Nonadiabatic Landau-Zener tunneling in Fe8 molecular nanomagnets

The Landau-Zener method allows to measure very small tunnel splittings Delta in molecular clusters Fes. The observed oscillations of Delta as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Studies of the temperature dependence of the Landau-Zener transition rate P gives access to the topological quantum interference between excited spin levels. The influence of nuclear spins is demonstrated by comparing P of the standard Fe-8 sample with two isotopically substituted samples. The need of a generalised Landau-Zener transition rate theory is shown.