title: Giant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8.
abstract: Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe 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.
authors.affiliation: Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands.
authors.forename: M
authors.initials: M
authors.lastname: Evangelisti
authors.forename: F
authors.initials: F
authors.lastname: Luis
authors.forename: F L
authors.initials: FL
authors.lastname: Mettes
authors.forename: R
authors.initials: R
authors.lastname: Sessoli
authors.forename: L J
authors.initials: LJ
authors.lastname: de Jongh