Sr0.75Ba0.25Fe12O19 hexagonal ferrite has attracted much attention due to its large (BH)MAX values and workability. We have prepared sheet magnets by the Dr. Blade method. To examine the effects of additives, such as SiO2, TiO2, Al2O3, and Cr2O3, on magnetic properties of sheet magnets, we used VSM magnetometer, x-ray diffraction, and Mössbauer spectrometer. The crystal structure is found to be a magnetoplumbite of typical M-type hexagonal ferrite, but the α-Fe2O3 phase develops with increasing additives concentration. Using our refined computer program, we have analyzed the Mössbauer spectra in the temperature range from 13 to 800 K. The Mössbauer spectra indicate that the line intensity for the 12k site is reduced with increasing SiO2 concentration, which is different from the reports of Fe-substituted Ba ferrite. This suggests that the developing α-Fe2O3 phase is related to 12k sites. The isomer shifts show the charge states of Fe ions is ferric. When the additives concentrations increase, the Curie temperatures, Tc go down. One sextet for α-Fe2O3 phase still persists above Tc, so it suggests that the high-Tc values do not result from α-Fe2O3. While Al2O3 and Cr2O3 additives increase coercive force Hc, the other additives reduce it.

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