Superparamagnetic-like behavior in an octanuclear iron cluster

Using high-frequency EPR spectroscopy we have found that a cluster comprising eight iron(III) ions, $\rm Fe_8$, which is essentially flat, has a ground S= 10 state and an Ising-type anisotropy. For the first time both ac susceptibility and Mössbauer spectroscopy could be used in order to monitor the relaxation time of the magnetization, which was found to follow a thermally activated behavior, as in a superparamagnet, with $\tau_0=1.9\times 10^{-7}\,{\rm s}$ and an energy barrier of 22.2 K. The set of data allowed us to conclude that the origin of the anisotropy in nanosize molecular clusters is associated with the single ion contributions and not with the shape of the clusters