Magnetic properties of mechanochemically prepared iron–wüstite (Fe–Fe yO) nanocomposites

Abstract

In this work, iron–wüstite (Fe–Fe y O) nanocomposites have been prepared via high-energy ball milling (HEBM), using high-purity hematite (α-Fe 2O 3) and iron (Fe) powders as the raw materials with different Fe/Fe 2O 3 mole ratios (MR)=0.6, 0.9, 1.0, 2.3, 4.9 and 13.6. X-ray diffraction studies of the as-milled powders show that a single-phase wüstite was formed for the lowest mole ratio (MR=0.6) and mixtures with MRs higher than 0.6 result in iron–wüstite nanocomposites, except for MR=13.6 that is dominantly a pure iron phase. The mean crystallite sizes of the iron and wüstite in the nanocomposites have been calculated by Scherrer's formula, which were 9±1 and 7±1 nm, respectively. Using the formula a=3.856+0.478 y, for Fe y O, where “ a” is the lattice parameter of wüstite, it is possible to estimate the value of “ y” for different nanocomposites and a composition of Fe 0.93O was estimated for the wüstite single phase (MR=0.6). In addition, a gradual decrease in “ y” from 0.87 to 0.83 was obtained by increasing MR values from 0.9 to 4.9, respectively. The room-temperature Mössbauer spectrum of the single-phase wüstite shows considerable asymmetry due to two overlapping quadrupole doublets. For higher MRs, room-temperature Mössbauer spectra exhibit sextets, which confirm the existence of iron in the samples. The Mössbauer spectrum of the sample with the highest mole ratio (MR=13.6) shows only a sextet related to α-Fe without any detection of wüstite, which is in agreement with the XRD results. The nanosized prepared wüstite shows ferrimagnetic like behavior, which was interpreted according to spinel-like defect clusters. The M s values obtained from VSM measurements and those calculated based on the Mössbauer data and chemical reaction are in good agreement. By increasing MR from 0.6 to 2.3, the coercivity ( H c) increases sharply to its maximum value at about MR=2.3, for which the value of Fe content is 45% and then drops off. This behavior is discussed based on α-Fe contents in the nanocomposites and percolation threshold.