Proton NMR and μSR inMn12O12acetate: A mesoscopic magnetic molecular cluster

The spin dynamics of Mn spins in the dodecanuclear manganese cluster of formula [Mn{sub 12}O{sub 12}(CH{sub 3}COO){sub 16}(H{sub 2}O){sub 4}]{center_dot}2CH{sub 3}-COOH{center_dot}4H{sub 2}O(Mn{sub 12}) has been investigated by {sup 1}HNMR and muon spin-lattice relaxation rate 1/T{sub 1} as a function of temperature (10{endash}400 K) and external magnetic field (0{endash}9.4 T). At room temperature, the proton 1/T{sub 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{sub 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 the 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{sub 1} an effect that could be related to the superparamagnetic behavior of the Mn{sub 12} molecule. {copyright} {ital 1998} {ital The American Physical Society}