三角形Mn（III）3团簇的魔角效应：单分子磁体的解构
Magic angle effects in a trigonal Mn(III)3 cluster: deconstruction of a single-molecule magnet
摘要：
我们对一个具有特殊结构的三角Mn 3簇单晶进行了角度依赖的高频EPR研究，其中Mn（III）离子的局部磁易磁化轴明显偏离分子C3轴，向魔角54.7度倾斜，导致与铁磁耦合的总自旋ST = 1相关的2阶轴向磁各向异性的几乎完全抵消。6基态。这与过去已经深入研究的许多相关Mn 3单分子磁体（SMM）中的情况形成对比，其中局部MnIII各向异性张量合理地平行，导致磁化弛豫（Ueff = 30至35 cm-1）的实质性势垒和低于约2.5 K的磁化阻断。在这种情况下，2阶各向异性的抑制导致ST = 6基态的零场分裂（ZH）由4阶和更高阶相互作用主导的情况。这提供了一个独特的机会，深入研究分子几何形状如何影响这些相互作用，负责在高对称性SMM的磁化量子隧穿。角度相关的EPR测量提供了一个完整的映射的分子磁各向异性。同时，采用不可约张量算符（ITO）方法，以获得直接将分子各向异性与微观物理联系起来的解析表达式，即，与单个MnIII离子相关的双张量，它们的取向，以及三个自旋之间的交换耦合。我们发现魔角倾斜导致ST = 6基态能级图的大规模压缩和自旋投影态之间的强混合。虽然这些特性是对立的SMM行为，他们提供了重要的见解多核分子纳米磁体的物理。
英文摘要：
We present angle-dependent high-frequency EPR studies on a single-crystal of a trigonal Mn3 cluster with an unusual structure in which the local magnetic easy-axes of the constituent Mn(III) ions are tilted significantly away from the molecular C3 axis towards the magic-angle of 54.7 degrees, resulting in an almost complete cancelation of the 2nd-order axial magnetic anisotropy associated with the ferromagnetically coupled total spin ST = 6 ground state. This contrasts the situation in many related Mn3 single-molecule magnets (SMMs) that have been studied intensively in the past, for which the local MnIII anisotropy tensors are reasonably parallel, resulting in substantial barriers to magnetization relaxation (Ueff = 30 to 35 cm 1) and magnetization blocking below about 2.5 K. The suppression of the 2nd-order anisotropy in the present case results in a situation in which the zero-field splitting (ZFS) of the ST = 6 ground state is dominated by 4th- and higher-order interactions. This provides a unique opportunity to study in depth how molecular geometry influences these interactions that are responsible for quantum tunneling of magnetization in high-symmetry SMMs. Angle-dependent EPR measurements provide a full mapping of the molecular magneto-anisotropy. Meanwhile, irreducible tensor operator (ITO) methods are employed in order to obtain analytic expressions that directly relate molecular anisotropy to the microscopic physics, i.e., the ZFS tensors associated with the individual MnIII ions, their orientations, and the exchange coupling between the three spins. We find that the magic-angle tilting leads to a massive compression of the ST = 6 ground state energy level diagram and strong mixing between spin projection states. Although these characteristics are antagonistic to SMM behavior, they provide important insights into the physics of polynuclear molecular nanomagnets.
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