  Current theories still fail to give a satisfactory explanation of the
observed quantum phenomena in the relaxation of the magnetisation of the
molecular cluster Mn12 acetate. In the very low temperature regime, Prokof'ev
and Stamp recently proposed that slowly changing dipolar fields and rapidly
fluctuating hyperfine fields play a major role in the tunnelling process. By
means of a faster relaxing minor species of Mn12ac and a new experimental 'hole
digging' method, we measured the intrinsic line width broadening due to local
fluctuating fields, and found strong evidence for the influence of nuclear
spins on resonance tunnelling at very low temperatures (0.04 - 0.3K). At higher
temperature (1.5 - 4K), we observed a homogeneous line width broadening of the
resonance transitions being in agreement with a recent calculation of
Leuenberger and Loss.
