THERMAL FLUCTUATIONS IN THE MAGNETIC GROUND-STATE OF THE MOLECULAR CLUSTER MN12O12 ACETATE FROM MU-SR AND PROTON NMR RELAXATION

Measurements of the spin-lattice relaxation rate are reported for muon s and protons as a function of temperature for different values of the applied magnetic field in the Mn12O12, molecular cluster. Strongly fi eld dependent maxima in the relaxation rate versus temperature are obs erved below 50 K. The results are explained in terms of thermal fluctu ations of the total magnetization of the cluster among the different o rientations with respect to the anisotropy axis. The lifetimes of the different m components of the total spin, S-T = 10, Of the molecule ar e obtained from the experiment and shown to be consistent with the one s expected from a spin-phonon coupling mechanism. No clear evidence fo r macroscopic quantum tunneling was observed in the field dependence o f the proton relaxation rate at low T. [S0031-9007(98)07391-8].