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 $\left[{\mathrm{Mn}}_6{\mathrm{O}}_2{\left({\mathrm{O}}_2\mathrm{C}\mathrm{Me}\right)}_2{\left(\text{salox}\right)}_6{\left(\mathrm{Et}\mathrm{O}\mathrm{H}\right)}_4\right]\bullet 4\mathrm{Et}\mathrm{O}\mathrm{H}$ (in short ${\mathrm{Mn}}_6$ acetate) and $\left[{\mathrm{Mn}}_6{\mathrm{O}}_2{\left({\mathrm{O}}_2\mathrm{C}\mathrm{Ph}\right)}_2{\left(\text{salox}\right)}_6{\left(\mathrm{Et}\mathrm{O}\mathrm{H}\right)}_4\right]\bullet 4\mathrm{Et}\mathrm{O}\mathrm{H}$ (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 $U_{\mathrm{eff}}\sim 28\mathrm{K}$. The main characteristics of the ${}^1\mathrm{H}$ NMR spectra (measured between $1.5\mathrm{K}$ 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 $(T<3.5\mathrm{K})$, 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 $T_2^{-1}$ 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 ${\omega }_c\left(T\right)$, 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