Chapter 1 Introduction
1.5 Currently used spinel ferrites for microwave applications
follows: (Fe3+)[ Ni2+Fe3+]O4. In mixed spinel, different ions of mixed valance state occupy both A and B site.[80] The schematic representation of normal and inverse spinel is illustrated in Figure 1-11.
Figure 1-11. The schematic illustration of unit cell structure of (a) Normal, (b) Inverse spinel.
Figure 1-12. The number of articles published in the last few years on spinel ferrites for microwave applications.
As the spinel structure comes under cubic structure, a relatively small effective magnetocrystalline anisotropy energy is observed that corresponds to a low magnetic anisotropy field (HA). Since the ferromagnetic resonance frequency (FMR) is strongly dependent on HA, spinel ferrites' zero field FMR frequency falls in lower GHz frequency, typically in the range of X-band, Ku band, or lower.
As the FMR frequency range largely determines the operating frequency range, the spinel ferrites are mainly used in the frequency range of Ku, X, S, and C - bands.[86] Ji et.
al.[87] performed FDTD analysis of Y-junction microstrip circulator with Ni-Zn ferrite sphere. They observed the transition loss of 2 dB, the ferrite sphere was operational on the resonant mode in the unmagnetized case, and their resonant standing wave pattern was rotated in the magnetized case. Liu et. al.[88] reported the microwave response of low temperature fired gyromagnetic ferrite (Ni-Cu-Zn) with Bi2O3 additive. They reported resonance linewidth (16 kA/m), dielectric loss (5.7E-4), and Ms (337 kA/m) and showed
the potential applicability for the passive integrated substrate and microwave chip circulator. Kurlyandskaya et. al.[89] studied the zero field absorption and microwave resonant behavior of pure and dopped Ni-Cu-Zn ferrites. They observed a sizable loss at zero field for the dopped sample and the effect increased with an increase in reaction temperature. Akhtar et. al.[90] reported the magnetic and morphological response of Cu substituted Ni-Zn ferrite. The objective of Cu doping was to check the suitability of the material for various microwave applications such as circulators, phase shifter and multilayer chip inductors. Saha et. al.[91] developed unique metamaterial (Ni-Zn-Cu in polyvinyl fluoride) for enhanced microwave absorption. The soft magnetic behavior modulated by PVDF enhanced the magnetic and microwave properties, which will be useful in designing next-generation devices. The high permittivity of Polyvinylidene fluoride (PVDF) and permeability of Ni-Zn-Cu made it a perfect EMI absorber in the GHz frequency range. Reddy et al. studied the electromagnetic properties of MnFe2O4 prepared by spark plasma sintering (SPS) and investigated the effect of sintering temperature on the synthesized materials' microwave absorption, Ms, Hc, and particle size.[92] The core-shell structured polythiophene nanofibers layered MnFe2O4/Fe3O4 were synthesized by Hosseni et. al.[93] and microwave absorption, conductive, and magnetic response were investigated.
The core-shell structure promoted the interphase interactions at the surface of two ferrite phases. They achieved a minimum reflection loss (RL) of -21 dB at 12 GHz for 1.5 mm thickness. Gong et.al.[94] synthesized the phase transition enabled MnFe2O4 nanoparticles for enhancing electrical transport properties to be regulated using high pressure. They observed the hybridized enhancement between O‒2p and Fe‒3d orbitals. Further, they found the increment in interface density that led to an improvement in electrical properties.
The enhancement in conductivity of MnFe2O4 due to the annealing of the pressure cycle provides a new feasible pathway to expand their application in microwave and electronic
devices. Mishra et. al.[95] studied the Ms, Ferromagnetic resonance (FMR) linewidth, and Gilbert damping parameters of MnFe2O4/rGO to check the material's applicability for microwave applications. The composite layer was loaded on the top of a coplanar waveguide transmission line and the experimental values of FMR linewidth was analyzed by different micromagnetic models. They obtained FMR absorption of -28 dB at 22 GHz and suggested the new composite for microwave signal processing devices. Actinomorphic tubular ZnO/CoFe2O4 composites were synthesized in large scale by Cao and his group.[96]
The microwave absorption efficiency was analyzed by the radar absorbing materials reflectivity far field radar cross section method. And, a maximum microwave absorption of -28 dB was obtained at 8.5 GHz which shows that the composite can be used as efficient microwave absorber. Gandhi et al.[97] studied the microwave absorption, dielectric and thermal properties of CoFe2O4-polyaniline composites synthesized by one step chemical oxidative polymerization. The incorporation of CoFe2O4 nanoparticles led to high dipolar and interfacial polarization that contributed to high shielding effectiveness. More than 99.99% attenuation of microwaves (Shielding effectiveness due to absorption = 21.5 dB) was achieved in the frequency range of 12.4 ‒ 18 GHz. A high-performance microwave absorber composite (CoFe2O4/porous carbon nanosheet) was synthesized using instantaneous freezing assisted template by Xu.[98] It possess nanosheet-like pore walls with hierarchical porous structure. The 3D interconnected carbon helps to improve the dielectric loss of absorbers. A broadband absorption bandwidth of 5.36 GHz with RL minimum of -52.29 dB (2 mm thickness) was achieved. Lin et. al.[99] described the EM wave absorption capacity of composites of porous LiFe5O8 microspheres with reduced graphene oxide. It exhibited outstanding microwave absorbing performance having broad effective bandwidth of 3.5 GHz with minimum reflection loss of -53.4 dB at 12.2 GHz (coating thickness - 2.2 mm). The outstanding absorbing performance was assigned to the
magnetic micro flower with multi-interfaces that improved the impedance matching that led to high magnetic, electrical and relaxation loss. They confirmed that this particular composite could be a better choice for lightweight microwave absorbing materials. Li et.
al.[100] reported the microwave absorbing performance of tangled ZnFe2O4@carbon nano tube/PVDF composites. An appropriate combination of magnetic, dielectric and conduction loss led to the excellent absorption. The optimal RL of -54.5 dB was achieved at 10 GHz under a processing temperature of 60 C. Again, Gupta et. al.[101] analyzed the dominant EMI shielding of three-dimensional interconnected graphene aerogels decorated with ZnO nanorods and cobalt ferrite nanoparticles. The addition of magnetic cobalt ferrite enhanced the power absorption from ∼ 37.738% to ∼ 87.788% and further incorporation of ZnO nanorod enhanced the absorption power to ∼ 93.655%. A maximum shielding effectiveness (SE) of 48.56 dB was achieved with a sample thickness of 5 mm.