Measurement of the relaxation rate of the magnetization in Mn12O12-acetateusing proton NMR echo

We present a novel method to measure the relaxation rate W of the magnetiza tion of Mn12O12-acetate (Mn12) magnetic molecular cluster in its S = 10 gro und state at low T. It is based on the observation of an exponential growth in time of the proton NMR signal during the thermal equilibration of the m agnetization of the molecules, We can explain the novel effect with a simpl e model which relates the intensity of the proton echo signal to the micros copic reversal of the magnetization of each individual Mn12 molecule during the equilibration process. The method should find wide application in the study of magnetic molecular clusters in off-equilibrium conditions.