Hydrothermal hexagonal SrFe12O19 ferrite powders: Phase composition, microstructure and acid washing

A series of hexagonal m-type SrFe12O19 ferrite powders were hydrothermally synthesized, and their phase composition, microstructure and magnetic properties before/after acid washing were studied. In the synthesis of these specimens, the atomic ratio of Fe/Sr (RF/S) in starting materials was set to 4, 5 and 12, respectively. When RF/S = 12, the specimen has morphology of round flat cakes, not typical hexagonal plate-like structure. The results of SEM images and XRD patterns indicate that the specimen with RF/S = 12 was mostly composed of Fe2O3. When RF/S = 4 or 5, the hexagonal plate-like SrFe12O19 ferrite powders were successfully synthesized with only a small quantity of Fe2O3 and SrCO3 impurities. It is also found that acid washing can eliminate the impurities in as-synthesized specimens effectively, and also change their topography, which enhances the saturation magnetization. However, the coercivity changed irregularly after acid washing, which is ascribed to the combination of the changed morphology, introduced stress and lattice defects.