Evidence of existence of metastable SrFe12O19 nanoparticles

► SrFe12O19, as the majority phase, is obtained by heat treatment at 400 °C under oxygen flux. ► Hexaferrite nanoparticles have crystalline imperfections that make it an unstable phase. ► Together with these phase transformations, an increment in the amount of SrCO3 is detected. ► To our knowledge, this is the first report of the existence of metastable SrFe12O19.