Iron clusters supported in a zeolite matrix: Comparison of different magnetic characterizations

Metallic iron clusters dilutely supported in zeolite NaX have been studied by magnetization, ac susceptibility, and transmission electron microscopy. Their behavior is superparamagnetic at high temperatures with negligible intercluster interactions and the cluster magnetic moments become blocked at low temperatures. From the ac susceptibility data, at frequencies between 1 Hz and 1 kHz in the temperature range 1.8-300 K, the distribution of activation energies has been determined. The results have been used to check a previously proposed scaling of the ac susceptibility and justify the use of the Arrhenius law for these systems. Although the obtained information is limited by the temperature and frequency window of the experiment, the quantitative analysis of the ac susceptibility provides, with respect to the magnetization data, additional information about the actual distribution of cluster sizes and it is a valuable tool to avoid misinterpretations about the interaction effects.