Observation of Small Exchange Bias in Defect Wüsti

Authors: M Gheisari M Mozafari M Niyaifar J Amighian R Soleimani

Publish Date: 2012/12/28

Volume: 26, Issue: 2, Pages: 237-242

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Abstract

Wüstite nanoparticles have been prepared by mechanochemical processing MCP using highpurity hematite αFe2O3 and iron Fe powders as the raw materials In order to get a singlephase wüstite different mole ratios of Fe/Fe2O3 were milled Xray diffraction studies of the asmilled powders show that a singlephase wüstite was formed Using the formula a=4334−0478x for Fe1−x O where “a” is the lattice parameter of wüstite a nonstoichiometric composition of Fe093O 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 roomtemperature 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 asmilled 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 spineltype defect clusters in nonstoichiometry wüstite Room temperature magnetic measurements showed that nanosized prepared wüstite ferrimagneticlike behavior was interpreted according to spinellike 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 Fe093O nonstoichiometry structure there are 0712 molecules of FeO and 0072 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

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