Al3+ substituted U-type hexaferrites Ba4Co2Fe36−xAlxO60: structural, magnetic, electrical and dielectric properties†

Polycrystalline samples of Al³⁺-substituted barium–cobalt U-type hexagonal ferrites, with the chemical composition Ba₄Co₂Fe_36−xAl_xO₆₀ (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), were synthesised using a citrate gel auto-combustion route and subsequently heated at 1300 °C for 5 h. To study the influence of Al³⁺ substitution on structural, magnetic, and dielectric characteristics, FTIR, XRD, SEM, EDX, M–H loops, Mössbauer spectroscopy and low frequency (up to 2 MHz) dielectric measurements were performed. Biocompatibility evaluation was also carried out with 3T3 fibroblast cells, and antioxidant capacity and antimicrobial activity were assessed. Agglomerated grains with different surface morphologies were seen in SEM images. EDX examination of all compositions showed the existence of Ba, Fe, Co and Al ions. Saturation magnetisation (M_S) varied from 32.3 to 52.2 A m² kg⁻¹. A squareness ratio (M_r/M_S) of < 0.5 was obtained, signifying that all the samples have multi-domain structures. However, all samples were magnetically soft ferrites, with H_C kA m⁻¹ found to vary from 62.1 Oe to 78.6 Oe (4.94  kA m⁻¹ to 6.26 kA m⁻¹). Mössbauer spectra were fitted with five sextets of five magnetic sublattices, and the results were obtained with a variation in Al content at room temperature. The composition x = 0.4 showed the highest value of relative area (12k site), whereas x = 1.0 showed the minimum value at the 12k site, indicating that Al³⁺ begins to replace Fe³⁺ in the 12k site. For compositions x = 0.2 and 0.4, Al³⁺ begins to replace Fe³⁺ in the 4f₂ site, leading to an increase in the electron density in this site, but this electron density is reduced with further Al³⁺ substitution because as the number of Fe vacancies increases, Al³⁺ favours the 12k site. This results in a non-linear variation in magnetic properties with increasing aluminium substitution. Dielectric parameters such as dielectric constant, dielectric loss tangent, and AC conductivity were analysed as a function of frequency (10 Hz–2 MHz), and analysis results illustrate the typical behaviour of ferrimagnetic materials. The Cole–Cole type plot showed one semi-circle arc for all samples. Simulated impedance plots obtained through electrochemical impedance spectroscopy (EIS) software complied with the measured impedance of the samples and revealed a variation in simulated values of grain and grain boundary parameters, which was in agreement with SEM images. EIS generated simulated impedance plots that agreed with the measured impedance of the samples. The disclosed deviation in the simulated values of grain and grain boundary parameters was consistent with SEM micrographs. The grain morphology influenced the electrical parameters of ferrite samples, and dielectric relaxation was found to be prevalent in the samples.