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

Abstract

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−x(Co_x/2Al_x/2)O₁₉ (x = 0.00–1.00) hexaferrites were synthesized via 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 (P6₃/mmc) with systematic lattice contraction from 693.53 ų (x = 0.0) to 671.27 ų (x = 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 (M_s) decreased from 54.79 emu g⁻¹ (x = 0.0) to 41.80 emu g⁻¹ (x = 1.0), while coercivity (H_c) dropped from 1788.8 Oe to 77.7 Oe attributed to dilution of Fe³⁺–O–Fe³⁺ superexchange and reduced anisotropy constant (K₁ ≈ 10⁵ to 10³ erg cm⁻³). The optimized composition, x = 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 (ε′, ε″) and permeability (µ′, µ″) ensured impedance matching (Z_in ≈ Z₀), while enhanced attenuation constant (α) 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.