Low-energy spin excitations in Mn3×3 molecular nanomagnets

NMR and $\ensuremath{\mu}\mathrm{SR}$ relaxation measurements in the ${\mathrm{Mn}}_{9}{({\mathrm{C}}_{19}{\mathrm{H}}_{15}{\mathrm{N}}_{9}{\mathrm{O}}_{2})}_{6}{({\mathrm{ClO}}_{4})}_{6}.18{\mathrm{H}}_{2}\mathrm{O}({\mathrm{Mn}}_{3\ifmmode\times\else\texttimes\fi{}3})$ 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 ${\mathrm{Mn}}^{2+}$ 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 $\ensuremath{\mu}\mathrm{SR}$ relaxation rate allows one to derive the Zeeman splitting of the low-energy spin levels.