Landau-Zener Method to Study Quantum Phase Interference of Fe-8 Molecular Nanomagnets (invited)

We present details about an exptl. method based on the Landau-Zener model which allows one to measure very small tunnel splittings \u394 in mol. clusters Fe8. The measurements are performed with an array of microsuperconducting quantum interference devices. The obsd. oscillations of \u394 as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topol. 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 topol. part of the quantum spin phase (Berry phase) in a magnetic system. The influence of intermol. dipole interactions on the measured tunnel splittings \u394 is shown