A comparative study of the correlation among the phase formation, crystal stability and magnetic properties of SrFe12-xMxO19 (M=Al3+, Cr3+ and Mn3+, x=0–0.5) ferrite permanent magnets

Abstract

The aim of this study is to enhance the magnetic and thermal properties of strontium hexaferrites by substituted with Al, Cr, and Mn ions instead of Fe, and to elucidate the changes in their structural, morphological, and thermal characteristics, in detail. SrFe12-xMxO19 (M: Al, Cr, Mn; x = 0.1, 0.3, and 0.5) powders were produced by the mechanochemical synthesis method from mill scale, which is a solid waste in steel production. While the Al3+ substitution did not cause any change in the phase structure, the Cr3+ and Mn3+ caused the formation of nanosized α-Fe2O3 and MnδFe2-δO4 phases in the structure, respectively. Increasing the Al substitution ratio led to a decrease in crystal size, from 176 nm to 32 nm. Similarly, the substitution of Cr resulted in a decrease in crystal size to 78 nm. However, Mn substitution had the opposite effect, increasing the crystal size of the powders. Al3+, Cr3+, and Mn3+ substitutions led to various effects on magnetic properties due to their placement in different lattice sites in the SrFe12O19 hexagonal crystal. The residual or remanent magnetization (Mr) and saturation magnetization (Ms) were lower than the initial values of Mr = 41.76 and Ms = 69.94 emu/g depending on the type and ratio of substitution ions. The highest coercivity was observed as 7116 Oe for SrFe11.5Al0.5O19. The Curie temperature increased to 515 °C with Cr3+ substitution but decreased to 431 °C and 426 °C with Al3+ and Mn3+, respectively.