Tunneling of mesoscopic spins 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 comp lex Mn-12-ac. Most observations can be understood in the framework of the r educed Hilbert space dimension 2S + 1 = 21. Due to the large spin S = 10, t he energy barrier preventing spin rotation is large, and as a consequence, quantum relaxation is very slow. The application of a magnetic field of a f ew 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) be tween the two ground-states m approximate to -10 and m approximate to 10 if the field is transverse. The crossover temperature between ground-state an d thermally assisted tunneling in a longitudinal field extrapolates in zero field at approximate to 1.7 K. The observation of square root relaxation a t short-times/low-temperatures and of exponential relaxation at long-times/ high-temperatures, as observed previously above 1.5 K, confirms the importa nt 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 V-15, with resultant spin S = 1/2 is investigate d. Contrary to high spin molecules, the energy barrier of low spin molecule s is small or null, and the splitting between the symmetrical and anti-symm etrical states is sufficiently large to allow spin-phonon transitions durin g spin rotation. In low spin molecules the coupling to the environment is q uite different from the one found in large spin molecules in low fields.