Influence of Mg substitution on structural, magnetic and dielectric properties of X-type barium–zinc hexaferrites Ba2Zn2-xMgxFe28O46

Mg substituted X–type hexagonal ferrites with the chemical composition Ba2Zn2-xMgxFe28O46 (x = 0.0, 0.4, 0.8, 1.2, 1.6 and 2.0) were successfully synthesised by a sol–gel auto–combustion technique, in order to investigate the effect of Mg substitution on structural, magnetic and dielectric properties. XRD analysis of prepared samples revealed the formation of pure X–type phase. The variations (decreasing trend) in lattice parameters with Mg substitution indicate the incorporation of Mg substitution into the crystal structure. The average crystallite size of heated powders was found to be in the range of 16–22 nm. For the first time, the RT hysteresis loops of Ba2Zn2Fe28O46 (Zn2X), Ba2Mg2Fe28O46 (Mg2X) and Ba2Zn2-xMgxFe28O46 (x = 0.0, 0.4, 0.8, 1.2, 1.6 and 2.0) were measured. The saturation magnetisation showed an initial decrease with x = 0.4 − 1.2, and then an increase with x > 1.2, to the largest final values of 63.29 A m2 kg−1 for the fully Mg–substituted x = 2.0. The value of coercivity lies in the range of 89.9–209.3 kA m−1 (1130–2630 Oe), and the magnetic results suggest that the compositions x = 0.0, 0.4 and 1.2 possess multi-domain microstructures, while x = 0.8, 1.6 and 2.0 possess single domain microstructures. The room temperature Mӧssbauer spectra were analysed with six sextets of five magnetic sublattices, and the results are presented as a function of Mg substitution. It was found that initial levels of Mg substitution reduce the Fe populations at a and b (spin up) sublattices, but that with x > 1.2 these become repopulated at fIV (spin down) site, resulting in an increase in magnetisation. Dielectric parameters such as dielectric constant and loss factor were studied as a function of frequency, and results show normal behaviour for ferrimagnetic materials. At low frequencies (100 Hz–2 MHz), relative permittivity was constant between 0.3 and 1.5 above 20 kHz, with the higher values belonging to the more dense samples. All Mg substituted samples had lower losses above 20 kHz than the pure Zn2X. In complex measurements at microwave frequencies (500 MHz–13.5 GHz), all samples had a real permittivity of around 7.5, except for the fully Mg–substituted sample (x = 2.0), which had a lower ε′ of 5.5. For two samples (x = 0.4 and 1.6) we observed dielectric resonances between 12.85 and 13.25 GHz. All showed a steady real permeability of around 1.2–1.5 over the whole 1–13 GHz range, and ferromagnetic resonance (FMR) between 1 and 5 GHz, and ∼12.5 GHz.