Size-Controlled Magnetite Cores and Hybrid Rietveld Tracking of Oxidation Kinetics

Magnetite (Fe3O4) micro- and nanoparticles are critical for advanced core–shell applications, but their susceptibility to oxidation into maghemite (γ-Fe2O3) degrades magnetic performance. This study evaluates the influence of solvothermal duration and autoclave filling factor on the size, morphology, and magnetic response of magnetite particles and introduces an X-ray-diffraction-based method for quantifying oxidation. Across the evaluated conditions, the 18 h series consistently produced particles approaching the intended 250 nm size while retaining saturation magnetization values of 80–85 A·m2kg−1. The 18 h series with 40% autoclave filling factor (18h40) yielded a particle diameter of ~243 nm, comparable to the 24 h samples, indicating no size benefit from extending the synthesis to 24 h. To track phase, a hybrid Rietveld refinement protocol was implemented to deconvolute peak overlap between the isomorphic phases. A 60-day aging study of the 18h40 condition demonstrated progressive oxidation, with the maghemite fraction increasing from 1.64 wt.% to 22.76 wt.%. VSM corroborated this degradation, showing a 9.1% reduction in saturation magnetization. A 60-day control study showed that sealed storage limited oxidation, with the particles containing only 0.9 wt.% maghemite. These findings support further investigation of the 18 h synthesis conditions for future core–shell studies. The localized Rietveld methodology provides a tool for monitoring magnetite-to-maghemite oxidation.