Hydrothermal synthesis of SrFe12O19 nanoparticles: effect of the choice of base and base concentration

Platelet shaped strontium hexaferrite (SrFe12O19) crystallites were hydrothermally synthesized in an autoclave to study the effect of (a) Fe/Sr molar ratio, (b) choice of base NaOH/KOH and (c) base concentration. The influence of these parameters on the final product is evaluated with regards to phase composition, structure and magnetic properties. Rietveld refinements were performed on powder x-ray diffraction (PXRD) data to determine the phase composition, structural changes, crystallite sizes, and preferred orientation, while the magnetic properties were measured using a vibrating sample magnetometer. When NaOH is used as the base, the samples consist mostly (>95 wt.%) of SrFe12O19 up to the same molar ratio of Fe/Sr = 8, independent of the concentration of the base. In contrast, when using KOH, the phase composition depends on both the molar ratio of Fe/Sr and the concentration of KOH. High concentrations of Sr2+ and OH- (Fe/Sr = 1 and OH-/NO3- = 4) result in the growth of wide crystallites (>400 nm). The thickness of the crystallites are in all cases around 40 nm causing the crystallites to have an anisotropic shape, which can align without applying an external magnetic field. In the case of KOH as base instead of NaOH, an expansion of the unit cell is observed, which can be attributed to K+ substituting Sr2+ in the structure. This is corroborated by increasing microstrain when increasing the KOH/NO3- ratio. Variations in the observed coercivity may be attributed to substitution of Sr2+ by K+. The present study illustrates that meticulous control of all reaction parameters and a meticulous analysis of the crystal structure is key for preparing and understanding hard-magnetic SrFe12O19.