Spin Excitations and Dynamics in Molecular Nanomagnets

在分子纳米磁体中，十几个磁性金属离子通过有机配体连接在一起，从而形成明确的结构并经历海森堡交换相互作用。这些分子是理想的磁性纳米团簇，因为它们不表现出形式或形状分散，并允许通过测量大量样品来研究单分子效应。本计画旨在探索及了解这些“零维”或“介观”量子自旋系统中可能存在的新颖磁性现象及复杂的多体波函数。在前一个项目中，我们记录了两种化合物Fe 9和Mn 12轮的非弹性中子散射（INS）数据。然而，由于这两个系统的复杂性，数据建模变得非常具有挑战性。我们将继续努力，发展磁性模型和对这些分子磁性的物理理解。在反铁磁环Fe 9中，奇数个金属中心导致自旋挫折的情况下，我们以前的结果表明，Dzyaloshinski-Moriya相互作用的影响必须另外考虑。理解自旋挫折和Dzyaloshinski-Moriya相互作用的相互作用及其物理后果将是有趣的。单分子磁体Mn 12轮的特征在于不寻常的磁化量子隧穿，其涉及不同总自旋的状态之间的隧穿，但其潜在的物理机制存在争议。对我们先前记录的数据的分析将进一步了解该星团的磁性并解决讨论。我们将研究Mn 19分子，它引起了巨大的关注，因为它表现出分子磁性簇的最大基态自旋（S = 83/2）。然而，我们对这种材料中的自旋激发一无所知，我们将通过INS进行研究。从初步工作中，我们预计该簇中（铁磁）交换相互作用的特定拓扑结构会导致低位激发模式，该模式更好地描述为“集体”激发，因为许多跃迁都有贡献，这与自旋簇常见的“离散”激发形成鲜明对比。最后，我们将研究一个新合成的Fe 7团簇与一个新的交换耦合拓扑结构，这可能是一个三叶螺旋桨。我们的初步分析的磁性数据和初步的数值模拟表明，一个非常罕见的自旋挫折的情况下，在这个分子。因此，我们将通过INS实验研究激励，并开发适当的物理模型。这些系统使我们能够从不同的角度研究海森堡交换相互作用的相互作用和自旋中心的底层晶格的拓扑结构，并将推进我们对小量子自旋团簇中的磁性的不完全理解，特别是分子纳米磁体。

In molecular nanomagnets a dozen or so magnetic metal ions are linked together by organic ligands such as to form well defined structures and experience Heisenberg exchange interactions. These molecules are ideal magnetic nanoclusters as they don't exhibit form or shape dispersion and allow studying single-molecule effects by measuring bulk samples. This project aims at exploring and understanding the novel magnetic phenomena and complex many-body wave functions which can exist in these "zero-dimensional" or "mesoscopic" quantum spin systems. In the preceding project we recorded excellent inelastic neutron scattering (INS) data on the two compounds Fe9 and Mn12wheel. However, the modeling of the data turned out to be very challenging because of the complexity of both systems. We will continue our efforts and develop the magnetic model and physical understanding of the magnetism in these molecules. In the antiferromagnetic ring Fe9 the odd number of metal centers leads to a spin frustration situation, where as our previous results show the effects of Dzyaloshinski-Moriya interactions have additionally to be considered. It will be interesting to understand the interplay of spin frustration and Dzyaloshinski-Moriya interactions and the physical consequences thereof. The single-molecule magnet Mn12wheel is distinguished by unusual quantum tunneling of the magnetization which involves tunneling between states of different total spin, but the underlying physical mechanism is discussed controversially. The analysis of our previously recorded data will further the understanding of the magnetism in this cluster and resolve the discussion. We will investigate the Mn19 molecule, which has attracted huge attention as it exhibits the largest ground-state spin (S = 83/2) known for a molecular magnetic cluster. However, nothing is known about the spin excitations in this material, which we will study by INS. From preliminary work we expect that the particular topology of the (ferromagnetic) exchange interactions in this cluster leads to a low-lying excitation mode which is better described as a "collective" excitation as many transitions contribute in contrast to the "discrete" excitations common for spin clusters. Finally, we will study a newly synthesized Fe7 cluster with a novel exchange coupling topology, which may be described as a three-bladed propeller. Our preliminary analysis of the magnetic data and preliminary numerical simulations suggest a highly uncommon spin-frustration situation in this molecule. We hence will study the excitations experimentally by INS and develop the appropriate physical model. These systems allow us to study the interplay of the Heisenberg exchange interactions and the topology of the underlying lattice of spin centers from various perspectives, and will advance our incomplete understanding of the magnetism in small quantum spin clusters in general and molecular nanomagnets in particular.