Low-temperature NMR study of quantum tunneling of magnetization in the molecular magnet Mn12-ac

Abstract We present an ultra-low temperature (down to T=20 mK ) NMR study on the 55 Mn nuclei in Mn 12 -ac molecular magnets. The data below T≈0.8 K show that the nuclear spin–lattice relaxation is driven by tunneling fluctuations of the cluster's electronic spin. We also show that the tunneling rate is influenced by the magnetization state of the sample, i.e. the distribution of quasi-static dipolar fields acting on the electronic spin levels.