Nonadiabatic Landau-Zener tunneling in Fe-8 molecular nanomagnets

The Landau-Zener method allows to measure very small tunnel splittings Delt a in molecular clusters Fes. The observed oscillations of Delta as a functi on of the magnetic field applied along the hard anisotropy axis are explain ed in terms of topological quantum interference of two tunnel paths of oppo site windings. Studies of the temperature dependence of the Landau-Zener tr ansition rate P gives access to the topological quantum interference betwee n excited spin levels. The influence of nuclear spins is demonstrated by co mparing P of the standard Fe-8 sample with two isotopically substituted sam ples. The need of a generalised Landau-Zener transition rate theory is show n.