Proton NMR and μSR in Mn12O12 acetate:: A mesoscopic magnetic molecular cluster

The spin dynamics of Mn spins in the dodecanuclear manganese cluster of formula [Mn12O12(CH3COO)(16)(H2O)(4)]. 2CH(3)-COOH-4H(2)O(Mn-12) has been investigated by H-1 NMR and muon spin-lattice relaxation rate 1/T-1 as a function of temperature (10-400 K) and external magnetic field (0-9.4 T). At room temperature, the proton 1/T-1 depends on the measuring frequency. The results can be interpreted in terms of a slow decay of the Mn electronic-spin autocorrelation function, a feature characteristic of the almost zero dimensionality of the system. As the temperature is lowered, 1/T-1 displays a critical enhancement that can be related to the slowing down of the local spin fluctuations as the cluster approaches the condensation into thr total spin S = 10 ground-state configuration. It is found that the application of an external magnetic field greatly depresses the enhancement of 1/T-1 an effect that could be related to the superparamagnetic behavior of the Mn-12 molecule.