MAGNETIC HYSTERESIS AND MOSSBAUER STUDIES IN ULTRAFINE IRON PARTICLES

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mössbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000-1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mössbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ-Fe 2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α-Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri- or antiferromagnetic oxide layer.