Low-energy spin excitations in Mn3Ã3 molecular nanomagnets

NMR and µSR relaxation measurements in the Mn9(C19H15N9O2)6(ClO4)6.18H2O(Mn3×3) molecular grid are presented. The temperature dependence of the relaxation rates can be conveniently described by a single correlation time for the spin fluctuations at temperatures of the order of the exchange coupling among Mn2+ ions. The inverse of the correlation time is observed to reach the MHz range at low temperature and its temperature dependence can be justified on the basis of a theoretical model where the electron spin fluctuations are driven by spin-phonon coupling. Finally, it is pointed out that the magnetic field dependence of the µSR relaxation rate allows one to derive the Zeeman splitting of the low-energy spin levels