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Samples of Ni0.6Zn0.3Co0.1NdxFe2-xO4 (0.00 ≤ x ≤ 0.10) ferrites were synthesized by the sol-gel auto-combustion method and microwave sintering technology. The effects of Nd substitution on the crystal structure, magnetic, and electrical properties have been investigated. The XRD results showed that the main phase of sintered samples was a standard spinel structure. When x > 0.02, the second phase appeared in the sintered samples, which led to the phenomenon that the lattice constant of the sample increased and then decreased. Overall, the saturation magnetization of the samples showed a downward trend, and the value of coercivity increased with the amount of substitution. In terms of electrical performance, the dielectric constant of the samples showed an overall increasing trend with the amount of substitution after a sharp drop in the low frequency band and tended to be stable in the high frequency band. At x = 0.02, the ferrite sample exhibited the lowest dielectric constant (e', 41.63), the highest dc resistivity (ρ, 4.973 × 106 Ω∙m), relatively small coercivity (Hc, 20.878 Oe) and relatively high saturation magnetization (Ms, 79.214 emu/g).

Ni06Zn0.3Co0.1YxFe2-xO4 ferrites (0 ≤ x ≤ 0.1) were fabricated by a sol–gel autocombustion method using Microwave Sintering Technology (MST) and Conventional Sintering Technology (CST). With increased Y3+ ions, the structure and electromagnetic performance of all MST samples were better than those of CST. In addition, the MST and CST samples formed a second phase (YFeO3) when x > 0.025 because the radius of the Y3+ ion is larger than that of the Fe3+ ion. Compared with the CST samples, the MST samples showed increased densification and average grain size. Meanwhile, excellent electromagnetic performance of MST was achieved when the amount of Y substitution was 0.025, with high saturation magnetization (Ms, 154.19 emu/g), the lowest coercivity (Hc, 20.21 Oe), the lowest dielectric constant (e′, 10.82@1 MHz), lower dielectric loss (tanδ, 0.0093@1 MHz) and the highest direct-current resistivity (ρ, 2.706 × 106 Ω⋅m). These excellent results also confirm the good prospects of microwave sintering, which can better meet the needs of modern electronic devices for high frequency and low loss.

HfO2 doped NiZnCo bulk ferrites, Ni0.5Zn0.3Co0.2Fe2O4 with different HfO2 dopant content (0 ≤ x ≤ 0.1 wt%), were fabricated by the solid states reaction method. We investigated the influence of HfO2 doping on structure and electromagnetic properties. The XRD pattern shows that as the doping of HfO2 increases, the lattice parameter decreases and the X-ray density increases. There is an extra peak in each sample due to generation of NiCo2O4 impurity phase. The SEM images demonstrate the grain size slightly decreases with increasing HfO2 content. As the amount of HfO2 doping increases, the saturation magnetization increases linearly and the coercivity decreases. The polarization of all ferrite samples is due to the fact that the electronic hopping frequency between Fe2+ and Fe3+ ions can't keep up with external applied frequency. As the amount of HfO2 doping increases, the dielectric constant increases when x