Thermal fluctuations in the magnetic ground state of the molecular cluster Mn12O12 acetate from μSR and proton NMR relaxation

Measurements of the spin-lattice relaxation rate are reported for muons and protons as a function of temperature for different values of the applied magnetic field in the Mn(12)O(12), molecular cluster. Strongly field dependent maxima in the relaxation rate versus temperature are observed below 50 K. The results are explained in terms of thermal fluctuations of the total magnetization of the cluster among the different orientations with respect to the anisotropy axis. The lifetimes of the different m components of the total spin, S(T) = 10, Of the molecule are obtained from the experiment and shown to be consistent with the ones expected from a spin-phonon coupling mechanism. No clear evidence for macroscopic quantum tunneling was observed in the field dependence of the proton relaxation rate at low T. [S0031-9007(98)07391-8].