Quantum tunneling of the magnetization in molecular nanomagnets: The crucial role of S mixing

Hysteresis curves of molecular nanomagnets display a step-like behavior typical of the relaxation through quantum tunneling of the magnetization. The relaxation rate depends on the value of the so-called tunnel splitting, i.e., the gap between quasi-degenerate states in correspondence of a level anticrossing. In the case of Fe8, the magnitude of this gap deduced by Landau–Zener measurements seems incompatible with the value calculated by using the Hamiltonian determined by spectroscopic measurements. We show that the commonly neglected quantum fluctuations of the magnitude of the total spin of the molecule (S mixing) hugely affect the tunnel splitting, and allow the above-mentioned discrepancy to be solved. Since the degree of S mixing is strongly influenced by the topology of the molecule, we suggest that cluster topology is one of the key ingredients in designing new nanomagnets.

The role of S mixing in the quantum tunneling of the magnetization has been investigated for [Fe8O2(OH)12(tacn)6]8+. The effect of S mixing on the tunnel splitting Δ is huge and the discrepancy (more than 3 orders of magnitude in the tunneling frequency) between spectroscopic and relaxation measurements can finally be resolved.Figure optionsDownload as PowerPoint slide