Phase evolution and magnetic properties of Co/α-Fe2O3 powder mixtures with different molar ratios treated by mechanical alloying

In this work, we report the phase formation and magnetic properties of Co–hematite powder mixtures with two different molar ratios: Co/α-Fe2O3: 1/0.7 and 1/1.3 subjected to high-energy mechanical milling using metallic cobalt and hematite powder as the initial raw material in ambient air atmosphere. The samples were activated with a ball to powder weight ratio (BPR) of 10 and the milled powders were collected after 0, 1, 5, 15, 25 and 30 h. Various characterization techniques such as XRD, HRTEM, VSM and Mössbauer spectroscopy were utilized to study the prepared samples. For the samples with Co/α-Fe2O3: 1/0.7 milled for 1 and 5 h the formation of cobalt ferrite was confirmed. However, this was not the case for the samples milled above 5 h for whose both Mössbauer and XRD results confirmed the phase decomposition taken place for the previously formed cobalt ferrite phase. Further, the formation of superparamagnetic nanoclusters of iron oxide, a wustite-like Fe1−XCoXO phase and the existence of small amounts of metallic Fe/Fe1−XCoX phase/s were also detected for these samples. The presence of the latter phase is not believed to be solely related to contamination from the steel vial/balls used. A mechanochemical-reduction process is assumed to be also possibly responsible for the formation of the observed reduced phases. For the powder mixture with Co/α-Fe2O3: 1/1.3, however, increased formation of cobalt ferrite phase was observed by increasing the milling time. The highest maximum magnetization (53 e.m.u/g) and coercive field (500 Oe) was obtained for the sample milled for 25 h among various samples of this series of powder mixture. The lower magnetization obtained for this sample compared to that of the bulk is attributed to the size effect. Furthermore, the structural–magnetic properties relationship of the various powders prepared is discussed in detail.