Nuclear magnetic resonance study of spin dynamics in molecular nanomagnets

Molecular nanomagnets are new class of materials made of a transition metal magnetic core achieved by large organic groups which isolate magnetically one molecule from another. These novel materials have been investigated here for the first time with a local microscopic technique i.e. Nuclear Magnetic Resonance. Various interesting aspects of spin dynamics in the finite size molecular nonomagnets have been studied in detail.;At high temperature the nuclear spin lattice relaxation (NSLR) measurements have revealed the persistence at long time of the spin-spin time correlation function of the spins of the metallic ions in clusters such as Mn12 and rings such as Cu6, Cu8, F6, Fe10. On lowering the temperature, when correlation effects become important, it has been shown that critical slowing down of the fluctuations occur in a way similar to the one occurring at magnetic phase transition in macroscopic systems. At even lower temperature the molecular nanomagnets have been investigated in their magnetic ground state manifold. Several interesting information were obtained in this quantum regime. The life time broadening of the magnetic levels was obtained indirectly from NSLR results in Mn12 nanomagnet. Cross relaxation effects were observed as a function of the applied magnetic fielding the rings with a singlet ground state, Fe10 and Fe6. Finally it was discovered that by monitoring the amplitude of the NMR signal as a function of time when magnetization of the nanomagnet evolves towards equilibrium at low temperature one obtains a microscopic monitoring of the slow spin reorientation in the superparamagnetic phase. As a extension of the research on the molecular nanomagnets, the core of the magnetic biomolecule ferritin has been studied with NMR. The direct experimental evidence of an antiferromagnetic transition in the ferritin core is reported for the first time.;Besides having evidenced some interesting novel effects, the NMR investigation has proved the potential and limits of the NMR technique for investigating the spin dynamics and magnetic properties of these new class of materials.