Magnetic properties of mechanochemically prepared iron-wustite (Fe-FeyO) nanocomposites

Abstract

In this work, iron-wustite (Fe-FeyO) nanocomposites have been prepared via high-energy ball milling (HEBM), using high-purity hematite (alpha-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 wustite was formed for the lowest mole ratio (MR = 0.6) and mixtures with MRs higher than 0.6 result in iron-wustite nanocomposites, except for MR = 13.6 that is dominantly a pure iron phase. The mean crystallite sizes of the iron and wustite 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 wustite, it is possible to estimate the value of y'' for different nanocomposites and a composition of Fe0.93O was estimated for the wustite 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 Mossbauer spectrum of the single-phase wustite shows considerable asymmetry due to two overlapping quadrupole doublets. For higher MRs, room-temperature Mossbauer spectra exhibit sextets, which confirm the existence of iron in the samples. The Mossbauer spectrum of the sample with the highest mole ratio (MR = 13.6) shows only a sextet related to alpha-Fe without any detection of wustite, which is in agreement with the XRD results. The nanosized prepared wustite shows ferrimagnetic like behavior, which was interpreted according to spinel-liked defect clusters. The M-s values obtained from VSM measurements and those calculated based on the Mossbauer 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 alpha-Fe contents in the nanocomposites and percolation threshold. (C) 2009 Elsevier B. V. All rights reserved.