Spin Excitations and Dynamics in Molecular Nanomagnets

在分子纳米磁体中，十几个左右的磁性金属离子通过有机配体连接在一起，形成明确的结构并经历海森堡交换作用。这些分子是理想的磁性纳米团簇，因为它们不表现出形状或形状的分散，并且允许通过测量整体样品来研究单分子效应。本项目旨在探索和理解这些零维或介观量子自旋系统中可能存在的新颖的磁现象和复杂的多体波函数。在前面的项目中，我们记录了Fe9和Mn12车轮这两种化合物的出色的非弹性中子散射(INS)数据。然而，由于这两个系统的复杂性，数据的建模被证明是非常具有挑战性的。我们将继续努力，发展磁性模型和对这些分子中磁性的物理理解。在反铁磁环Fe9中，奇数个金属中心导致自旋受挫，正如我们先前的结果所表明的那样，必须另外考虑Dzyaloshinski-Moriya相互作用的影响。了解自旋受挫和Dzyaloshinski-Moriya相互作用的相互作用及其物理后果将是很有趣的。单分子磁体Mn12车轮的独特之处在于磁化强度的不同寻常的量子隧道效应，这涉及到不同总自旋的状态之间的隧道效应，但其潜在的物理机制仍存在争议。对我们以前记录的数据的分析将进一步加深对这个星系团中的磁性的理解，并解决讨论。我们将研究Mn19分子，它是已知的分子磁团簇中最大的基态自旋(S=83/2)，引起了人们的极大关注。然而，我们对这种材料中的自旋激发一无所知，我们将用INS进行研究。根据初步工作，我们预计该团簇中(铁磁)交换相互作用的特殊拓扑导致了一种低位激发模式，这种模式更好地被描述为“集体”激发，因为与自旋团簇常见的“离散”激发相比，许多跃迁都有贡献。最后，我们将研究一个新合成的Fe7团簇，它具有一种新颖的交换耦合拓扑结构，可以描述为一个三叶螺旋桨。我们对磁性数据的初步分析和初步的数值模拟表明，这种分子中存在非常罕见的自旋受挫情况。因此，我们将用INS对激发进行实验研究，并建立合适的物理模型。这些系统使我们能够从不同的角度研究海森堡交换作用的相互作用和自旋中心的基本晶格的拓扑结构，并将促进我们对小量子自旋团簇特别是分子纳米磁体中的磁性的不完全理解。

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.