NMR in Magnetic Molecular Rings and Clusters

Abstract

Molecular nanomagnets (MNM) are magnetic molecular clusters con- taining a limited number of transition ions in a highly symmetric configuration and coupled by strong exchange interaction (either ferromagnetic (FM) or more often antiferromagnetic (AFM)). The magnetic intermolecular interaction is very weak and thus the clusters behave as single nanosize units. NMR has proved to be an excellent probe to investigate the static magnetic properties and the spin dynamics of this new fascinating class of magnetic materials. The chapter contains a compre- hensive review of the work performed in the last few years by the present authors with only a brief reference to work performed by other researchers. Most of the NMR measurements were performed on protons but important results were obtained also using other nuclei like 55 Mn, 57 Fe, 7 Li, 23 Na, 63 Cu, 19 F. In some cases the NMR was observed at low temperature in zero external field. Some novel NMR phenom- ena specific of the systems investigated were discovered and explained. For example in the anisotropic ferrimagnetic clusters Mn12 and Fe8, the ground state is a high total spin S = 10 state whereby the crystal field anisotropy generates an energy barrier typical of superparamagnets. It is shown how NMR and relaxation measure- ments can detect the microscopic local spin configuration in the ground state and the dynamics of quantum tunnelling of the magnetization (QMT). Another exam- ple is the case of the AFM rings, Fe10, Fe6 and Cr8, in which the ground state is a singlet, S = 0, separated from the first triplet excited state by an energy gap of about 5-10 K. By applying a magnetic field one can observe level crossing effects. These effects were studied by proton NMR and relaxation measurements vs field at low temperature (1.5-3 K). Finally, the nuclear relaxation rate as a function of temperature in the above mentioned AFM rings displays a field dependent peak at a temperature of the order of the exchange constant J ,w hich can be fi tted with a general scaling law. From these data, the lifetime broadening of the energy levels can be determined.