Characterization of Cobalt Co-Substituted M-Type Hexaferrites BaFe12−x−yCoxMyO19 with M = Sn, Zr, Hf

The study investigates Co2+/M4+ (Sn, Zr, Hf)-substituted M-type barium ferrites to understand phase formation, structural evolution and magnetic behavior. Ferrites with the general composition BaFe12−x−yCoxMyO19 were synthesized via sodium carbonate flux and analyzed using powder and single-crystal X-ray diffraction, wavelength dispersive X-ray spectroscopy, X-ray absorption spectroscopy and magnetic measurements. Structural analysis showed increasing lattice parameters with increasing degree of substitution, confirming incorporation of the substituting tetravalent metals. Differing maximum substitution levels were determined for the different systems, with wavelength dispersive X-ray spectroscopy providing the most reliable compositional data. A slight excess of the tetravalent metals Sn4+, Zr4+ and Hf4+ relative to Co2+ was frequently observed. X-ray absorption spectroscopy and wavelength dispersive X-ray spectroscopy analyses indicated negligible Fe2+ formation and no clear trends for formation of vacancies. Site occupancy analysis assigned tetravalent cations primarily to the Fe(4) site (4f2), with evidence that cobalt partially occupies the Fe(3) site (4f1). Magnetic measurements revealed decreasing saturation magnetization, remanence and coercivity at room temperature with increasing substitution level, while low-temperature measurements showed enhanced remanence and coercivity.