Properties of nanocomposites of α-Fe and Fe₃O₄

Abstract

Composites of α-Fe and Fe₃O₄ having dimensions in the range 10-20 nm have been prepared by subjecting micrometer-sized α-Fe₂O₃ powders to a reduction treatment in hydrogen at a temperature 683 K for a duration from 5 to 40 min. The specimens have been characterized by Mossbauer spectroscopy. The latter shows the presence of a substantial amount of superparamagnetic particles. The fraction of these particles increases as the reduction treatment is enhanced. The incorporation of hydrogen atoms during the reduction process appears to break down the precursor oxide particles. Magnetic susceptibility shows a maximum at around 125 K which arises due to Verwey transition in Fe₃O₄ particles. Magnetization measurements on zero-field cooled and field cooled specimens indicate a blocking temperature close to 300 K. Coercivity variation with temperature shows an unusual behaviour. Exchange coupling between the ferro- and ferri-magnetic particles seems to be the reason behind this. The presence of such nearest neighbouring pairs of α-Fe/Fe₃O₄ is confirmed by the DC resistivity data which show electron tunnelling between metal islands separated by Fe₃O₄ granules.