Structural elucidation and dielectric behavior evaluation of sol-gel synthesized Co–Al co-substituted M-type hexaferrite materials

In this study, the sol-gel method was employed for the preparation of M-type hexaferrites Sr1-xAlxFe12-yCoyO19. Annealing of these samples was done at 1200 °C for 4 h. The single phase structure of all the prepared samples of M-type hexaferrites was observed by the X-ray diffraction (XRD) plots. The changes were examined in lattice parameters (a and c), cell volume, as well as in c/a ratio. The variations in c/a ratio were attributed to the changes in lattice parameters. The volume of the cell was found within the range 676.822–667.036 Å3. Debye Scherrer's formula was used for the measurement of crystallite size, which was found in the range of 26.026 to 22.634 nm. The variations were also perceived in bulk density (δb) and porosity (%) with the incorporation of Al–Co concentration. There was reduction in X-ray density (δx), due to the increment in substitution concentration. Fourier transform infrared spectroscopy (FTIR) was performed in the range 400–3000 cm−1, which displayed specific absorption bands regarding hexagonal ferrites. Dielectric parameters including dielectric constant, dielectric loss, tan loss, and AC-conductivity revealed decreasing behavior, due to Al–Co substitution. The slope “n” was measured from graph of log(σac) as a function of log(ω), and found within the range 0.697–0.972. The grain boundary contribution of all the synthesized samples was evidenced via Cole-Cole plots, and extraordinary contribution of grain boundaries in conduction phenomenon was established by the presence of only one semi-circle in Cole-Cole plots. Therefore, the smaller dielectric parameters of the investigated M-type hexaferrites recommended their feasible role in large frequency applications (e.g., dielectric resonators, microwave absorbers, phase shifters, and stealth technology).