Superspin glass behavior of amorphous FeO-SiO₂ thin films

We have prepared amorphous xFeO"(100 % x)SiO₂ (x = 38.2, 54.8, and 67.5 in mol%) thin films by a pulsed laser deposition method and examined their magnetic properties. The temperature dependence of zero-field-cooled magnetization manifests a cusp-like peak, i.e., spin-freezing in a low temperature region. The spin-freezing temperature depends on the frequency of ac magnetic field, and the dependence is explainable in terms of the scaling law. The aging-memory effect characteristic of spin glass is also observed. Furthermore, superparamagnetic behavior is found in the variation of magnetization with magnetic field at high temperatures. These observations indicate that there exist magnetic clusters of Fe²⁺ ions in the amorphous FeO-SiO₂ thin films and that the intercluster interactions bring about the superspin glass transition at low temperatures.