High-efficiency X-band electromagnetic wave absorption in Co–Al engineered Ba–Sr hexaferrites: role of cationic substitution and microstructural tuning

The growing demand for high-performance electromagnetic (EM) absorbers and interference (EMI) shielding materials has motivated the design of structurally tunable ferrites with balanced dielectric and magnetic losses. In this study, crystalline Ba_0.5Sr_0.5Y_1.0Fe_11-<i>x</i> (Co _<i>x</i>/2Al _<i>x</i>/2)O₁₉ (<i>x</i> = 0.00-1.00) hexaferrites were synthesized <i>via</i> a sol-gel auto-combustion route to investigate the correlation between multi-cation substitution, microstructure, and microwave absorption. Rietveld-refined X-ray diffraction confirmed single-phase M-type hexagonal symmetry (<i>P</i>6₃/<i>mmc</i>) with systematic lattice contraction from 693.53 ų (<i>x</i> = 0.0) to 671.27 ų (<i>x</i> = 1.0) due to smaller Co²⁺ and Al³⁺ ionic radii. Field emission scanning electron microscopy and energy-dispersive X-ray analyses revealed dense, homogeneous microstructures with uniform dopant distribution. Magnetic measurements showed a progressive softening with substitution saturation magnetization (<i>M</i> _s) decreased from 54.79 emu g^-1 (<i>x</i> = 0.0) to 41.80 emu g^-1 (<i>x</i> = 1.0), while coercivity (<i>H</i> _c) dropped from 1788.8 Oe to 77.7 Oe attributed to dilution of Fe³⁺-O-Fe³⁺ superexchange and reduced anisotropy constant (<i>K</i> ₁ ≈ 10⁵ to 10³ erg cm^-3). The optimized composition, <i>x</i> = 0.50, achieved outstanding microwave absorption with a minimum reflection loss (RL_min) of -47.58 dB at 9.27 GHz (6 mm thickness) and an effective absorption bandwidth of 0.609 GHz, confirming nearly complete EM attenuation. Dielectric and magnetic spectra revealed that balanced permittivity (<i>ε</i>', <i>ε</i>″) and permeability (<i>µ</i>', <i>µ</i>″) ensured impedance matching (<i>Z</i> _in ≈ <i>Z</i> ₀), while enhanced attenuation constant (<i>α</i>) and reduced eddy current loss promoted multi-mechanistic energy dissipation. These findings demonstrate that Co-Al co-substitution effectively tailors lattice strain, anisotropy, and interfacial polarization, providing a rational design strategy for high-efficiency microwave absorbers and EMI-shielding hexaferrites in the X-band region.