Comparative structural and magnetic study of Cr-substituted Sr-hexaferrites synthesized by sol–gel and solid-state routes

Magnets have become significantly important materials due to their high demand in modern technology. However, the use of rare earth elements (REEs) presents significant challenges, including high cost, limited availability, and environmental concerns. M-type hexaferrite (Ba/SrFe₁₉O₁₂) magnets have much smaller magnetic performance than REE magnets, but due to their low cost, they constitute the most produced type of magnets globally. Manufacture as nanostructures or metal-ion substitution in the ferrites constitutes state-of-the-art strategies to improve the magnetic properties of hexaferrites, as part of a shift toward developing cost-effective, rare-earth-free alternatives for permanent magnetic materials. In this work, a comparative study of the structural, morphological, and magnetic properties of Cr-doped Sr-hexaferrite prepared by two synthesis routes, sol–gel (SG) and solid-state methods (SSMs), is performed. The study primarily focuses on multiple aspects of fabricated oxides: Cr doping (SrFe_12-xCr_xO₁₉ with x=0.2, 0.4, 0.6) and the calcination temperature (1000 °C and 1100 °C). We have performed magnetisation studies that include hysteresis loops, Curie temperatures, and the anisotropy field. In general, magnetic studies indicate that the SSM route enables us to obtain a single phase, whereas two magnetic phases are observed in the SG samples. Furthermore, the specific saturation magnetisation of the ferrites made by SG is smaller than that of the SSM oxides. X-ray diffraction analysis and electron dispersive X-ray spectroscopy are performed to investigate differences in the phase formation and composition. The studies demonstrate that the Cr-substitution and temperature calcination weakly affect the properties of the oxides. However, the Cr-substituted ferrites with x=0.6 exhibit the largest specific magnetisation, 70 Am²/kg, and the largest coercive field of 0.56 T, when calcined at 1100 °C and 1000 °C, respectively. The enhanced magnetic moment and coercivity in ferrites prepared by SSM make them a suitable candidate for future permanent magnets.