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

Current theories still fail to give a satisfactory explanation of the obser ved quantum phenomena in the relaxation of the magnetisation of the molecul ar cluster Mn-12 acetate. In the very low-temperature regime, Prokof'ev and Stamp recently proposed that slowly changing dipolar fields and rapidly fl uctuating hyperfine fields plap 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 loa temperatures (0.04-0.3 K ). At higher temperature (1.5-4 K), we observed a homogeneous linewidth bro adening of the resonance transitions being in agreement with a recent calcu lation of Leuenberger and Loss.