Combined magnetic and structural investigation of nano crystalline iron oxides: The interplay between crystal size and phase composition during FeS2 oxidation

The oxides that form during thermal oxidation of natural FeS2 (pyrite and marcasite) consist of nanometer-sized crystals of α-Fe2O3 and γ-Fe2O3. This is shown with heating experiments that were made up to 650 °C, which resembles temperatures used in metallurgical processes. It is shown that magnetic measurements can play a key role in the investigation of this reaction, due to the unwanted blurring effects associated with finite crystal sizes if other methods are used. According to Mössbauer spectra combined with pXRD, many α-Fe2O3 crystals are in a stable magnetic state only due to the formation of bridging superexchange interactions in between them, but the γ-Fe2O3 experiences super-paramagnetic relaxation ceasing first at 20 K. Magnetisation measurements were used for two main purposes (1) determination of the amounts of γ-Fe2O3 in the products, and (2) for characterization of γ-Fe2O3 with respect to crystal size and possible magnetic surface effects such as spin-glass. It is proven that fine FeS2 grains produce more γ-Fe2O3 than coarse. At 500 °C the fine FeS2 grains oxidised into c. 30% γ-Fe2O3 and ca. 70% α-Fe2O3. At 525 °C, the γ-Fe2O3 amounts were also estimated in coarse oxidised FeS2, and results were ca. 20% and 10% γ-Fe2O3 for the fine and coarse FeS2 respectively. The γ-Fe2O3 crystal sizes were a function of both temperature and grain size, and it decreased with decreasing grain size, and upon rising the temperature from 450 to 550 °C. It is argued that the estimated errors during γ-Fe2O3 amount determination are due mainly to disordered magnetic sublattices at the crystal faces of γ-Fe2O3, giving an error of ca. 15% for those samples that have the smallest crystals.