Abstract
We report a detailed proton NMR study, as a function of temperature and external magnetic field, of two hexanuclear manganese magnetic molecule clusters with chemical formula (in short acetate) and (henceforth benzoate). Both clusters are characterized by a ferrimagnetic ground state with total spin and a large uniaxial anisotropy, which gives rise to an effective energy barrier for the relaxation of the magnetization of the order of . The main characteristics of the NMR spectra (measured between and room temperature for different fields) are explained in terms of the dipolar hyperfine interaction of the proton nuclei with the adjacent magnetic ions. At low temperatures , the spectra broaden significantly and become structured due to the slowing down of the local fluctuating fields at the proton sites, caused by the gradual freezing of the moments into the collective ground state. The spin dynamics of the exchange coupled magnetic ions was also probed by proton spin-spin relaxation rate and spin-lattice relaxation rate measurements. On decreasing the temperature, a gradual enhancement of both relaxation rates is observed, followed by a significant decrease of the signal intensity (wipe-out effect). The low frequency regime of the spin fluctuations as probed by , can be described and analyzed in terms of a single characteristic correlation frequency , which is interpreted as the lifetime broadening of the discrete magnetic energy levels due to spin-phonon interactions.
- Received 5 November 2006
DOI:https://doi.org/10.1103/PhysRevB.75.064414
©2007 American Physical Society