Influence of Co⁴⁺-Ca²⁺ substitution on structural, microstructure, magnetic, electrical and impedance characteristics of M-type barium–strontium hexagonal ferrites

Co4+-Ca2+ substituted M-type barium strontium hexagonal ferrites with chemical composition Ba0.25Sr0.75CoxCaxFe12-2xO19 (x = 0.0, 0.2, 0.4, 0.8, 1.2, 1.6 and 2.0) were prepared by sol-gel auto combustion technique and sintered at 1150 °C for 5 h. The structural, magnetic, electrical and impedance properties of prepared samples were characterized using FTIR, XRD, SEM, VSM, Mössbauer spectroscopy and impedance spectroscopy. The FTIR spectra displayed two absorption bands in a wave-number range from 600 to 550 cm−1 and 450 to 400 cm−1 that confirm the formation of hexaferrite. XRD analysis of x = 0.0 and x = 0.2 compositions show the formation of the majority of hexaferrite M-phase, while other samples show M-phase with other phases in the prepared compositions. The substitution of Co–Ca reduced the coercivity (HC) from 3135.45 Oe (x = 0.0) to 48.28 Oe (x = 0.8) and saturation magnetization (MS) decreased from 65.97 emu/g (x = 0.0) to 42.24 emu/g (x = 2.0). Mössbauer spectroscopic analysis showed that each sublattice has five sextets that can be attributed to the Fe+3 ions in the high spin state and few Fe3+ ions were converted into Fe2+ ions for compositions x ≥ 0.8. Single semicircle curves were observed in Nyquist plots, attributed to the contributions of grain boundaries. Impedance and Nyquist plots depicted non-Debye relaxation in the compositions. AC conductivity of all samples increases with the increase in frequency. The microstructure accompanied by grain and grain boundaries influenced the electrical and impedance properties.