Single crystalline magnetite, maghemite, and hematite nanoparticles with rich coercivity

Abstract

One-pot synthesis of amorphous iron oxide nanoparticles with two different dimensions (<5 nm and 60 nm) has been achieved using the reverse micelle method, with <5 nm nanoparticles separated from the stable colloid by exploiting their magnetic behaviour. The transformation of the as-prepared amorphous powders into Fe₃O₄ and Fe₂O₃ phases (γ and α) is achieved by carrying out controlled annealing at elevated temperatures under different optimized conditions. The as-prepared samples resulting from micellar synthesis and the corresponding annealed ones are thoroughly characterized by powder X-ray diffraction, transmission electron microscopy (TEM), and by Raman and X-ray photoelectron spectroscopies. Expectedly, the magnetic characteristics of Fe₃O₄ and Fe₂O₃ phase (γ and α) nanoparticles are found to have strong dependence on their phase, dimension, and morphology. The coercivity of Fe₃O₄ and Fe₂O₃ (γ and α) nanoparticles is reasonably high, even though high resolution TEM studies bring out that these nanoparticles are single crystalline. This is in contrast with previous reports wherein poly-crystallinity of iron oxides nanoparticles has been regarded as a prerequisite for high coercivity.