin rotation. In low spin molecules the coupling to the environment is quite different from the one found in large spin molecules in low fields. 
B. Barbara, I. Chiorescu, R. Giraud, A. G. M. Jansen, A. Caneschi