  This thesis contains a thorough investigation of the quantum spin dynamics in
Mn12-ac and Mn6 Single-molecule magnets. In particular, we have investigated
the interplay between quantum tunneling of magnetization and nuclear spin
dynamics in Mn12-ac by ultra-low temperature NMR experiments. We discuss the
effect of quantum tunneling on the nuclear spin-lattice relaxation, the nuclear
spin diffusion, the thermalization of the nuclear spins, the isotope effect,
the influence of strong perpendicular fields, and we propose a model to unify
all our observations. For the isotropic Mn6 molecule, we report the observation
of long-range ferromagentic ordering of the cluster spins, due to dipolar
interactions only. The nuclear spin dynamics provides an interesting comparison
with the case of Mn12-ac. Two large introductory chapters describe the design
of our ultra-low temperature setups, and the theoretical background necessary
to understand the interplay between nuclear spins and quantum tunneling, with
emphasis on the Prokof'ev-Stamp theory of the "spin-bath".
