探索最大单分子磁体的磁性
Exploring the magnetic properties of the largest single molecule magnets
摘要：
巨大的$\{ \mathrm{Mn}_{70} \}$和$\{ \mathrm{Mn}_{84} \}$轮是迄今为止合成的最大的核性单分子磁体，了解它们的磁性对理论提出了挑战。从第一性原理计算出发，我们利用有效自旋哈密顿量来探讨这些轮子的磁性和激发。我们发现，不寻常的几何形状的超交换途径导致弱耦合的$\{ \mathrm{Mn}_{7} \}$亚基携带有效的$S=2$自旋。光谱表现出层次的能量尺度和大规模的简并性，与最低的能量激发所产生的海森堡环样激发的$\{ \mathrm{Mn}_{7} \}$子单元周围的车轮，在能量与所观察到的磁化率的温度依赖性一致。我们进一步提出了一个重要的作用弱长程耦合在选择精确的自旋基态的$\mathrm{Mn}$车轮出近简并基态带。
英文摘要：
The giant $\{ \mathrm{Mn}_{70} \}$ and $\{ \mathrm{Mn}_{84} \}$ wheels are the largest nuclearity single-molecule magnets synthesized to date and understanding their magnetic properties poses a challenge to theory. Starting from first principles calculations, we explore the magnetic properties and excitations in these wheels using effective spin Hamiltonians. We find that the unusual geometry of the superexchange pathways leads to weakly coupled $\{ \mathrm{Mn}_{7} \}$ subunits carrying an effective $S=2$ spin. The spectrum exhibits a hierarchy of energy scales and massive degeneracies, with the lowest energy excitations arising from Heisenberg-ring-like excitations of the $\{ \mathrm{Mn}_{7} \}$ subunits around the wheel, at energies consistent with the observed temperature dependence of the magnetic susceptibility. We further suggest an important role for weak longer-range couplings in selecting the precise spin ground-state of the $\mathrm{Mn}$ wheels out of the nearly degenerate ground-state band.
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