Low temperature spin dynamics in Cr7Ni-Cu-Cr7Ni coupled molecular rings

Proton Nuclear Magnetic Resonance (NMR) relaxation measurements have been performed down to very low temperature (50 mK) to determine the effect of coupling two Cr7Ni molecular rings via a Cu2+ ion. No difference in the spin dynamics was found from nuclear spin lattice relaxation down to 1.5 K. At lower temperature, the 1H-NMR line broadens dramatically indicating spin freezing. From the plot of the line width vs. magnetization, it is found that the freezing temperature is higher (260 mK) in the coupled ring with respect to the single Cr7Ni ring (140 mK).