Observation of Small Exchange Bias in Defect Wüstite (Fe0.93O) Nanoparticles

Abstract

Wustite nanoparticles have been prepared by mechanochemical processing (MCP), using high-purity hematite (α-Fe2O3) and iron (Fe) powders as the raw materials. In order to get a single-phase wustite, different mole ratios of (Fe/Fe2O3) were milled. X-ray diffraction studies of the as-milled powders show that a single-phase wustite was formed. Using the formula a=4.334−0.478x, for Fe1−x O, where “a” is the lattice parameter of wustite, a nonstoichiometric composition of Fe0.93O was estimated for the wustite single phase. A mean crystallite size of 13±1 nm was calculated for the single phase wustite, using Scherrer’s formula. The morphology of the powders was also checked by TEM. The room-temperature Mossbauer spectra of the samples supported the presence of Fe3+ in octahedral sites of wustite phase, which is a sign of its nonstoichiometry. Hysteresis loops of the as-milled powders at 5 K and room temperature have been obtained by SQUID and by VSM systems, respectively. The loops show nonzero coercivity, in contrast to the bulk wustite. The observed magnetizations can be explained by a model based on the spinel-type defect clusters in nonstoichiometry wustite. Room temperature magnetic measurements showed that nanosized prepared wustite ferrimagnetic-like behavior was interpreted according to spinel-like defect clusters. Therefore, small exchange bias effects 20 Oe and 38 Oe were observed in the magnetization curves at room and 5 K temperatures, respectively. According to the Dimitrov model, in the Fe0.93O nonstoichiometry structure, there are 0.712 molecules of FeO and 0.072 molecules of Fe3O4, which the interaction between the antiferromagnetic (FeO) and ferrimagnetic (Fe3O4) phases in the Fe1−x O can be the cause of the observed exchange bias effect in the hysteresis loops.