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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.

Ni0.6Zn0.4Co0.2Fe1.8O4 ferrites doped with x wt.% K0.5Na0.5NbO3 (0.00≤x≤1.00) were successfully prepared by solid-state reaction method. The lattice parameters a decreased and the diffraction peak of (311) shifted to higher angle with the increase of K0.5Na0.5NbO3 (KNN) content. The grain size D initially increased to 3.12 μm (x = 0.50) and then reduced to 2.66 μm (x=1.00). The study also showed the addition of KNN effectively improved magnetic and electrical properties of NiZnCo ferrites. The saturation magnetization Ms decreased from 60.59 to 46.11 emu/g and the coercivity Hc overall showed a decreasing trend from 84.64–67.00 Oe. The dielectric constant e´ of prepared samples increased when x≤0.75, then decreased when x>0.75, and all prepared samples had low loss tangent tanδ at high frequency. In addition, all samples exhibited high resistivity ρ and activation energy Eρ.

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

NiZnCe ferrite, Ni0.4Zn0.6−xCexFe2O4 (0 ≤ x ≤ 0.20), were prepared by the sol-gel self-spreading process combing with the solid reaction process. The crystal structure and physical properties of NiZnCe ferrite with different amount of Ce substitution have been studied. The lattice parameter and density rise and the grain size is decreasing with Ce substitution increasing because the ionic radius of Ce3+ ion is much bigger than that of Zn2+ ions, and the atomic weight of Ce3+ ions is heavier than Zn2+ ions. Ms initially increases with increasing amount of Ce substitution because of the increase of density and reduction of porosity, then it decreases when Ce substitution excesses 0.15 which contributes to the increase of the net magnetic moment in tetrahedral site and porosity and the apparence of CeO2 phase. Hc monotonously decreases with Ce substitution increasing due to the mono-domain structure of grains. The initial permeability initially increases at room temperature with the Ce substitution content increasing and has the maximum when the replacement content is 0.15. But it sharply drops when the amount of Ce replacement is 0.20 because the porosity increasing and impurity phase appearance. The Curie temperature has been greatly improved with Ce substitution due to the enhancements of A-B superexchange interactions. The dc resistivity gradually decreases at room temperature due to the fact that Ce substitution reduces the activation energy, and it obeys an exponential decay with the temperature increasing.

Zr-substituted NiZnCo ferrite nanopowders, Ni 0.4− x Zn 0.5 Zr x Co 0.1 Fe 2.0 O 4 (0 ≤ x ≤ 0.20), were synthesized by the sol-gel auto-combustion method. The effects of Zr substitution on the structural, magnetic and electrical properties have been investigated. The DTA and TG results indicate that there are three steps of combustion process. The X-ray diffraction patterns show that the lattice parameter and the average crystallite size increase with the increase of Zr substitution. The saturation magnetization increases with the increase of Zr substitution when x ≤ 0.05, and then decreases when x > 0.05. Meanwhile, the coercivity initially decreases with the increase of Zr substitution when x ≤ 0.05, and then increases when x > 0.05. The polarization behavior for all the samples in the test frequency range from100 Hz to10 MHz obeys the charge polarization mechanism, which happens since the frequency of the hopping of electron exchange between Fe 2+ and Fe 3+ ions are far from the frequency of alternating-current field. And the dielectric constant increases with the increase of Zr substitution. The relaxation peak of the frequency dependence of dielectric loss is observed for x ≥ 0.10, which is due to the frequency of charge hopping between the Fe 2+ and Fe 3+ exactly matches with the frequency of the external applied field. Electrical transport behavior of the ferrite nanopowders is found to follow the impurity semiconductor, and the effect of Zr substitution on the temperature dependence of dc resistivity is observed.

Co-substituted NiCu ferrite nanopowders with the chemical formula Ni0.5−xCu0.5−xCo2xFe2O4 (0 ≤ x ≤ 0.50) were synthesized by sol-gel auto-combustion method. The effects of Co substitution on the cation distribution, structural and magnetic properties of the NiCu ferrite nanopowders have been investigated. Differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM) measurements were used to characterize the chemical, structural and magnetic properties of the ferrite nanopowders, respectively. The DTA-TG results indicate that there are three steps of the combustion process. XRD results indicate that there are Fe2O3 and CuO impurity phases when x ≤ 0.10. Furthermore, the lattice parameter increases, and the X-ray density and the average crystallite size decrease with increasing Co substitution. And the obtained particle size from TEM image is in very good agreement with the average crystallite size estimated by XRD measurements. The saturation magnetization and coercivity monotonically increase with the increase of Co substitution. The increase of the saturation magnetization is due to the substitution of Ni2+ and Cu2+ ions with lower magnetic moment by Co2+ ions with higher magnetic moment on the octahedral sites. And the increase of the coercivity is mainly due to the increase of magnetocrystalline anisotropy energy.

Al-substituted NiZnCo ferrite, Ni0.4Zn0.5Co0.1AlxFe2−xO4 (0 ≤ x ≤ 0.20), were synthesized by the sol–gel auto-combustion method. The effects of Al substitution on the structural, magnetic and electrical properties have been investigated. The X-ray diffraction patterns show that the lattice parameter decreases with the increase of Al substitution. The SEM images show that the grain size decreases with the increase of Al substitution. The initial magnetic permeability reaches the maximum value when Al substitution is 0.05. The resonance frequency is inversely proportional to initial magnetic permeability which obeys Snoek's law. The temperature coefficient reaches the maximum value when Al substitution is 0.05, and it has the minimum value when Al substitution is 0.20. The Curie temperature decreases with the increase of Al temperature due to the number of nonmagnetic Al3+ ions increase results in the decrease of superexchange interaction. The saturation magnetization and coercivity of the ferrite decrease linearly with the increase of Al substitution. The real dielectric constant of Ni0.4Zn0.5Co0.1AlxFe2−xO4 ferrite decreases exponentially as frequency increases, and the dielectric constants decrease with the increase of Al substitution. The direct-current resistivity decreases and follows an exponential decay with the increase of temperature, and it increases with Al substitution.

Zn-substituted Cu ferrite nanopowders, Cu1−xZnxFe2O4 (0≤x≤1.0), were synthesized by the sol–gel auto-combustion method. The effect of Zn substitution on the structural, magnetic and electrical properties have been investigated. The DTA and TG results indicate that there are three steps of combustion process. The X-ray diffraction patterns show that there are Fe2O3, CuO and CuFeO2 secondary phases and tetragonal structure when x=0.0. Furthermore, the lattice parameter and the average crystallite size decrease, and the X-ray density increases with the increase of Zn substitution. The saturation magnetization increases with the increase of Zn substitution when x≤0.40, and decreases when x>0.40. Meanwhile, the coercivity monotonically decreases with the increase of Zn substitution. The polarization behavior for all the samples in the test frequency range of 100 Hz to 10 MHz obeys the charge polarization mechanism. Electrical transport behavior of the ferrites nanopowders is same with the impurity semiconductor, and the effect of Zn substitution on the temperature dependence of dc resistivity is observed.

In this work, we investigate the dynamic magnetic properties and their thermostability in the temperature range of 300 to 420 K for FeSiAl–SiO2 thin films fabricated by hybrid oblique co-sputtering technique systematically. The magnetic anisotropy field HK and ferromagnetic resonance frequency fFMR can be tuned from 14.66 to 85.32 Oe and 1.13 to 2.62 GHz respectively, by adjusting the oblique angle β, which can be elucidated with respect to the contribution of reinforced shape and stress-induced anisotropies. Moreover, the thermostability of FeSiAl–SiO2 films in terms of HK and fFMR are enhanced with the increase of β up to 35° while the thermostability of effective Gilbert damping factor αeff and the maximum imaginary permeability μ″max are enhanced with the increasing of β up to 45° and the involved mechanisms are analyzed in detail.

Electrical conduction and polarization behaviors of the Sr 1− x La x Fe 12− x Co x O 19 ( x = 0–0.3) hexaferrites for use in low temperature co-fired ceramics (LTCC) technology were investigated according to their temperature dependence of resistivity and complex dielectric permittivity, as well as the polarization versus electric field ( P - E ) curves. The direct-current (DC) resistivity from room temperature (RT) to 750 °C is mainly dominated by metallic-conduction mechanism and semi-conduction mechanism, which can be responsible for the single metal-semiconductor (M − S) transition behavior. But the double M − S transitions and conduction relaxation are observed with alternating current (AC) measurement in a frequency range of 1 kHz–10 MHz. These abnormal electrical transport behaviors are suggested to be attributed to the coexistence of conduction electrons with different activation energy. The multi particle polarization behavior is obtained in the ferrites from the variation of real permittivity and dielectric loss with temperature, and the space charge polarization mechanism is found to determine the non-linear relationship of P - E . Furthermore, the La 3+ -Co 2+ substitution exhibits significant effects on the electrical transport and dielectric characteristics for the ferrites. Typically, when the substitution amount increases from 0 to 0.3, the activation energy E a increases from 0.331 to 1.068 eV, the RT resistivity ρ d increases from 2.2 × 10 9 to 3.7 × 10 9 Ω cm, and the average value of real permittivity e ′ (RT-100 °C) measured at 1 kHz decreases from 4.15 to 2.29.

The Sr 1− x La x Fe 12− x Cu x O 19 ( x =0–0.5) ferrites for use in low temperature co-fired ceramics (LTCC) technology were prepared by improved solid phase method. Dependence of their crystal structure, magnetic, electrical, and dielectric properties on La–Cu substitution amount were investigated. Pure M-type phase is obtained for the ferrites with La–Cu substitution amount x ≤0.3. With x further increasing to 0.5, the multiphase structure is formed, where the M-type phase coexists with the LaFeO 3 phase and Sr 2 FeO 4− x phase. In single M-type phase region with x ≤0.3, the varied magnetic properties can be well explained considering the occupancy effects of La 3+ and Cu 2+ ions in magnetoplumbite structure. Their electrical transport behavior is found to be correlated with La–Cu substitution amount. Single metal–semiconductor (M–S) transition is clearly observed in the ferrites with a high doping amount as x =0.3. The polarization behavior from 1 kHz to 10 MHz follows the charge polarization mechanism, and the temperature dependence of real permittivity ( e ′– T curves) and dielectric loss (tg δ – T curves) strongly suggests the complicated multiparticle polarization and relaxation.

The La3+–Co2+ substituted Sr0.7La0.3Fe11.7Co0.3O19 ferrites with 2–5 wt% Bi2O3·B2O3·SiO2·ZnO (BBSZ) glass were synthesized by conventional sintering (CS) method, microwave sintering (MS) method, and hot press sintering (HPS) method at a low sintering temperature of 850 °C compatible with the low temperature co-fired ceramics (LTCC) systems, respectively. These ferrites were chiefly characterized by X-ray diffraction, precision density balance, and vibrating sample magnetometer. It is observed that the addition of BBSZ can promote the low temperature sintering of the ferrites, as well as the formation of M-type phase. But the excessive addition of non-magnetic BBSZ is inclined to reduce the magnetic properties for the ferrites. Detailed effects of the BBSZ amount on their low temperature sintering characteristics are revealed and explained. The proper addition of BBSZ is suggested to be 4, 4, and 5 wt% for the CS ferrites, MS ferrites, and HPS ferrites, respectively. Notably, the obtained MS ferrites can provide relatively high saturation magnetization of 65.3 emu/g, magnetic anisotropy field of 1528.8 kA/m, and intrinsic coercivity of 336.1 kA/m as low temperature sintered M-type hexaferrites for use in microwave LTCC circulators.

Temperature dependence of resistivity from room temperature to 750 °C at different frequency for the low temperature sintered Sr 1− x La x Fe 12− x Co x O 19 ( x =0–0.3) ferrites was investigated, and a theory model based on a hybrid grain structure was established to explain the observed complex electrical transport behavior. It indicates that the formation of surface phase associated with non-uniform La 3+ –Co 2+ substitution is crucial to the hybrid grain structure, which consists of body phase, surface phase, and interface. The abnormal double metal-semiconductor (M–S) transitions in La 3+ –Co 2+ substituted ferrites are suggested to arise from the competition of lattice vibration scattering effect and thermal excitation of different conduction electrons strongly correlated with the hopping of electrons between Fe 2+ and Fe 3+ at octahedral sites in body phase and surface phase.

The La 3+ and Co 2+ substituted Sr 1− x La x Fe 12− x Co x O 19 ( x = 0–0.5) ferrites with Bi 2 O 3 sintering aid were prepared by microwave sintering method at a low sintering temperature of 870 °C compatible with low temperature co-fired ceramics (LTCC) systems. Detailed effect of La–Co substitution amount on their crystal structure and magnetic properties were mainly investigated. The results show that the pure M-type crystal phase is successfully obtained for the ferrites with La–Co substitution amount x not higher than 0.3. With further increased substitution amount, the multiphase structure is formed, where the non-magnetic α-Fe 2 O 3 phase and LaCoO 3 phase coexist with the M-type phase. Appropriate La–Co substitution improves the magnetic properties of the ferrites, including the saturation magnetization M s , effective magnetic anisotropy constant K eff , magnetic anisotropy field H a , and intrinsic coercivity H ci . Besides, the Curie temperature T C of the ferrites exhibits a rapid decrease when the La–Co substitution amount exceeds 0.3. It is suggested that the La 3+ –Co 2+ ions can partially substitute the Sr 2+ –Fe 3+ ions for the ferrites, leading to enhanced magnetic properties. Typically, the obtained Sr 1− x La x Fe 12− x Co x O 19 ( x = 0.2–0.3) ferrites can provide improved M s above 64 emu/g, H a above 1510 kA/m, and H ci above 330 kA/m as low temperature sintered M-type hexaferrites, which are promising for use in future design and realisation of self-biased circulators in LTCC systems.

The La–Co substituted Sr 1− x La x Fe 12− x Co x O 19 ( x =0–0.5) ferrites with appropriate Bi 2 O 3 additive were prepared at a low sintering temperature of 890 °C compatible with LTCC (low temperature co-fired ceramics) systems, and the effect of La–Co substitution on their crystal structure and magnetic properties was investigated. The results show that the pure M-type phase is successfully obtained when the La–Co substitution amount x does not exceed 0.3. However, the single M-type phase structure transforms to multiphase structure with further increased x , where the α-Fe 2 O 3 phase and La 2 O 3 phase coexist with the M-type phase. Moreover, the saturation magnetization M s , magnetic anisotropy field H a , intrinsic coercivity H ci , and Curie temperature T C of the ferrites depend on the La–Co substitution amount strongly, which are suggested to be determined by the partially substitution of La 3+ –Co 2+ ions for Sr 2+ –Fe 3+ ions with x not higher than 0.3. It is found that the obtained Sr 1− x La x Fe 12− x Co x O 19 ( x =0.2 and 0.3) ferrites can provide improved magnetic properties ( M s >62 emu/g, H a >1400 kA/m, and H ci >320 kA/m) as low temperature sintered M-type hexaferrites for microwave LTCC applications.

The La3+ and Co2+ substituted Sr1−xLaxFe12−xCoxO19 (x=0−0.5) ferrites with Bi2O3 additive were prepared by hot press sintering method at a low sintering temperature of 870 °C compatible with LTCC (low temperature co-fired ceramics) systems, and their crystal structure and magnetic properties were mainly investigated. The results show that the pure M-type crystal phase is successfully obtained for the ferrites with La–Co substitution amount x not higher than 0.3. When the substitution amount further increases to 0.4 and 0.5, the α-Fe2O3 phase and La2O3 phase are found to coexist with the M-type phase, thus the multiphase structure is formed. Besides, the saturation magnetization Ms, intrinsic coercivity Hci, and Curie temperature TC of the ferrites are strongly correlated with the La–Co substitution amount. Enhanced Ms and Hci are obtained when the substitution amount varies from 0.2 to 0.4, which can exceed 3600 Gs and 4000 Oe, respectively. However, the TC decreases rapidly when the substitution amount exceeds 0.3, and a value under 410 °C is observed. It is suggested that the La3+–Co2+ ions can partially substitute the Sr2+–Fe3+ ions for the hot press sintered ferrites at low sintering temperatures, leading to improved Ms and Hci for use in microwave LTCC ferrite devices.

Cr-substituted NiZnCo ferrite nanopowders, Ni 0.5− x Zn 0.5 Cr x Co 0.1 Fe 1.9 O 4 (0≤ x ≤0.20), were synthesized by sol–gel auto-combustion method. The effect of Cr substitution on the structural and magnetic properties have been investigated. Differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD), Fourier transform infrared spectro-scopy (FT-IR) and vibrating sample magnetometer (VSM) measurements were used to characterize chemical, structural and magnetic properties. The DTA-TG results indicate that there are three steps of combustion process. FT-IR spectra confirm the formation of spinel structure during the self-propagation combustion. XRD results indicate that the lattice parameter and the X-ray density decrease, and the average crystallite size increase with increasing Cr substitution. And Fe 2 O 3 secondary impurity phase formed with excess Cr substitution. The saturation magnetization increases with the increase of Cr substitution when x ≤0.05, and decreased when x >0.05. Meanwhile, the coercivity monotonically decreases with the increase of Cr substitution.

Zr-substituted NiZn ferrite thin films, Ni 0.5 Zn 0.5 Zr x Fe 2− x O 4 (0 ⩽ x ⩽ 0.2), were synthesized by sol–gel process. The structure and magnetic properties of Ni 0.5 Zn 0.5 Zr x Fe 2− x O 4 ferrite thin films have been investigated. The diffraction peak shifted towards the lower angle and the lattice parameter increased with Zr substitution. The average grain size of Zr-substituted NiZn ferrtie thin films increased with Zr substitution. The saturation magnetization gradually increased with the increase of Zr substitution when x ⩽ 0.10, and decreased when x > 0.10. Meanwhile, the coercivity initially decreased with the increase of Zr substitution when x ⩽ 0.10, and increased when x > 0.10.

Strontium ferrites with different Bi2O3 content are prepared by the solid phase method, and their magnetic properties are investigated primarily. The Bi2O3 additive and sintering temperature separately exhibit a strong effect on the sintering density, crystal structure, and magnetic properties of the ferrites. As to the ferrites with 3 wt% Bi2O3, the relatively high sintering density ρs, saturation magnetization Ms, and intrinsic coercivity Hci can be obtained at a low sintering temperature of 900°C even much lower. Furthermore, the effective magnetic anisotropy constant Keff and magnetic anisotropy field Ha of the ferrites are calculated from the magnetization curve by the law of approach to saturation. It is suggested that the low-temperature sintered SrFe12O19 ferrites with Ms of 285.6 kA/m and Ha of 1564.6 kA/m possess a significant potentiality for applying in the self-biased low-temperature co-fired ceramics circulators from 34 to 40 GHz.