Low-energy spin excitations inMn3×3molecular nanomagnets

Abstract

NMR and $\mu \mathrm{SR}$ relaxation measurements in the ${\mathrm{Mn}}_9{\left({\mathrm{C}}_{19}{\mathrm{H}}_{15}{\mathrm{N}}_9{\mathrm{O}}_2\right)}_6{\left({\mathrm{ClO}}_4\right)}_6.18{\mathrm{H}}_2\mathrm{O}\left({\mathrm{Mn}}_{3\times 3}\right)$ 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 $\mu \mathrm{SR}$ relaxation rate allows one to derive the Zeeman splitting of the low-energy spin levels.