Influence of Diamagnetic Mg²⁺ Ion Substitution on Crystal Structure, Optical Properties, Magnetic Features, and Magnetocaloric Effect in M-Type Ba–Ca Hexaferrites

In this study, we investigate the impact of diamagnetic Mg substitution on the crystal, magnetic and magnetocaloric effect (MCE) properties of the novel M-type Ba_0.5Ca_0.5Fe_12-xMg_xO₁₉ (x = 0.3, 0.4 and 0.5) BCFMO hexaferrites. The three series were synthetically prepared using the conventional solid-state process. X-ray diffraction (XRD) combined with Rietveld refinement, confirm the magnetoplumbite structure, alongside a secondary phase with space group R-3 m. The hexaplate-like grain sizes varied in the range of 2.37–1.9 µm, while the crystallite sizes are in the range of 119–104 nm. The FTIR spectra revealed two positions at 573 cm⁻¹ and 494 cm⁻¹ corresponding to the tetrahedral and octahedral sites, respectively, which confirmed the M-type hexagonal structure. The direct optical band gap energy (E_g) of BCFMO increases from 2.91 eV to 3.14 eV with varying Mg²⁺ concentration from 0.3 to 0.5. The presence of Fe in the + 3 oxidation state (Fe³⁺) and of Mg²⁺ is confirmed by XPS. The saturation magnetization M_s decreases from 111.18 to 72.34 emu/g which attributed to the addition of Mg²⁺ in Fe³⁺ in 12 k octahedral site and confirmed by DRX and XPS analyses. The increase of the coercivity H_C from 3.17 kOe to 5.81 kOe is due to the reduction in crystallite sizes. The anisotropy field H_a and the effective anisotropy constant K_eff rise from 12.13 to 21.78 kOe and 6.74 to 7.88 (10⁵emu/cm) for x = 0.3 to 0.5, respectively. The Para-Ferromagnetic phase transition at Curie Temperature T_C was identified for all compounds. The substitution by Mg²⁺ leads to a reduction of T_C from 730 to 693 K. The maximum entropy (− ΔS_m^max) decreases from 1.68 to 1.29 J/kg.K, while, the relative cooling power (RCP^max) increases from 110.58 J/kg to 217.9 J/kg for x = 0.3 to 0.5, respectively at µ₀H = 5 T. The results demonstrated that samples could be utilized for optoelectronic devices, permanent magnet, magnetic recording media and other magnetic applications.