Nuclear-spin–driven resonant tunnelling of magnetisation in Mn 12 acetate

Current theories still fail to give a satisfactory explanation of the observed quantum phenomena in the relaxation of the magnetisation of the molecular cluster Mn 12 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 Mn 12 ac and a new experimental "hole digging" method, we measured the intrinsic linewidth broadening due to local fluctuating fields, and found strong evidence for the influence of nuclear spins on resonance tunnelling at very low temperatures (0.04–0.3 K). At higher temperature (1.5–4 K), we observed a homogeneous linewidth broadening of the resonance transitions being in agreement with a recent calculation of Leuenberger and Loss.