Spin Dynamics Study of Magnetic Molecular Clusters by Means Moessbauer Spectroscopy

Spin dynamics of the two magnetic molecular clusters Fe4 and Fe8, with four and eight Fe(III) ions, respectively, was studied by means of Moessbauer spectroscopy. The transition probability is W's between the spin states of the ground multiplet were obtained from the fit of the spectra. For the Fe4 cluster we found that, in the range from 1.38 to 77 K, the trend of W's vs the temperature corresponds to an Orbach's process involving an excited state with energy of about 160 K. For the Fe8, which due to the presence of a low energy excited state, could not be studied at temperatures greater than 20 K, the trend of W's in the range from 4 to 18 K seems to correspond to a direct process. The correlation functions of the magnetization were then calculated in terms of the W's. They have an exponential trend fro the Fe4 cluster, while a small oscillating component is also present for the Fe8 cluster. For the first of the clusters, the decay time of the magnetization has a trend which, at low temperatures (T< 15 K), corresponds to an Arrhenius law with a potential barrier of the order of the energy difference between the state M=5 and M=0. Instead, for T> 15 K, the decay time follows the trend of 1/W. For the Fe8 the decay time follows an Arrhenius law, but with a prefactor which is smaller than the one obtained from susceptibility measurements.