On the anisotropy barrier reduction in fast relaxing Mn12 single-molecule magnets

TL;DR

This paper introduces a new high-frequency EPR technique for studying low-symmetry single-molecule magnets, revealing that increased transverse anisotropy and quantum tunneling significantly reduce the magnetization barrier in Mn12 SMMs.

Contribution

The study demonstrates how low symmetry and Jahn-Teller axis orientation affect magnetic relaxation, highlighting the role of transverse anisotropy and quantum tunneling in barrier reduction.

Findings

Reduced symmetry increases transverse anisotropy E.

Quantum tunneling dominates barrier reduction.

Barrier Ueff is decreased by nearly 40% in low-symmetry Mn12.

Abstract

A novel angle-swept high-frequency EPR (HFEPR) technique is described that enables in-situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based method involves recording spectra at fixed frequency and field, while sweeping the field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the spin S = 10 Mn12 SMMs (e.g. Mn12-acetate). The low-symmetry complex is also an SMM, but with a significantly reduced barrier to magnetization reversal (Ueff ~ 43 K) and, hence, faster relaxation at low temperature in comparison with the high-symmetry species. Mn12 complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Mn(III) Jahn-Teller axes to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive HFEPR study of [Mn12O12(O2CCH2But)16(H2O)4].CH2Cl2.MeNO2 is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier. The reduction in the axial anisotropy, D, is found to be insufficient to account for the nearly 40% reduction in Ueff. However, the reduced symmetry of the Mn12 core gives rise to a very significant 2nd order transverse anisotropy, E ~ D/6. This, in turn, causes a significant mixing of spin projection states well below the top of the classical barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the barrier reduction in fast relaxing Mn12 SMMs.