MAGNETIC-PROPERTIES OF FE CLUSTERS IN NAY ZEOLITE

Clusters of up to 28 Fe0 atoms have been introduced into the"supercages" of NaY zeolite by ion exchange of Fe+2 for Na+ followed by reduction to Fe0 with solvated electrons and Na-1 ions. The temperature-dependent ac and dc susceptibilities, which exhibit Curie-Weiss behavior before reduction, change to something approximating superparamagnetic behavior afterwards. The blocking temperature, Tb shows a strong dependence on magnetic field but weak dependence on frequency. There is at best a very weak remanence and coercive field while the onset of irreversibility occurs at temperatures well above Tb. In addition, the real and imaginary parts of the ac susceptibility show essentially similar temperature dependence. Some of this anomalous behavior can be attributed to a distribution of particle sizes. To the best of our knowledge these preliminary data - while poorly understood - represent the first measurements of the temperature, frequency, and field dependence of the magnetic properties of such small clusters.