Observation of Small Exchange Bias in Defect Wüstite (FeO) Nanoparticles.

Gheisari, M.; Mozafari, M.; Niyaifar, M.; Amighian, J.; Soleimani, R.

Wüstite nanoparticles have been prepared by mechanochemical processing (MCP), using high-purity hematite ( α-FeO) and iron (Fe) powders as the raw materials. In order to get a single-phase wüstite, different mole ratios of (Fe/FeO) were milled. X-ray diffraction studies of the as-milled powders show that a single-phase wüstite was formed. Using the formula a=4.334−0.478 x, for FeO, where ' a' is the lattice parameter of wüstite, a nonstoichiometric composition of FeO 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 Fe 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 FeO nonstoichiometry structure, there are 0.712 molecules of FeO and 0.072 molecules of FeO, which the interaction between the antiferromagnetic (FeO) and ferrimagnetic (FeO) phases in the FeO can be the cause of the observed exchange bias effect in the hysteresis loops.

NANOPARTICLES; HEMATITE; IRON powder; MORPHOLOGY; MAGNETIZATION; ANTIFERROMAGNETIC materials

Journal of Superconductivity & Novel Magnetism, 2013, Vol 26, Issue 2, p237

1557-1939

Academic Journal

10.1007/s10948-012-1821-9