Measurement of the Relaxation Rate of the Magnetization inMn12O12-Acetate Using Proton NMR Echo

We present a novel method to measure the relaxation rate $W$ of the magnetization of $\mathrm{Mn}{}_{12}\mathrm{O}{}_{12}$-acetate (Mn12) magnetic molecular cluster in its $S=10\mathrm{}$ ground 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 magnetization of the molecules. We can explain the novel effect with a simple model which relates the intensity of the proton echo signal to the microscopic 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.