Giant Isotope Effect in the Incoherent Tunneling Specific Heat of the Molecular Nanomagnet Fe-8

Time-dependent specific heat experiments on the molecular nanomagnet Fe-8 and the isotopic enriched analogue Fe-57(8) are presented. The inclusion of the Fe-57 nuclear spins leads to a huge enhancement of the specific heat below 1 K, ascribed to a strong increase in the spin-lattice relaxation rate Gamma arising from incoherent, nuclear-spin-mediated magnetic quantum tunneling (MQT) in the ground doublet. Since Gamma is found comparable to the expected tunneling rate, the MQT process has to be inelastic. A model for the coupling of the tunneling spins to the lattice is presented. Under transverse field, a crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.