Proton NMR wipeout effect due to slow fluctuations of the magnetization in single molecule magnets

We report a detailed H1 NMR study on the spin dynamics of single molecule magnets as a function of temperature and external magnetic field. A gradual loss of the H1 NMR signal intensity (wipeout effect) is observed on decreasing the temperature for all the investigated ferromagnetic clusters. This effect is accompanied by a simultaneous enhancement of the spin-spin and spin-lattice relaxation rate T2-1 and T1-1, respectively. The complications entered in the interpretation of the signal loss by the wipeout effect are overcome, and the information about the spin dynamics is retrieved, by implementing a simple and intuitive model that captures the main physical characteristics of the problem and reveals a universal behavior of the spin dynamics for all the clusters. According to our analysis the origin of the wipeout effect as well as the enhancement of the relaxation rates T1-1 and T2-1 in the FM clusters is related to a decrease of the lifetime broadening parameter of the magnetic energy levels, down to the range of the H1 Larmor frequency. The temperature dependence of the lifetime broadening can be described at intermediate temperatures by a power law dependence on T similar to that observed in antifferomagnetic rings [S. H. Baek, Phys. Rev. B 70, 134434 (2004)].