Investigation of structural, magnetic and dielectric properties of gallium substituted Z-type Sr3Co2-xGaxFe24O41 hexaferrites for microwave absorbers

Gallium substituted Z-type hexagonal ferrites with chemical composition Sr3Co2-xGaxFe24O41 (x = 0.0,0.4, 0.8, 1.2, 1.6, and 2.0) were successfully synthesised in air at 1200 degrees C for 5 h using the sol-gel autocombustion technique, in order to investigate the effect of gallium substitution on structural, magnetic and dielectric properties. X-ray Diffraction (XRD) analysis of all samples reveals the formation of mixed hexaferrite phases, with Z ferrite as the major phase (72-90%).The average crystallite size of heated powders was found to be in the range of 21-40 nm. The saturation magnetisation decreases after gallium substitution, with the lowest values of 64 Am-2 kg(-1) for composition x = 1.6, which also hasthe highest value of coercivity (28.3 kA m(-1)). Nevertheless, all were soft ferrites, with H-c between 3.4 and 28.3 kA m(-1).The M-r/M-S ratio of all samples was found to be less than 0.5, suggesting that all the compositions possess multi-domain microstructures. Mossbauer spectroscopic analysis confirmed that the Fe ions were found in the 3 + high spin state for compositions below x <= 0.4, whereas similar to 1.5% of the Fe ions were converted into Fe2+ high spin state beyond x >= 0.8 compositions, as Ga3+ began to substitute for Fe3+, forming Fe2+ in the cobalt positions. The average hyperfine magnetic field (<H-hf>) was found to be decreased with Ga-substitution. Dielectric parameters such as dielectric constant and loss factor were studied as a function of frequency, and their results show normal behaviour for ferrimagnetic materials. In complex measurements at microwave frequencies (8 GHz-12.5 GHz, the X-band), all samples had a real permittivity of around 8-14. For sample x = 2.0, a dielectric resonance peak was observed around 12.15 GHz. All showed a real permeability of around 1.0-1.4 over the frequency of 8 GHz-12.5 GHz range, and ferromagnetic resonance (FMR) was observed in x = 0.0 and 2.0 samples, at around 11 and 12 GHz, respectively. This suggests that the prepared samples can be used as microwave absorbers/EMI shielding at specific microwave frequencies. The co-existence of FMR and dielectric resonance at the same frequency of 12.15 GHz for x = 2.0 could lead to the coupling of these resonances and the development of potential metamaterials. (C) 2019 Elsevier B.V. All rights reserved.