Proton NMR study in hexanuclear manganese single-molecule magnets

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 [Mn6O2(O2CMe)(2)(salox)(6)(EtOH)(4)]center dot 4EtOH (in short Mn-6 acetate) and [Mn6O2(O2CPh)(2)(salox)(6)(EtOH)(4)]center dot 4EtOH (henceforth Mn-6 benzoate). Both clusters are characterized by a ferrimagnetic ground state with total spin S-T=4 and a large uniaxial anisotropy, which gives rise to an effective energy barrier for the relaxation of the magnetization of the order of U-eff similar to 28 K. The main characteristics of the H-1 NMR spectra (measured between 1.5 K 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 (T < 3.5 K), 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 Mn3+ moments into the S-T=4 collective ground state. The spin dynamics of the exchange coupled magnetic ions was also probed by proton spin-spin relaxation rate T-2(-1) and spin-lattice relaxation rate T-1(-1) 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 T-1(-1), can be described and analyzed in terms of a single characteristic correlation frequency omega(c)(T), which is interpreted as the lifetime broadening of the discrete magnetic energy levels due to spin-phonon interactions.