Nonadiabatic Landau Zener Tunneling and Quantum Phase Interference in Fe8 Molecular Nanomagnets

We present details about an experimental method based on the Landau Zener model which allows to measure very small tunnel splittings .DELTA. in molecular clusters Fe8. The measurements are performed with an array of micro-SQUlDs. 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. Transitions between the quantum numbers M=-S and (S-n), with n even or odd, revealed a parity (symmetry) effect which is analogous to the suppression of tunneling predicted for half integer spins. This observation is the first direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system. The influence of nuclear spins and intermolecular dipole interactions on the measured tunnel splittings .DELTA. are demonstrated. Studies of the temperature dependence of the Landau Zener transition rate yield a deeper insight into the spin dynamics of the Fe8 cluster. We show the need of a generalized Landau Zener transition rate theory taking into account environmental effects such as hyperfine coupling and temperature.