Landau-Zener method to study quantum phase interference of Fe8 molecular nanomagnets (invited)

We present details about an experimental method based on the Landau-Zener model which allows one to measure very small tunnel splittings Delta in molecular clusters Fe-8. The measurements are performed with an array of microsuperconducting quantum interference devices. 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 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 intermolecular dipole interactions on the measured tunnel splittings Delta is shown. (C) 2000 American Institute of Physics. [S0021-8979(00)31608-5].