Design and development of Ga-substituted Z-type hexaferrites for microwave absorber applications: Mössbauer, static and dynamic properties

Gallium substituted Z-type Sr3GaxCo2-xFe24O41 (x = 0.0–2.0 in steps of 0.4) hexaferrites were synthesised by the sol-gel auto-combustion process, and sintered at 1150 °C. The structural, morphology, magnetic, Mössbauer, dielectric and microwave absorption properties were examined. XRD results of x = 0.0, 0.4, 0.8, and 1.2 samples show the formation of a single Z-type hexagonal phase. The samples x = 1.6 and 2.0 show the formation of Z and M phases. Hysteresis loops analysis suggest that samples x < 1.6 possess a soft magnetic nature, while the samples x = 1.6 and 2.0 show a hard ferrite characteristics. All samples possess multi-domain microstructures. The composition x = 0.4 [maximum MS = 97.94 Am2kg−1] was fitted with seven sextets (Fe3+) and a paramagnetic doublet-A (Fe3+), while beyond x ≥ 0.8 two more doublets (Fe2+) were observed along with seven sextets in Mössbauer spectra. The maximum values of Fe2+ ions (1.26%) and relative area of paramagnetic doublets (1.91%) were observed for x = 1.6 composition, which is also responsible for the lowest value of MS (69.99 Am2kg−1) for this composition. The average hyperfine magnetic field was found to decrease, whereas average quadrupole splitting was found to increase, with Ga-substitution. The substitution of Ga ions enhanced permeability, dielectric constant, magnetic loss and dielectric loss, in a non-linear fashion. The reflection loss was maximum at lower frequencies for samples x = 0.0 and 0.8, and decreases with frequency. Sample x = 0.8 has maximum reflection loss of −12.44 dB at 8 GHz, a measured thickness of 3 mm, and a bandwidth of −10 dB at 1.18 GHz. The observed absorption has been discussed with the help of the input impedance matching mechanism and quarter wavelength mechanism. The observed coercivity in different samples also influenced microwave absorption which demonstrated potenial in microwave absorber applications.