快弛豫Mn12单分子磁体的各向异性势垒降低
On the anisotropy barrier reduction in fast relaxing Mn12 single-molecule magnets
摘要：
描述了一种新的角扫描高频EPR（HFEPR）技术，该技术能够原位对准包含低对称性磁性物质的单晶样品，例如单分子磁体（SMM）。这种基于腔的方法涉及在固定频率和场下记录光谱，同时扫描场方向。该方法适用于研究自旋S = 10 Mn 12 SMM（例如Mn 12-醋酸盐）的低对称性Jahn-Teller变体。低对称性复合物也是SMM，但具有显著降低的磁化反转势垒（Ueff ~ 43 K），因此与高对称性物质相比在低温下更快弛豫。在较低对称性结构中结晶的Mn 12络合物表现出一个或多个Mn（III）Jahn-Teller轴异常取向的趋势，这被认为是更快弛豫的原因。本文对[Mn_（12）O_（12）（O_2CCH_2But）_（16）（H_2 O）_4]·CH_2Cl_2·MeNO_2体系进行了大量的HFEPR研究，以考察反常取向的Jahn-Teller轴对有效势垒的影响。发现轴向各向异性D的减少不足以解释Ueff减少近40%。然而，Mn 12芯的降低的对称性引起非常显著的二阶横向各向异性，E ~ D/6。这反过来又导致了自旋投射态在经典势垒顶部以下的显著混合。因此，磁量子隧穿是导致快速弛豫Mn 12 SMM中势垒降低的主导因素。
英文摘要：
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.
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