Observation of Small Exchange Bias in Defect Wüstite (Fe_0.93O) Nanoparticles

Abstract

Wüstite 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 wüstite, different mole ratios of (Fe/Fe₂O₃) were milled. X-ray diffraction studies of the as-milled powders show that a single-phase wüstite was formed. Using the formula a=4.334−0.478x, for Fe_1−x O, where “a” is the lattice parameter of wüstite, a nonstoichiometric composition of Fe_0.93O was estimated for the wüstite single phase. A mean crystallite size of 13±1 nm was calculated for the single phase wüstite, using Scherrer’s formula. The morphology of the powders was also checked by TEM. The room-temperature Mössbauer spectra of the samples supported the presence of Fe³⁺ in octahedral sites of wüstite 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 wüstite. The observed magnetizations can be explained by a model based on the spinel-type defect clusters in nonstoichiometry wüstite. Room temperature magnetic measurements showed that nanosized prepared wüstite 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 Fe_0.93O 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 Fe_1−x O can be the cause of the observed exchange bias effect in the hysteresis loops.