Fabrication and microwave absorption performances of hollow-structure Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/PANI microspheres

The hollow polyaniline (PANI) microspheres were prepared by controlling the mass ratio of the aniline to polystyrene (PS) via a template method, and Fe₃O₄/PANI composite microspheres have been fabricated by blending the hollow PANI microspheres with Fe₃O₄ magnetic particles. The effects of the mass ratio of aniline/PS on the microwave absorption performances of Fe₃O₄/PANI microspheres were investigated. It was found that the value of minimum reflection loss (RL_min) of the microspheres were respectively ?14.06, ?22.34 and ?24.3 dB, corresponding to the mass ratio of aniline/PS of 1:1.5, 1:3, and 1:6. In addition, when the mass ratio of aniline/PS was 1:6, with the thickness of 1.5 and 2.0 mm, the bandwidth below ?10dB were respectively 2.48 GHz (15.52–18 GHz) and 4.64 GHz (11.04–15.68 GHz), indicating that the Fe₃O₄/PANI microspheres could be a potential electromagnetic wave absorbing material in X (8–12 GHz) and Ku (12–18 GHz) bands.     

Synthesis and characterization of magnetic and microwave absorbing properties in polycrystalline cobalt zinc ferrite (Co<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>) composite

In this research work, magnetic and microwave absorption loss and other response characteristics in cobalt zinc ferrite composite has been studied. Cobalt zinc ferrite with the composition of Co_0.5Zn_0.5Fe₂O₄ was prepared via high energy ball milling followed by sintering. Phase characteristics of the as-prepared sample by using XRD analysis shows evidently that a high crystalline ferrite has been formed with the assists of thermal energy by sintering at 1250 °C which subsequently changes the magnetic properties of the ferrite. A high magnetic permeability and losses was obtained from ferrite with zinc content. Zn substitution into cobalt ferrite has altered the cation distribution between A and B sites in spinel ferrite which contributed to higher magnetic properties. Specifically, Co_0.5Zn_0.5Fe₂O₄ provides electromagnetic wave absorption characteristics. It was found that cobalt zinc ferrite sample is highly potential for microwave absorber which showed the highest reflection loss (RL) value of ??24.5 dB at 8.6 GHz. This material can potentially minimize EMI interferences in the measured frequency range, and was therefore used as fillers in the prepared composite that is applied for microwave absorbing material.     

Hierarchical structures based on Li<Subscript>0.35</Subscript>Zn<Subscript>0.3</Subscript>Fe<Subscript>2.35</Subscript>O<Subscript>4</Subscript>/polyaniline nanocomposites: synthesis and excellent microwave absorption properties

For the first time, the hierarchical structures of Li_0.35Zn_0.3Fe_2.35O₄(LZFO)/polyaniline nanocomposites were successfully synthesized by interfacial polymerization. Firstly, the LZFO particles were prepared by the sol–gel method, and subsequently the PANI nanorods, composed of nanoneedle-like PANI, were grafted on the surface of the LZFO. A novel microtopography, urchin-like, of LZFO/PANI was prepared by a simple, efficient and controllable two-step method. The crystal structure, chemical bonding states and morphology of samples were characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Scanning and Transmission electron microscopy (SEM/TEM). The bandwidth of reflection loss exceeds 10 dB in the frequency was 5.56 GHz (3.36–8.48, 10.32–10.76 GHz), and the maximum reflection loss can reach ??49.4 dB at 4.96 GHz with the thickness of 5.1 mm. The enhanced microwave absorption properties of LZFO/PANI nanocomposites are mainly ascribed to the multi-level structure and the improved impedance matching, and make it a potential candidate for microwave absorption materials.     

Synthesis and microwave absorption properties of sandwich-type CNTs/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/RGO composite with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> as a bridge

A novel sandwich-type CNTs/Fe₃O₄/RGO composite with Fe₃O₄ as a bridge was successfully prepared through a simple solvent-thermal and ultrasonic method. The structure and morphology of the composite have been characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. This new structure can effectively prevent the agglomeration of GO and the combination of CNTs/Fe₃O₄ and RGO shows a strong reflection loss (R_L) (?50 dB) at 8.7 GHz with absorber thickness of 2.5 mm. Moreover, compared with CNTs/Fe₃O₄/GO composite, it is found that the thermal treating process is beneficial to enhance the microwave absorption properties, which may be attributed to high conductivity of RGO. On this basis, the microwave absorbing mechanism is systematically discussed. All the data show that the CNTs/Fe₃O₄/RGO composite exhibits excellent microwave absorption properties with light density and is expected to have potential applications in microwave absorption.     

The microwave absorbing properties of CoFe<Subscript>2</Subscript> attached single walled carbon nanotube/BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanocomposites

The CoFe₂ attached single-walled carbon nanotubes (CoFe₂@SWCNTs) and BaFe₁₂O₁₉ ferrite nanocomposites with different CoFe₂@SWCNTs weight ratios (1, 3, 5, 7 wt%) were synthesized by a simple combination process. Then, the electromagnetic and microwave absorption properties were systematically investigated by a vector network analyzer in the frequency range of 2–18 GHz. High-quality CoFe₂@SWCNTs were prepared by a direct current arc discharge method in one-step. BaFe₁₂O₁₉ nanocrystals were synthesized by a nitrate citric acid sol–gel auto-ignition method. The CoFe₂@SWCNT/BaFe₁₂O₁₉ nanocomposites exhibited an efficient reflection loss (RL) and a wide absorption bandwidth. The minimum RL of ?54.13 dB was observed at 11.84 GHz for the nanocomposite (5 wt% CoFe₂@SWCNTs) with a thickness of 2.8 mm, 3.4 times greater than those without CoFe₂@SWCNTs, and a broad absorption bandwidth of 4.64 GHz (<?10 dB) was achieved. In addition, the nanocomposite (1 wt% CoFe₂@SWCNTs) shows a broader effective microwave absorption bandwidth of 7.12 GHz with a thickness of 1.9 mm. The experimental results reveal that the absorbing properties of the nanocomposites are greatly improved by controlling the CoFe₂@SWCNTs weight ratio and the matching thickness of the absorber. This CoFe₂@SWCNT/BaFe₁₂O₁₉ nanocomposite is anticipated to be applied in advanced microwave absorbers.     

Synthesis and excellent microwave absorption property of polyaniline nanorods coated Li<Subscript>0.435</Subscript>Zn<Subscript>0.195</Subscript>Fe<Subscript>2.37</Subscript>O<Subscript>4</Subscript> nanocomposites

In this work, Li_0.435Zn_0.195Fe_2.37O₄ (LZFO) was firstly prepared by the sol–gel process, and then core–shell polyaniline (PANI) nanorods/LZFO composites were successfully synthesized by interfacial polymerization. The structures and morphologies of samples were characterized by means of X-ray diffraction, Fourier transform infrared spectra, and scanning electron microscopy. The average size of the pure ferrites was about 0.75 μm and the size distribution was 0.3–1.0 μm. The results indicated that the composites with different morphologies and structures resulted in different electromagnetic properties. The electromagnetic absorption test demonstrated that the PANI nanorods/LZFO possessed the best absorption property. The value of the minimum reflection loss was ?51.1 dB at 9.86 GHz, and the absorption bandwidth exceeding ?10 dB was 2.3 GHz (from 3.7 to 6 GHz) and 2 GHz (from 15.7 to 17.7 GHz). The excellent electromagnetic wave absorption properties of the nanocomposites were attributed to the improved impedance matching and the enhanced interfacial effects.     

Preparation of TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/CF composites for enhanced microwave absorbing performance

To meet the demand of electromagnetic absorption, cheap and easily available microwave absorbents are urgently required. As an important functional material, carbon fibers (CFs) have been widely reported, however, too high conductivity easily leads to the impedance mismatch, which is not favorable to the microwave absorbing performance (MAP). To address this challenge, in this study, novel TiO₂/Fe₃O₄/CF composites with tunable magnetic were synthesized by hydrothermal method and characterized by SEM, XRD, XPS and VSM. As absorbents, the minimum reflection loss (R_L) value is ??41.52 dB at a thickness of 2.1 mm, and the corresponding bandwidth with effective attenuation (R_L?<???10 dB) is up to 5.65 GHz (4.54–10.19 GHz). More importantly, the plausible mechanisms for the enhanced MAP are explored.     

Preparation and microwave absorption properties of BiFeO<Subscript>3</Subscript> and BiFeO<Subscript>3</Subscript>/PANI composites

BiFeO₃(BFO) particle was successfully synthesized by normal citric acid sol–gel method and the size of BiFeO₃ particle is about 200 nm. BiFeO₃/polyaniline (PANI) composites with the different weight ratio were synthesized by in situ emulsion polymerization. The citric acid doped PANI is fibrous and form a loose structure outside the BFO particle. With the increasing of PANI, the conductivity value of composites are increasing to 9.34?×?10^?2 S/cm. Moreover, the permittivity also enhance with the increasing of conductivity, which contribute to the improvement of dielectric loss. Microwave absorbing properties were investigated with a vector network analyzer in 1–18 GHz. The minimum reflection loss (RL) value is about ?40.2 dB at 8.3 GHz when the thickness is 3.5 mm, and the maximum bandwidth less than ?10 dB is 3.5 GHz (from 13.5 to 18 GHz) at the thickness of 2 mm. 3 mm millimeter-wave-attenuation properties were also tested, and the maximum attenuation value of BFO/PANI composites reach 15.71 dB. The composites can dissipate microwave energy into heat effectively by dielectric relaxation because of the suitable conductivity. The interface scattering and multiple reflections also play a important role because of the increasing of a loose structure. The BFO/PANI composite can be taken as a promising lightweight and multiband microwave absorber.     

Microwave absorption characteristics of CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript> perovskite/carbon nanotube composites

The CH₃NH₃PbI₃ (MAPbI₃) and CH₃NH₃PbI₃/carbon nanotube (MC) composite have been successfully synthesized by a facile in situ solution method, which are investigated as the microwave absorption materials. For the MAPbI₃ particles, the minimum reflection loss is only ?4.9 dB around 16.4 GHz due to the poor relative complex permittivity. Then, the relative complex permittivity of MC composites could be adjusted by changing the mass fraction of CNTs in composite, which is a vital role for the dielectric loss. The reflection loss of MC-5 composite (MAPbI₃/CNT, 5:1 wt%) can be improved to ?35.7 dB with thickness of 1.3 mm at 13.1 GHz. When the thickness is <3.0 mm, the microwave absorption bandwidth of MC-5 is 11.8 GHz (5.0–16.8 GHz) under the reflection loss lower than ?20 dB. The quarter-wavelength (λ/4) matching model is used to discuss the microwave absorption mechanism of MC composites. These results indicate that MC-5 composite could be used as the microwave absorption materials with strong reflection loss, lightweight and broad bandwidth.     

Effect of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> addition on microwave absorption property of plasma sprayed Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>/NASICON coatings

The work attempted to develop a kind of high temperature microwave absorption coating. The Ti₃SiC₂/NASICON composite coatings with different Ti₃SiC₂ concentrations were fabricated by atmospheric plasma spraying. The effect of Ti₃SiC₂ addition on phase, density, microstructure, dielectric property and microwave absorption property of as-sprayed coatings was investigated. Results show that the complex permittivity increases with increasing the content of Ti₃SiC₂ due to the enhanced space charge polarization, decreased porosity and increased conduction loss. When the content of Ti₃SiC₂ increases to 30 wt%, the coating exhibits the optimal microwave absorption property with a bandwidth (below ??5 dB) of 4.01 GHz and lowest reflection loss of ??12.4 dB at 9.63 GHz in 1.4 mm thickness. It indicates that the Ti₃SiC₂/NASICON composite coating can be a potential candidate for microwave absorption.     

Facile synthesis and excellent electromagnetic wave absorption properties of flower-like porous RGO/PANI/Cu<Subscript>2</Subscript>O nanocomposites

Novel porous ternary nanocomposite systems containing reduced graphene oxide (RGO)/polyaniline (PANI)/cuprous oxide (Cu₂O) were prepared via one-step in situ redox method. The RGO/PANI/Cu₂O nanocomposites present a flower-like structure with an average size of 2.0 μm in diameter. The morphologies and properties of the products can be controlled by adjusting the molar ratios of aniline to Cu²⁺. When the molar ratio of aniline to Cu²⁺ is 1:1, the product exhibits excellent microwave absorption property in the frequency range of 2–18 GHz. It can be seen that the maximum reflection loss (RL) of the ternary composite is up to ?52.8 dB at 2.7 GHz with a thickness of only 2 mm, and the absorption bandwidth corresponding to ?10 dB (90% of EM wave absorption) is 13.2 GHz. The microwave absorption property of ternary RGO/PANI/Cu₂O composite is significantly improved due to its special flower-like porous structure, dielectric loss property and well impedance matching characteristics, which is 8.12 times than that of pure RGO and 5.28 times than that of pure PANI. Therefore, our study paves a new way to prepare the promising lightweight and high-performance composite materials combined with the characteristics of three components for electromagnetic absorption.     

Removal and reuse of Ag nanoparticles by magnetic polyaniline/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanofibers

For the treatment of wastewater containing Ag nanoparticles (NPs), PANI/Fe₃O₄ nanofibers were firstly prepared by a novel self-assemble. And then, the efficiency for the removal of Ag NPs from wastewater was investigated. The magnetic performance of PANI/Fe₃O₄ nanofibers could be optimized by adjusting the pH of the self-assemblied system. Under pH of 3, the as-prepared nanofibers exhibited the highest magnetism and also displayed good efficiency (>?12 mg g^?1) for the removal of Ag NPs. Importantly, the resulted product (PANI/Fe₃O₄/Ag composite) could act as a catalysis for cleaning durable pollutant, 4-nitrophenol. After 10 cycles, only slight decrease in rate constant was found, indicating excellent reusability. Those approaches provide a new way to merge the recovery of Ag NPs as pollutants and reuse of recovered Ag NPs as recyclable material for environmental remediation.     

Low temperature sintering and dielectric properties of Ba<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript> microwave ceramics using Co<Subscript>2</Subscript>O<Subscript>3</Subscript> additives

The Ba₃(VO₄)₂–x wt% Co₂O₃ (x?=?0.5–5) ceramics were prepared by the solid state reaction method in order to reduce the sintering temperature. The effects of the Co₂O₃ additions on the phase composition, microstructures, sintering characteristics and microwave dielectric properties of Ba₃(VO₄)₂ ceramics are investigated by an X-ray diffractometer, a scanning electron microscope and a network analyzer. As a result, the Q?×?f value of 54,000 GH, the ε _r of 14.6 and the τ_f value of +58.5 ppm/°C were obtained in the sample of the Ba₃(VO₄)₂–3 wt% Co₂O₃ ceramic sintered at the temperature of 925 °C, which is capable to co-fire with electrode metal of high conductivity such as Ag (961 °C). Moreover, the Q?×?f values of the sample with Co₂O₃ higher than that of 3 wt% additions decreased because of the formation of Ba₂V₂O₇ phase.     

Light weight RGO/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposite for efficient electromagnetic absorption coating in X-band

Reduced graphene oxide (RGO)/magnetite (Fe₃O₄) nanocomposite has been synthesized by an in-situ facile hydrothermal method. The XRD pattern reveals the development of nanocomposite in which both phases are coexistent. Raman Spectroscopy shows the main characteristics peaks of D and G bands at 1349 cm^?1 and 1595 cm^?1 for graphitic structures. The intensity ratio (I_D/I_G) is also calculated, which indicate the degree of defects in the material. This ratio (I_D/I_G), increases from 0.84 for GO to 0.91 for RGO/Fe₃O₄ nanocomposite and promotes the defects which are beneficial for electromagnetic (EM) absorption. The SEM image depicts that, Fe₃O₄ spherical nanoparticles are dispersed over the surface of graphene sheets and provide a thermal conducting path for heat dissipation between different layers of graphene. The EM absorption properties have been analyzed at 2–18 GHz of RGO and RGO/Fe₃O₄. The addition of proper content of Fe₃O₄ magnetic nanoparticles in RGO sheets improved the Reflection Loss (RL) from ??13.5 dB to ??20 dB at a frequency of 9.5 GHz. Moreover, due to magnetic loss and interfacial polarization, the effective bandwidth increases from 2.5 GHz to 3.8 GHz at a coating thickness of 1.5 mm. Hence this light weight nanocomposite is an excellent material for strong EM absorption in X-band.     

Enhanced electromagnetic wave absorption properties of polyaniline-coated Fe3O4/reduced graphene oxide nanocomposites

Fe₃O₄-reduced graphene oxide-polyaniline (Fe₃O₄–RGO–PANI) ternary electromagnetic wave absorbing materials were prepared by in situ polymerization of aniline monomer on the surface of Fe₃O₄–RGO nanocomposites. The morphology, structure and other physical properties of the nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, vibration sample magnetism, etc. The electromagnetic wave absorbing properties of composite materials were measured by using a vector network analyzer. The PANI–Fe₃O₄–RGO nanocomposites demonstrated that the maximum reflection loss was ?36.5 dB at 7.4 GHz with a thickness of 4.5 mm and the absorption bandwidth with the reflection loss below ?10 dB was up to 12.0 GHz with a thickness in the range of 2.5–5.0 mm, suggesting that the microwave absorption properties and the absorption bandwidth were greatly enhanced by coating with polyaniline (PANI). The strong absorption characteristics of PANI–Fe₃O₄–RGO ternary composites indicated their potential application as the electromagnetic wave absorbing material.     

Structural,dielectric, ac conductivity and electromagnetic shielding properties of polyaniline/Ni<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> composites

A conducting polymer, polyaniline (PANI)/Ni_0.5Zn_0.5Fe₂O₄ composites with high dielectric absorbing properties and electromagnetic shielding effectiveness at low frequencies were successfully synthesized through a simple in situ emulsion polymerization. PANI was doped with hydrochloric acid to improve its electrical properties and interactions with ferrite particles. PANI/Ni_0.5Zn_0.5Fe₂O₄ composites were characterized by X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analysis. Frequency dependence of dielectric and ac conductivity (σ_ac) studies have been undertaken on the PANI/Ni_0.5Zn_0.5Fe₂O₄ composites in the frequency range 50 Hz–5 MHz. The electrical conduction mechanism in the PANI/Ni_0.5Zn_0.5Fe₂O₄ is found to be in accordance with the electron hopping model. Further, frequency dependence of electromagnetic interference (EMI) shielding effectiveness (SE) is studied. The EMI shielding effectiveness is found to decrease with an increase in the frequency. The maximum value 55.14 dB of SE at 50 Hz was obtained at room temperature for PANI/Ni_0.5Zn_0.5Fe₂O₄ composites in the 50 Hz–5 MHz frequency range. PANI/Ni_0.5Zn_0.5Fe₂O₄ composites were demonstrated as a promising functional material for the absorbing of electromagnetic waves at low frequencies because of a large amount of dipole polarizations in the polymer backbone and at the interfaces of the Ni–Zn ferrite particles and PANI matrix.     

Cu doping effect on the resistive switching behaviors of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films

Spin-coated Cu_xCo_1?xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, and 0.8) thin films were prepared on Pt/TiO₂/SiO₂/Si substrates. Pt/Cu_xCo_1?xFe₂O₄/Pt structures were fabricated to investigate the effect of Cu doping concentration on the resistive switching behaviors. Structural and morphology characterizations revealed that Cu doping improved the crystallization of the thin films as compared to undoped CoFe₂O₄. Current–voltage characterization showed that all Cu_xCo_1?xFe₂O₄ thin films showed unipolar resistance switching, but the distribution range of the set voltage, reset voltage, and resistances were much reduced by Cu doping. Clear improvement in the stability of these parameters started to appear with x = 0.4, and the optimized performance was observed in the Pt/Cu_0.6Co_0.4Fe₂O₄/Pt structure. The improved stability of the switching parameters was attributed to the enhancement of hopping process between the Fe ions and the Cu ions in the spinel lattice. Our results indicated that appropriate adjustment of the doping elements in oxides can be a feasible approach in achieving stable resistance switching memory devices.     

Fast microwave synthesis of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Ag magnetic nanoparticles using Fe<Superscript>2+</Superscript> as precursor

A simple and quick microwave method to prepare high performance magnetite nanoparticles (Fe₃O₄ NPs) directly from Fe has been developed. The as-prepared Fe₃O₄ NPs product was fully characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the as-prepared Fe₃O₄ NPs are quite monodisperse with an average core size of 80 × 5 nm. The microwave synthesis technique can be easily modified to prepare Fe₃O₄/Ag NPs and these NPs possess good magnetic properties. The formation mechanisms of the NPs are also discussed. Our proposed synthesis procedure is quick and simple, and shows potential for large-scale production and applications for catalysis and biomedical/biological uses.     

The synergetic electromagnetic properties and enhanced microwave absorption of BiFeO<Subscript>3</Subscript>/BaFe<Subscript>7</Subscript>(MnTi)<Subscript>2.5</Subscript>O<Subscript>19</Subscript> composite

The effective microwave absorption materials could contribute to alleviating the electromagnetic wave pollution. However, conventional microwave absorption materials usually suffer from insufficient absorption intensity and the narrow effective absorption bandwidth. Herein, BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composites are proposed to address these issues through offering synergetic electromagnetic properties and proper electromagnetic properties. BiFeO₃ combined with BaFe₇(MnTi)_2.5O₁₉ exhibits dielectric multiple relaxation behaviors, strong ferromagnetic resonance and electromagnetic matching, ensuring increased multi-band microwave absorption. Accordingly, the minimum reflection loss (RL) of the composite with volume ratio of 1.5:1 reaches ??48 dB, and the bandwidth less than ??10 dB covers multi-frequencies at C, X and Ku band. These results suggest that BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composite could be a promising microwave absorption material in imaging, healthcare, information safety and military fields.     

Effects of Mg<Superscript>2+</Superscript> ion substitution on the structural and electric studies of spinel structure of Co<Subscript>1−x</Subscript>Mg<Subscript>x</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>

In this work, we have prepared Mg substituted CoFe₂O₄ from the transition metals by Sol–Gel Auto combustion method for different concentration (0.00, 0.05, 0.10, 0.15, 0.20 and 0.30) the aqueous materials involved Cobalt Nitrate, Magnesium Nitrate, Ferric nitrate and used a citric acid as a fuel. The final product was calcinated at 850 °C. Powder XRD studies confirmed the crystal structure, FTIR revealed the presence of various functional groups, FE-SEM showed the morphology of the prepared sample. EDAX shows percentage of chemical composition in series of Co ferrites. Dielectric constant, AC Conductivity, Cole–Cole plots, Activation Energy were analysed for the prepared nanoparticles for various frequencies at different temperatures. The dielectric constant values are higher for Mg substituted CoFe₂O₄. The activation energies were also calculated which ranged from 0.1552 to 0.1753 eV for CoFe₂O₄ and Co_1?xMg_xFe₂O₄ materials.