Experimental study on oxidation of synthetic and natural magnetites monitored by magnetic measurements

Magnetite is one of the most important iron oxides due to its relatively widespread natural occurrence, simple synthesis methods, and unique magnetic properties. In addition, when the size of its particles decreases to nanoscale, magnetite acquires new characteristics, becoming useable for environmental remediation, biomedicine, data storage, and catalysis. However, the successful use of magnetite nanoparticles in these areas strongly depends on its chemical stability and magnetic properties under various conditions. In this article, the experimental oxidation of various types of magnetite in air was studied using thermomagnetic, thermogravimetric, and X-ray diffraction analyses, along with magnetometry. The results showed that natural and synthetic micro-sized magnetites oxidized to form hematite at 200 degrees C, while the formation of other phases was not detected. The complete conversion of magnetite to hematite was achieved after heating at 600-700 degrees C for 60 min. In contrast, synthetic nano-sized magnetites oxidized via the formation of maghemite-magnetite solid solution, revealing an increase in the Curie temperature and only a slight decrease in the mass magnetization at heating to 500 degrees C. Above 500 degrees C, the magnetization decreased due to the transformation of maghemite to hematite or, possibly, amorphous iron oxide. Also, the formation of an intermediate phase with Curie temperatures of 325 -425 degrees C was observed at a later stage of the oxidation of synthetic nano-sized magnetite. This can be explained by maghemite phase associated with small residual clusters that have not yet undergone conversion. The oxidation of magnesiomagnetite with 9 wt% of MgO proceeded differently due to the stabilizing effect of Mg ions. A gradual decrease in Curie temperature from 508 to 315 degrees C and magnetization from 65 to 25 A m(2) kg(-1) indicate the oxidation of Fe2+ to Fe3+ and cation redistribution. The sample after heating at 900 degrees C for 60 min was represented by the spinel phase with magnetic properties close to magnesioferrite, while hematite formation was not detected. The phase transformations of magnetite with different grain sizes and composition can be used for the synthesis of iron oxide nanoparticles (IONs) with tunable magnetic properties and improved stability. (C) 2020 Elsevier B.V. All rights reserved.