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

Abstract

We report a detailed $^{1}\mathrm{H}$ NMR study on the spin dynamics of single molecule magnets as a function of temperature and external magnetic field. A gradual loss of the $^{1}\mathrm{H}$ 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 ${T}_{2}^{\ensuremath{-}1}$ and ${T}_{1}^{\ensuremath{-}1}$, 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 ${T}_{1}^{\ensuremath{-}1}$ and ${T}_{2}^{\ensuremath{-}1}$ 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 $^{1}\mathrm{H}$ 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 et al., Phys. Rev. B 70, 134434 (2004)].