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

In this work, iron–wüstite (Fe–FeyO) nanocomposites have been prepared via high-energy ball milling (HEBM), using high-purity hematite (α-Fe2O3) and iron (Fe) powders as the raw materials with different Fe/Fe2O3 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+0.478y, for FeyO, where “a” is the lattice parameter of wüstite, it is possible to estimate the value of “y” for different nanocomposites and a composition of Fe0.93O 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 Ms 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 (Hc) 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.