NMR study of the molecular nanomagnet [Fe-8(N3C6H15)(6)O-2(OH)(12)]center dot[Br-8 center dot 9H(2)O] in the high-spin magnetic ground state - art. no. 094439

The magnetic molecular cluster [Fe-8(N3C6H15)(6)O-2(OH)(12)](8+)[Br-8.9H(2) O](8-), in short Fe8, has been investigated at low temperature by H-1-NMR a nd relaxation measurements. Some measurements of D-2-NMR in partially deute rated Fe8 clusters will also be reported. Upon decreasing temperature the N MR spectra display a very broad and structured signal which is the result o f the internal local fields at the proton sites due to the local moments of the Fe(III) ions in the total S=10 magnetic ground state. The proton and d euteron NMR spectra have been analyzed and the different resonance peaks ha ve been attributed to the different proton groups in the molecule. The simu lation of the spectra by using a dipolar hyperfine field and the accepted m odel for the orientation of the Fe(III) local moments do not agree with the experiments even when the magnitude of the local Fc(III) moments is allowe d to vary, It is concluded that a positive contact hyperfine interaction of the same order of magnitude as the dipolar interaction is present for all proton sites except the water molecules. The temperature and magnetic field dependence of the nuclear spin-lattice relaxation rate is ascribed to the fluctuations of the local Fe(III) moments, which follow rigidly the fluctua tions of the total ground state magnetization of the nanomagnet. By using a simple model already utilized for the Mn 12 cluster, we derive the value o f the spin phonon coupling constant which determines the lifetime broadenin g of the different magnetic quantum number m substates of the S=10 ground s tate. It is shown that the lifetime broadening decreases rapidly on lowerin g the temperature. When the lifetime becomes longer than the reciprocal of the frequency shift of the proton lines a structure emerges in the NMR spec trum reflecting the "frozen" local moment configuration.