Landau Zener method to study quantum phase interference of Fe8 molecular nanomagnets

TL;DR

This paper introduces an experimental method using the Landau Zener model and micro-SQUIDs to measure tiny tunnel splittings in Fe8 molecular nanomagnets, revealing quantum interference effects and the Berry phase.

Contribution

It provides the first direct evidence of the topological quantum spin phase (Berry phase) in a magnetic system through detailed measurements of tunnel splittings.

Findings

Oscillations of tunnel splitting Delta as a function of magnetic field observed.

Parity effects in tunneling related to spin symmetry demonstrated.

Intermolecular interactions influence measured tunnel splittings.

Abstract

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-SQUIDs. 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 $\Delta$ are shown.