Anisotropy barrier reduction in fast-relaxing Mn12 single-molecule magnets

An angle-swept high-frequency electron paramagnetic resonance (HFEPR) technique is described that facilitates efficient in situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based technique involves recording HFEPR spectra at fixed frequency and field, while sweeping the applied field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the extensively studied spin S=10 Mn-12 SMMs (e.g., Mn-12-acetate). The low-symmetry complex also exhibits SMM behavior, but with a significantly reduced effective barrier to magnetization reversal (U-eff approximate to 43 K) and, hence, faster relaxation at low temperature in comparison with the higher-symmetry species. Mn-12 complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Jahn-Teller axes associated with the Mn-III atoms to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive multi-high-frequency angle-swept and field-swept electron paramagnetic resonance study of [Mn12O12(O2CCH2But)(16)(H2O)(4)]center dot CH2Cl2 center dot MeNO2 is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier to magnetization reversal. The reduction in the axial anisotropy, D, is found to be insufficient to account for the nearly 40% reduction in U-eff. However, the reduced symmetry of the Mn-12 core gives rise to a very significant second-order transverse (rhombic) zero-field-splitting anisotropy, E approximate to D/6. This, in turn, causes a significant mixing of spin projection states well below the top of the classical anisotropy barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the effective barrier reduction in fast relaxing Mn-12 SMMs.