M-type hexaferrites SrxBa1-xFe12O19 as new magnetic pigments: The role of cationic substitution on the magnetic and colour characteristics

Magnetic pigments composed of SrxBa1-xFe12O19 were prepared using a modified Pechini method. The influence of the calcination temperature and the composition of the solid solution on the structural, colorimetric, and magnetic properties were investigated. XRD patterns showed pigments crystallizing in a magnetoplumbite-type structure after calcination above 900 degrees C. The formation of alpha-Fe2O3 as a secondary phase was also observed in some compositions. Raman and Mossbauer spectroscopy indicated the presence of Fe in different coordination sites, which strongly influenced the magnetic and colorimetric properties of the pigments. A brown colour was obtained for all the magnetic pigments, with a higher saturation when calcination was performed at 900 degrees C. A meaningful increase in saturation was attained for the sample with a Sr0.50Ba0.50Fe12O19 composition, which was related to the short-range disorder and occupancy of Fe3+ cations at different sites. The addition of Sr2+ (up to x = 0.50) to the solid solution improved the intensity of the brown colour by increasing the a* and b* colour coordinates and decreasing L*. Magnetic measurements revealed saturation magnetization values of 56.8 emu g(-1) for BaFe12O19. Upon the addition of Sr2+, an increase in the ferromagnetic behaviour was observed for barium-containing pigments, which indicates the possibility of their use as magnetic pigments in high-density recording media or paints for construction.