Unravelling the structure–magnetism coupling in Al-substituted BaSrCa (1:1:2)-based ferrite nanoparticles

Ba[Formula: see text]Sr[Formula: see text]Ca[Formula: see text]Fe[Formula: see text]Al x O[Formula: see text] ([Formula: see text]–3.0) M-type hexaferrites were synthesized by the sol–gel autocombustion technique to investigate the effect of Al substitution on structural and magnetic behavior. X-ray diffraction confirmed a single-phase hexagonal structure (P63/mmc, JCPDS No. 84-0757) with lattice parameters decreasing slightly from [Formula: see text] [Formula: see text]Å, [Formula: see text] [Formula: see text]Å to [Formula: see text] [Formula: see text]Å, [Formula: see text] [Formula: see text]Å, indicating lattice contraction with Al[Formula: see text] substitution. The crystallite size reduced from 62.22[Formula: see text]nm to 49.95[Formula: see text]nm (Scherrer method) and from 73.13[Formula: see text]nm to 58.31[Formula: see text]nm (Williamson–Hall method), whereas microstrain and porosity improved from [Formula: see text] to [Formula: see text] and from 11.70% to 14.11%, respectively. SEM micrographs revealed hexagonal platelet-like grains with sizes lessening from 300[Formula: see text]nm to 450[Formula: see text]nm ([Formula: see text]) to 150–250[Formula: see text]nm ([Formula: see text]), confirming grain growth suppression by Al[Formula: see text]. Magnetic characterization exhibited a decrease in saturation magnetization (Ms) from 36.98[Formula: see text]emu/g to 30.41[Formula: see text]emu/g and an increase in coercivity (Hc) from 3.112[Formula: see text]kOe to 3.827[Formula: see text]kOe up to [Formula: see text], followed by a minor decline at [Formula: see text]. Curie temperature also decreased with increasing Al doping concentration due to the dilution of magnetic Fe[Formula: see text] ions. These results demonstrate that moderate Al substitution improves coercivity and anisotropy while sustaining structural stability, making the composition a promising contender for high-coercivity and microwave device applications.