Spin Excitations and Dynamics in Molecular Nanomagnets

• Applicant: Professor Dr. Oliver Waldmann
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2008 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 84866817

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.

• DFG Programme: Research Grants