MAGNETIC-PROPERTIES OF ULTRAFINE IRON PARTICLES

The magnetic and morphological properties of fine Fe particles have been studied. Ultrafine particles of Fe were prepared using a vapor deposition technique under an argon atmosphere. The argon pressure was varied from 0.5 to 8 Torr during evaporation, and samples with a median diameter in the range 50-200 angstrom were obtained having a log-normal distribution. The dependence of magnetic properties on particle size and temperature (10 K < T < 300 K) were studied using superconducting-quantum-interference-device magnetometry and Mossbauer spectroscopy. Samples with particle diameter below 90 angstrom showed a superparamagnetic behavior below room temperature. The saturation magnetization of the particles varied from 25 to 190 emu/g, with the higher values corresponding to larger particles. For these larger particles, a coercivity of 1.05 kOe was obtained at room temperature. The magnetic and structural data suggested a core-shell type of structure, where the core consists of metallic Fe and the shell is composed of Fe oxides.