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

We report a detailed H-1 NMR study on the spin dynamics of single molecule magnets as a function of temperature and external magnetic field. A gradual loss of the H-1 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 T-2(-1) and T-1(-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 T-1(-1) and T-2(-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 H-1 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.