Proton NMR study in hexanuclear manganese single-molecule magnets

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 $[{\mathrm{Mn}}_{6}{\mathrm{O}}_{2}{({\mathrm{O}}_{2}\mathrm{C}\mathrm{Me})}_{2}{(\text{salox})}_{6}{(\mathrm{Et}\mathrm{O}\mathrm{H})}_{4}]∙4\mathrm{Et}\mathrm{O}\mathrm{H}$ (in short ${\mathrm{Mn}}_{6}$ acetate) and $[{\mathrm{Mn}}_{6}{\mathrm{O}}_{2}{({\mathrm{O}}_{2}\mathrm{C}\mathrm{Ph})}_{2}{(\text{salox})}_{6}{(\mathrm{Et}\mathrm{O}\mathrm{H})}_{4}]∙4\mathrm{Et}\mathrm{O}\mathrm{H}$ (henceforth ${\mathrm{Mn}}_{6}$ 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 ${U}_{\mathrm{eff}}\ensuremath{\sim}28\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The main characteristics of the $^{1}\mathrm{H}$ NMR spectra (measured between $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $(T<3.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, 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 ${\mathrm{Mn}}^{3+}$ 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 ${T}_{2}^{\ensuremath{-}1}$ and spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ 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 ${T}_{1}^{\ensuremath{-}1}$, can be described and analyzed in terms of a single characteristic correlation frequency ${\ensuremath{\omega}}_{c}(T)$, which is interpreted as the lifetime broadening of the discrete magnetic energy levels due to spin-phonon interactions.