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Research and utilization of nanotechnology are growing exponentially in every aspect of life. The constant growth of applications for magnetic nanoparticles, specifically nanoferrites, attracted many researchers. Among them, nanocobalt ferrite is the most crucial and studied magnetic nanoparticle. Environmentally benign synthetic methods became necessary to minimize environmental and occupational hazards. Green synthesis approaches in science and technology are now widely applied in the synthesis of nanomaterials. Herein, we reviewed recent advances in synthesizing nanocobalt ferrites and their composites using various scientific search engines. Subsequently, various applications were discussed, such as environmental (treatment of water/wastewater, photocatalytic degradation of dyes, and nanosorbent for environmental remediation) and biomedical (nanobiosensors for cancer diagnosis at the primary stage, effective targeted drug delivery, magnetic resonance imaging, hyperthermia, and potential drug candidates against cancer and microbial infections). This review offers comprehensive knowledge on how to choose appropriate natural resources for the green synthesis of nanocobalt ferrite and the benefits of this approach compared to conventional methods.

The gadolinium substituted cobalt ferrite (CoFe1.9Gd0.1O4) nanoparticles and CoFe1.9Gd0.1O4/Polyaniline (PANI) microwave absorber were synthesized by sol-gel auto combustion technique using lemon juice and in-situ polymerization method respectively. X-ray patterns confirmed the formation of single phase cubic structure. The crystallite size of the synthesized CoFe1.9Gd0.1O4 nanoparticles are within the range of 15–68 nm. The saturation magnetization of CoFe1.9Gd0.1O4 ferrite/Polyaniline (PANI) composite was reduced due to nonmagnetic PANI. The reflection loss for microwave absorbing properties of CoFe1.9Gd0.1O4 ferrite nanoparticles and CoFe1.9Gd0.1O4/PANI nanocomposite were investigated and minimum value of reflection loss was found to be −16.85 dB at 13.52 GHz for nanoparticles of thickness 2.5 mm and −25.59 dB at 11.92 GHz for CoFe1.9Gd0.1O4/PANI nanocomposite of thickness 2.0 mm) respectively. The prepared samples have low density, high surface resistivity and enhanced attenuation constant. The nanocomposite exhibits excellent absorption performance over a broad band range in the radar band.

TiO2 (0-10 wt %)-doped nanocrystalline Ni0.4Cu0.3Zn0.3Fe2O4 (Ni-Cu-Zn) ferrites were synthesized using the sol-gel route of synthesis. The cubic spinel structure of the ferrites having the Fd3m space group was revealed from the analysis of Rietveld refined X-ray diffraction (XRD) data. The secondary phase of TiO2 with a space group of I41/amd was observed within the ferrites with doping, x > 3 wt %. The values of lattice parameter were enhanced with the addition of TiO2 up to 5 wt % and reduced further for the highest experimental doping of 10 wt %. Field emission scanning electron microscopy (FESEM) images exhibit the spherical shape of the synthesized particles with some agglomeration, while the compositional purity of prepared ferrite samples was confirmed by energy-dispersive X-ray spectroscopy (EDX) and elemental mapping. The cubic spinel structure of the prepared ferrite sample was confirmed by the Raman and Fourier transform infrared (FTIR) spectra. UV-visible diffuse reflectance spectroscopy was utilized to study the optical properties of the ferrites. The value of band gap energy for the pristine sample was less than those of the doped samples, and there was a decrement in band gap energy values with an increase in TiO2 doping, which specifies the semiconducting nature of prepared ferrite samples. A magnetic study performed by means of a vibrating sample magnetometer (VSM) demonstrates that the values of saturation magnetization of the ferrites decrease with the addition of TiO2 content, and all investigated ferrites show the characteristics of soft magnetic materials at room temperature. The Mossbauer study confirms the decrease in the magnetic behavior of the doped ferrites due to the nonmagnetic secondary phase of TiO2.

The nanocrystalline samarium substituted Co-Zn ferrites with chemical formula Co0.7Zn0.3SmyFe2–yO4 (where y = 0, 0.01, 0.02, 0.03, 0.04) were synthesized by sol-gel autocombustion route. The analysis of X-ray diffractograms (XRD) reveals the formation of cubic spinel structure. The planes indexed from XRD analyses were confirmed in the selected area electron diffraction (SAED) image of the sample. Nanocrystalline nature of the particles in the ferrite samples was confirmed by TEM. The morphology was analyzed by scanning electron microscopy (SEM). Magnetic measurements show an increase in the magnetization for x ≤ 0.03. The decrease in magnetization due to spin canting is observed for x = 0.04. The coercivity depends on Sm3+ doping concentration, grain size and saturation magnetization. The complex permeability of the ferrites was analyzed as the function of frequency and Sm3+ composition (y). The real part of complex permeability varies linearly with the grain size.

Nanomaterials have recently gained significant interest as they are believed to offer an outstanding prospect for use in environmental remediation. Among many possible candidates, due to their useful properties including magnetic nature, wide surface area, and high absorptivity, ferrite materials hold tremendous appeal, allowing them to be used for multifaceted applications. In the present study, using a sol–gel auto combustion process, a magnetically separable Zn1−xCo0.5xMg0.5xFe2O4 (x = 0.0, 0.25, 0.50, 0.75, 1.0) ferrite with superior photocatalytic activity for dye degradation was manufactured. Rietveld refinement and FTIR studies confirm that a single-phase cubic spinel system was built for all samples with crystallite sizes of 34–57 nm. VSM has determined the magnetic properties of the samples at room temperature. With the introduction of Mg2+ and Co2+ in the Zn ferrites, a transformation from the soft superparamagnetic activity to the hard ferromagnetic character was reported. Considering the band structure in the visible region, the photocatalytic activities of the Zn1−xCo0.5xMg0.5xFe2O4 ferrites for the degradation of the MB dye under natural sunlight were investigated. Zn0.25Co0.375Mg0.375Fe2O4 showed an efficiency of degradation of 99.23% for MB dye with a quick 40 min irradiation period with high reusability of up to four cycles.

In the present study, we examined the effects of doping with Zn–Zr on the structural, optical, electrical, and mechanical properties of cobalt ferrite. Nanocrystalline cobalt ferrites comprising CoZnxZrxFe2-2xO4 (x = 0.0 to 0.4, Δx = 0.1) were prepared via the sol–gel route. X-ray diffraction analysis indicated the formation of a spinel phase in the ferrites. An additional ZrO2 phase was observed for x = 0.3. Characteristic absorption bands at frequencies of 580–610 cm−1 and 375–400 cm−1 related to spinel structures were observed in the Fourier transform infrared FTIR spectra obtained for the ferrites. The elastic constants decreased as the Zn–Zr contents increased. The longitudinal and shear velocities of ultrasound through the ferrite materials were measured using the ultrasonic pulse transmission method. The variations in the ultrasound velocities could be attributed to the changes in density. The dielectric parameters ɛ′ (dielectric constant) and tanδ (dielectric loss tangent) were analyzed as functions of the frequency and Zn–Zr composition. Both ɛ′ and tanδ decreased as the frequency and Zn–Zr contents increase. The variations in the values of parameters ɛ′ and tanδ were explained using Maxwell–Wagner interfacial polarization according to Koops phenomenological theory. The values of the activation energy and Curie temperature were calculated based on plots of the thermal variation in the direct current resistivity. The activation energy values indicated that electron hopping was the mechanism responsible for conduction in the ferrites.

Nanocrystalline Ni0.4Cu0.3Zn0.3Fe2O4 ferrites doped with TiO2 (0–10 wt %) were prepared by the sol-gel method. Elastic properties of synthesized samples were studied with the help of ultras...

TiO2 (0–10 wt %)-doped nanocrystalline Ni0.4Cu0.3Zn0.3Fe2O4 (Ni–Cu–Zn) ferrites were synthesized using the sol–gel route of synthesis. The cubic spinel structure of the ferrites having the Fd3m space group was revealed from the analysis of Rietveld refined X-ray diffraction (XRD) data. The secondary phase of TiO2 with a space group of I41/amd was observed within the ferrites with doping, x > 3 wt %. The values of lattice parameter were enhanced with the addition of TiO2 up to 5 wt % and reduced further for the highest experimental doping of 10 wt %. Field emission scanning electron microscopy (FESEM) images exhibit the spherical shape of the synthesized particles with some agglomeration, while the compositional purity of prepared ferrite samples was confirmed by energy-dispersive X-ray spectroscopy (EDX) and elemental mapping. The cubic spinel structure of the prepared ferrite sample was confirmed by the Raman and Fourier transform infrared (FTIR) spectra. UV–visible diffuse reflectance spectroscopy was utilized to study the optical properties of the ferrites. The value of band gap energy for the pristine sample was less than those of the doped samples, and there was a decrement in band gap energy values with an increase in TiO2 doping, which specifies the semiconducting nature of prepared ferrite samples. A magnetic study performed by means of a vibrating sample magnetometer (VSM) demonstrates that the values of saturation magnetization of the ferrites decrease with the addition of TiO2 content, and all investigated ferrites show the characteristics of soft magnetic materials at room temperature. The Mössbauer study confirms the decrease in the magnetic behavior of the doped ferrites due to the nonmagnetic secondary phase of TiO2.

Rare earth (RE) ions are known to improve the magnetic interactions in spinel ferrites if they are accommodated in the lattice, whereas the formation of a secondary phase leads to the degradation of the magnetic properties of materials. Therefore, it is necessary to solubilize the RE ions in a spinel lattice to get the most benefit. In this context, this work describes the synthesis of Co–Zn ferrite nanoparticles and the Gd3+ doping effect on the tuning of their magnetic properties. The modified sol–gel synthesis approach offered a facile way to synthesize ferrite nanoparticles using water as the solvent. X-ray diffraction with Rietveld refinement confirmed that both pure Co–Zn ferrite and Gd3+ substituted Co–Zn ferrite maintained single-phase cubic spinel structures. Energy dispersive spectroscopy was used to determine the elemental compositions of the nanoparticles. Field and temperature dependent magnetic characteristics were measured by employing a vibration sample magnetometer in field cooled (FC)/zero field cooled (ZFC) modes. Magnetic interactions were also determined by Mossbauer spectroscopy. The saturation magnetization and coercivity of Co–Zn ferrite were improved with the Gd3+ substitution due to the Gd3+ (4f7)–Fe3+ (3d5) interactions. The increase in magnetization and coercivity makes these Gd3+ substituted materials applicable for use in magnetic recording media and permanent magnets.

A series of ferrite with a chemical composition Co 0.7 Zn 0.3 Gd x Fe 2−x O 4 (where x=0.0 to x=0.1) were prepared by sol-gel auto-combustion method. X-ray diffraction pattern were used to determine the crystal structure and phase formation of the prepared samples. Scanning electron microscopy is used to study the surface morphology of the prepared samples. Elastic properties were determined from the infrared spectroscopy. Debye temperature, wave velocities, elastic constants found to increase with the increase in Gd 3+ substitution. Dielectric properties such as dielectric constant and dielectric loss were studied as a function of Gd 3+ substitution and frequency. Dielectric constant decreased with the increase in frequency and Gd 3+ substitution. Behavior of dielectric properties was explained on the basis of Maxwell-Wagner interfacial polarization which in accordance with Koops phenomenological theory. Real and imaginary part of impedance was studied as a function of resistance and Gd 3+ substitution. The behavior of impedance is systematically discussed on the basis of resistance-capacitance circuit.

The structural modifications in Co–Zn ferrites due to the substitution of Gd3+ ions and their credible use as low loss dielectrics and H2S gas sensors are reported in the present study. The structural modifications lead to the enhancement in physical properties of the ferrites making it suitable for the applications. The nanocrystals of Co0·7Zn0·3Fe2-xGdxO4 (0 ≤ x ≤ 0.1, Δx = 0.025) were obtained using self-ignited citrate sol-gel route of synthesis. The X-ray diffractograms of the ferrites were refined by Rietveld method using the full-proof pdf software. The refinement asserts the creation of mono phase spinel ferrite crystals. The morphology of ferrite crystals analyzed from scanning electron microscopy (SEM) profiles depict that the ferrites exhibit the porous nature. The transmission electron microscopy (TEM) images confirms the nanocrystalline nature of the ferrites with an average dimension of 25 nm and high resolution TEM (HRTEM) confirms the spinel phase created within the ferrites. The frequency variation of dielectric parameters was analyzed to study the relaxation phenomenon in ferrites. The values of dielectric constant, dielectric loss and loss tangent were reported to decline with frequency and the Gd3+composition (x). The a.c. electrical conductivity increases with frequency and decreases with composition ‘x’. The room temperature gas sensing response of the ferrite for the hazardous H2S gas was much higher than that for LPG, SO2, NO2 and H2 gases. The ferrite sample with x = 0.025 exhibits a better sensor performance for H2S gas with small recovery and response time. The nanocrystalline nature, porosity and morphology of the ferrites bear control over response and other parameters.

Ho 3+ substituted cobalt-zinc ferrite with a chemical formula Co 0.7 Zn 0.3 Ho x Fe 2−x O 4 (0≤x≤0.1) were synthesized by sol-gel method. Synthesized samples were characterized by using X-ray diffraction, field emission scanning electron microscopy, Raman, Infrared and UV–visible spectroscopy. X-ray diffraction patterns show the formation of cubic spinel structure of all the synthesized samples. The gradual changes in the Raman spectra of Co-Zn ferrite were observed at higher level of Ho 3+ substitution. The Raman band shows that there is a strong electron-phonon coupling in the investigated ferrite system and is the highest for the T 3 2g mode as compared to A 1g and T 2 2 g modes. Fourier transform infrared spectroscopy shows that the vibration bands corresponding to Fe–O is slightly varied with the substitution of Ho 3+ substitution. UV–visible spectra was studied by using the optical absorbance measurements in the wavelength ranges from 500 to 800 nm. The estimated energy band gap is found to increase from 1.72 to 1.84 eV with the increase in Ho 3+ substitution. The elastic properties such as longitudinal velocity, shearing velocity, elastic constant, Poisson's ratio and Debye temperature were measured at room temperature by using the ultrasonic pulse transmission method. It is found that the elastic constant and Debye temperature decreased with the increase in Ho 3+ substitution. A linear relationship between Debye temperature, elastic constant and average sound velocity with the Ho 3+ substitution is observed and discussed in this manuscript. Dielectric properties have been investigated as function of frequency. The dispersion of the dielectric properties was discussed in the light of Koop's theory in accordance with the Maxwell-Wagner polarization.

The effect of Zn–Ti substitution on the infrared spectroscopy, elastic, dielectric, and resistivity properties of CoZnxTixFe2−2xO4 ferrites (where x= 0.0–0.4 in steps of 0.1) is investigated in the paper. All the samples were synthesized by the sol–gel method. Infrared spectroscopy shows two major absorption bands indicating that the prepared samples are in spinel structure. Elastic properties and dielectric properties are measured at room temperature and Zn–Ti is substitution dependent. The dc resistivity is measured using a two-probe method in the temperature range 300 to 650 K. The dielectric constant decreased whereas dc resistivity increased with the Zn–Ti substitution. The elastic constant decreases with increases in Zn–Ti ions.

Nanoparticles of Ho3+ substituted in Co–Zn ferrites were synthesised by sol–gel method. The phase formation of these samples has been confirmed by X-ray powder diffraction technique. XRD Rietveld refinement carried out using the FULLPROF program shows that the Co–Zn ferrite retains its single phase cubic structure with space group Fd3m for x ≤ 0.05. Occupancy of the cations is explained on the basis of site preference, size and valance of the substitution cations. The nanostructure and morphology of prepared samples were investigated by field emission scanning electron microscopy and transmission electron microscopy. The elemental percentage of the constituent ions was determined using energy dispersive spectroscopy. The magnetic interactions among the nanoparticles were analyzed by employing a temperature dependent vibration sample magnetometer, field cooled (FC)/zero field cooled (ZFC) measurements. The ZFC and FC curves diverge below the blocking temperature exhibiting a ZFC cusp at 195–225 K. The saturation magnetization of Co–Zn ferrite increased linearly with Ho3+ substitution for x ≤ 0.05 and almost remains constant thereafter. The frequency dependence of the AC susceptibility measurements was performed on the sample. It shows a peak at around spin freezing temperature, with the peak position shifting as a function of driving frequency, indicating a spin-glass-like transition of the sample.

The Zn-Zr co-substituted cobalt ferrite nanoparticles (CoZnxZrxFe2-2xO4, x = 0.0˗0.4) were synthesized by sol gel auto combustion route. The formation of cubic phase of Co ferrite was revealed from the X-ray diffractograms of the powder samples. The additional α-Fe2O3 and ZrO2 phases were occurred for ≥20% Zn-Zr substitution. The lattice parameters obtained by extrapolating Nelson–Riley function shows increase in its values with the Zn-Zr substitution. The particle size shows increasing trend with the Zn-Zr substitution. The cation distribution obtained from the Rietveld refinement of XRD estimate the equal preference of Zn ons preferred tetrahedral A site whereas most of the Zr ions occupy octahedral B site. The decrease in Fe ions with the Zn-Zr substitution resulted in the decrease in coercivity and saturation magnetization of the ferrites. Zero field cooled and field cooled magnetization plots of the ferrites reveals the ferromagnetic behaviour of the prepared ferrite samples. The blocking temperature does not vary significantly with the varying Zn-Zr substitution. The Mossbauer study confirms the weakening of the magnetic linkages between cations at A and B sites, due to the substitution of non-magnetic Zn-Zr ions for magnetic Fe ions. The decrease in coercivity with moderate saturation magnetization could make this material suitable for electronic devices.

Co-Zn spinel ferrites with lanthanum (La3+) ion substitutions having formula Co0.7Zn0.3LaxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.1) were prepared using sol gel auto ignition route. The structural characterization of the ferrites was performed by the x-ray diffraction (XRD) method. The parameters calculated from the XRD analysis include lattice parameter, density, porosity, crystal size and lattice strain. The nanocrystallinity of the ferrite samples was observed with the crystallites of 20–30 nm size. The scanning electron microscopy (SEM) was employed to analyse the morphology of the ferrite crystals. The size and shape of the ferrite nanocrystals were confirmed from transmission electron microscopy (TEM) images. The crystal planes revealed from XRD calculations were confirmed by the selected area electron diffraction (SAED) analysis of the ferrites. Vibrating sample magnetometer (VSM) measurements of the ferrite samples were performed to study the magnetic properties of the ferrites. The effects of the La3+ substitution were observed on the coercivity and saturation magnetization of the ferrites. The variation of dielectric properties of the ferrites within 50 Hz to 5 MHz frequency band were studied at room temperature. The dielectric relaxation in the ferrites was governed by the electron hopping between divalent (Fe2+ and Co2+) and trivalent (Fe3+ and Co3+) cations. Remarkable impact of La3+ composition and the crystal size was observed on dielectric constant as well as loss tangent.

Nanocrystalline Ce-substituted yttrium iron garnet (YIG) powders of different compositions, Y3−xCexFe5O12 (0 ≤ x ≤ 2.0), were synthesized by a combination of sol–gel auto-combustion and solid-state synthesis techniques. The as-obtained powder samples were sintered at 1150 °C for 10 h. The garnet structure formation is confirmed by the X-ray diffraction pattern, which shows that the calculated lattice parameter increased for x = 1.0 and shows a decreasing trend for x ≥ 1.0 with the addition of cerium ions. The lattice parameter increased from 12.38 A to 12.41 A for x ≤ 1.0 whereas it decreased from 12.412 A to 12.405 A with the cerium composition for x > 1.0. The average particle size determined by high resolution transmission electron microscopy is in the range of 50 to 90 nm and found to increase with the substitution of cerium ions in YIG. The room temperature magnetic parameters such as saturation magnetization, coercivity and remanence magnetization are greatly affected by the substitution of cerium ions. The values of saturation magnetization decrease from 25.5 to 15 emu g−1 whereas coercivity increases from 1 to 28 Oe with the substitution of cerium ions. The pure YIG sample shows polycrystalline nature that changed towards a single-crystal structure leading to a preferred-(100) orientation with the Ce substitution. The change from a ring to a spotty pattern observed in SAED confirmed the crystalline phase transformation and is well supported by HRTEM and magnetic measurements. The behavior of magnetic and electrical properties is well supported by the poly- and single-crystalline nature of YIG and Ce-YIG, respectively. The crystal structure transformation in YIG brought about by Ce substitution could unveil enormous opportunities in the preparation of single-crystal materials from their polycrystalline counterparts.

CoFe2O4, cobalt ferrite (CFO) nano-particles with composition CoZnxTixFe2−2xO4 (0 ≤ x ≤ 0.4) were synthesized by sol–gel autocombustion method. The effect of Zn2+–Ti4+ substitution on the structural, magnetic and frequency dependent permeability properties of the CFO nano-particles were investigated by X-ray diffraction, 57Fe Mossbauer spectroscopy, vibrating sample magnetometry, transmission electron microscopy and permeability analysis. The Rietveld refinement of XRD patterns confirm the single spinel phase and the crystallite size is found in the range of 22–32 nm. Cation distribution was estimated by refining the XRD pattern by Rietveld method, and shows Zn2+ ions at the tetrahedral A-sites, and Co2+ and Ti4+ ions at octahedral B-sites. The saturation magnetization (Ms) increased from 58 to 75 emu g−1 for up to x = 0.2 and then decreased, while the coercivity decreased continuously with Zn2+–Ti4+ substitution. Two distinct composition ranges with Zn2+–Ti4+ substitution are identified for which Ms variation with x is explained by the Neel and Yafet–Kittel models. The room temperature Mossbauer spectra are analyzed in detail for probing the magnetic properties of Fe based Zn2+–Ti4+ substituted CFO. The effect of Zn2+–Ti4+ substitution on various Mossbauer parameters, viz. hyperfine field distribution, isomer shift, quadrupole splitting, and line width, has also been studied. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of A–B and B–B supertransferred hyperfine interactions. The CFO nanoparticle is considered to possess a fully inverse spinel structure with a Neel-type collinear spin alignment, whereas the Zn2+–Ti4+ substitution in CFO is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. This study also demonstrates that one can tailor the magnetic properties of CFO particles by optimizing the Zn2+–Ti4+ substitution. The increase in the permeability, saturation magnetization and lower loss factor makes the synthesized materials suitable for applications in microwave devices and deflection yokes.

The bismuth (Bi3+)-doped cobalt ferrite nanostructures with dual phase, i.e. cubic spinel with space group Fd3m and perovskite with space group R3c, have been successfully engineered via self-ignited sol-gel combustion route. To obtain information about the phase analysis and structural parameters, like lattice constant, Rietveld refinement process is applied. The replacement of divalent Co2+ by trivalent Bi3+ cations have been confirmed from energy dispersive analysis of the ferrite samples. The micro-structural evolution of cobalt ferrite powders at room temperature under various Bi3+ doping levels have been identified from the digital photoimages recorded using scanning electron microscopy. The hyperfine interactions, like isomer shift, quadrupole splitting and magnetic hyperfine fields, and cation distribution are confirmed from the Mossbauer spectra. Saturation magnetization is increased with Bi3+-addition up to x = 0.15 and then is decreased when x = 0.2. The coercivity is increased from 1457 to 2277 G with increasing Bi3+-doping level. The saturation magnetization, coercivity and remanent ratio for x = 0.15 sample is found to be the highest, indicating the potential of Bi3+-doping in enhancing the magnetic properties of cobalt ferrite.

Rare earth Pr3+ ions with its larger ionic radii substituted CoFe2O4 nanoparticles with x ranging from 0.0 to 0.1 were synthesized by sol–gel auto-combustion chemical method. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR) and vibrating sample magnetometer (VSM) were employed to characterize the physical properties of these ferrite nanoparticles. XRD pattern reveals the formation of cubic spinel ferrite with the signature of PrFeO3 phases for x ⩾ 0.05. SEM images show that the synthesized samples are in good homogeneity with uniformly distributed grain. The results of IR spectroscopy analysis indicated that the functional groups of cobalt spinel ferrite were formed during the sol–gel process. The cations distribution between the tetrahedral (A-site) and octahedral sites (B-site) has been estimated by XRD analysis. Room temperature magnetic measurement shows saturation magnetization and coercivity increased from 54.7 to 64.2 emu/g and 644 to 1013 Oe, respectively with the increasing Pr3+ substitution.

The role of Bi2O3 addition on the structural and magnetic properties of NiCuZn-Tb ferrites with composition Ni0.5Cu0.25Zn0.25Tb0.1Fe1.9O4+xBi2O3 has been investigated, where values of x (in wt %) are 0, 1, 2 and 3. Samples were prepared by the sol-gel auto combustion synthesis. The crystallographic, structural and magnetic properties of the samples were investigated by X-ray diffraction (XRD), scanning electron micrography, vibrating sample magnetometer and permeability measurements. Cubic spinel structure was observed from the XRD pattern for all the samples, implying that the Bi2O3 additives did not form a visible second phase in ≤ 2 wt% of Bi2O3. Microstructural studies showed that Bi2O3 forms a liquid-like phase and promotes the grain growth. The average grain size increased for ≤ 2 and then decreased for > 2 wt% of Bi2O3 content. The maximal grain size appears with 2 wt% Bi2O3. These characteristics evidently affect the magnetic properties, such as saturation magnetization (Ms) and permeability of the NiCuZn-Tb ferrites. Bi2O3 increase the Ms, bulk density (dB) and permeability effectively, while excessive Bi2O3 concentration results in the abnormal grain growth and decrease of Ms and dB.

Samples of Zr4+ and Zn2+ co-substituted Li0.5Fe2.5O4 nanoparticles were prepared by the sol–gel auto-combustion method. The samples were obtained by annealing at relatively low temperature at 600 °C and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and Fourier transform infrared (FT-IR) spectroscopy. Lattice parameter, X-ray density, specific surface area and porosity were found to increase, whereas bulk density and crystallite size showed the decreasing trend with the Zr4+ and Zn2+ substitution. Cation distribution estimated from XRD results suggests that Zn2+ and Li+ occupy tetrahedral A- and octahedral B-site respectively, whereas Zr4+ and Fe3+ occupy both the available sites. The FT-IR spectra show two major absorption bands at 715–749 cm−1 (ν1) and 484–507 cm−1 (ν2) related to spinel structure of ferrite. Magnetization results exhibit collinear ferrimagnetic structure for x ≤ 0.2, and changed to the non-collinear for x > 0.2. Observed and calculated magneton number shows discrepancy in their values that gives rise to Yafet–Kittle angle for x > 0.2.

A simple and low cost effective method involving egg white (ovalbumin) is used to synthesize the Cu 0.7 Zn 0.3 Al x Fe 2− x O 4 nanoparticles. The structural and magnetic properties of the synthesized samples were studied as a function of Al content x . The formation of crystal phase was identified by the X-ray diffraction method. All the samples confirm the formation of single phase cubic spinel structure and lattice constant that were found to decrease with the increase in Al content x . The morphology and particle size were examined by scanning electron microscopy. We report the synthesis of nanoparticle with crystallite size ranging from 25 to 33 nm. A vibrating sample magnetometer was used to obtain the hysteresis parameters. The saturation magnetization and magneton number decreased with the substitution of Al ions. The Curie temperature was determined from a.c. susceptibility plots and it is found to decrease with Al substitution.

Stoichiometric compositions of ferrites with the chemical formula Li 0.5−0.5 x Co x Fe 2.4−0.5 x Dy 0.1 O 4 with x =0, 0.25, 0.5, 0.75, 1.0 were prepared by the standard double sintering ceramic method. X-ray diffraction analysis confirmed the cubic spinel structure of the prepared samples. The structural, morphological and magnetic properties were studied by X-ray diffraction, infra-red spectroscopy (IR), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM) and ac susceptibility measurements. Lattice constant, grain size and density increase whereas porosity decreases with the increase in Co 2+ substitution. IR measurements show the characteristic ferrite bands. Spectral absorption bands were observed in IR spectroscopic analysis at ν 1 =564−601 cm −1 , ν 2 =486−519 cm −1 and ν 3 =551−578 cm −1 . The cation distribution estimated by the X-ray diffraction is supported by magnetization and susceptibility studies. The saturation magnetization decreases from 44.25 to 17.14 emu/g whereas coercivity remarkably increases from 240.69 to 812.14 emu/g with increasing Co 2+ substitution. The mechanisms involved are discussed.

The most remarkable effect in spinel ferrites is the strong dependence of properties on the state of structural disorder and, in particular, on the cation distribution. The structural characterization of a Co-Zn ferrite nanoparticle sample was reported which prepared by wet chemical co-precipitation method. The samples were sintered at three different temperatures viz. 650 °C, 850 °C and 1 050 °C for 12 h. The structural details like: lattice constant and distribution of cations in the tetrahedral and octahedral interstitial voids have been deduced through X-ray diffraction (XRD) data analysis. Lattice constant was found to increase with the increase in Zn2+ ions and sintering temperature. Theoretical intensity ratios of (220), (400), (440) planes were considered, as these reflections are sensitive to cations on the A and B sites. Close agreement of the theoretical intensity ratio with the intensity ratio observed from XRD pattern supports the occupancy of Zn2+ ions and Co2+ ions on the octahedral and tetrahedral sites, respectively.

NiAl x Fe 2− x O 4 ( x = 0.0–1.0 in the step of 0.2) spinel ferrite system was prepared by the co-precipitation method. The elemental compositional stoichiometry, microstructure, infrared spectral and elastic properties have been carried out by means of energy dispersive analysis of X-ray (EDAX), scanning electron microscopy (SEM) and infrared spectroscopic (IR) measurements. Infrared spectra were carried out at room temperature in the wavenumber range of 300–800 cm −1 . The IR spectra show three major absorption bands. High frequency bands ‘ ν 1 ’ is assigned to the tetrahedral and low frequency bands ‘ ν 2 ’ is assigned to the octahedral complex. Small frequency bands ‘ ν 3 ’ is assigned to Al 3+ O −2 complexes. Force constant for the tetrahedral and octahedral site was determined by using IR data. Force constant used to calculate the stiffness constants ( C 11 and C 12 ). Using the values of stiffness constants; elastic moduli such as Young’s modulus, bulk modulus, modulus of rigidity, Poisson ratio and Debye temperature were calculated.

Ultrafine Ni1−xCdxFe2O4 (where x = 0.0–0.5 in steps of 0.1) ferrite nanoparticles have been synthesized by the citrate gel process. The X-ray analysis confirmed the single phase cubic spinel structure of the synthesized powder. The lattice constant is found to increase with the increase in concentration of cadmium. The energy dispersive analysis of X-ray (EDAX) shows that the theoretical and observed percentage of the elements; Ni2+, Cd2+ and Fe3+ and they are in the desired stoichiometric proportion. The distribution of metal ions in the spinel ferrite system is determined from the X-ray diffraction studies. The Fourier Transform Infrared Spectra (FT-IR) of the Ni–Cd ferrite system have been analysed in the frequency range of 350–800 cm−1. FT-IR spectra revealed the two prominent frequency bands, one is high frequency band ν1 and another is law frequency band ν2 which are assigned to the tetrahedral and octahedral metal complexes respectively. Saturation magnetization decreased with the Cd2+ substitution. Resistivity of the samples was measured using the two probe technique. AC susceptibility was used to determine the Curie temperature of the samples.

Nanocrystalline ferrites with general formula NiFe 2− x Cr x O 4 ( x =0.0−1.0) were prepared by the wet chemical method. Infrared spectra were scanned in the range of 300–800 cm −1 . Three absorption bands were observed in the infrared spectra, the high frequency band ν 1 is assigned to tetrahedral complex, low frequency band ν 2 to octahedral complex and a small band ν 3 is due to Fe 3+ O 2− complex. The force constant, bond length, and the Debye temperature was determined by infrared spectra analysis. The force constant and bond length decrease with the increase in Cr 3+ content x which suggests the weakening of the inter atomic bonding. The force constant, lattice constant, and pore fraction have been used to calculate elastic moduli such as stiffness constant, Young's modulus, bulk modulus, rigidity modulus, Poisson's ratio, wave velocity and the Debye temperature. The values of elastic moduli and the Debye temperature decrease with the increase in Cr 3+ content x whereas Poisson's ratio almost remains constant. Results have been explained on the basis of inter atomic bonding.

Ni–Zn ferrite samples with the chemical formula Ni 1− x Zn x Fe 2 O 4 ( x = 0.0 to x = 1.0) were prepared by solid state reaction. Using the Rietveld refinement the discrepancy factor, interatomic distance, atomic coordinates, cation occupancy and degree of inversion have been determined. SEM images revealed that the Zn 2+ promotes grain growth in NiFe 2 O 4 . The IR spectra show two absorption bands in the wave number range of 400–600 cm −1 . The IR data have been used to calculate elastic moduli such as stiffness constant ( C 11 , C 12 ), longitudinal wave velocity ( V l ), shear wave velocity ( V S ), mean wave velocity ( V m ), Young modulus ( E ), bulk modulus ( K ), rigidity modulus ( G ), Poisson ratio ( σ ) and Debye temperature ( θ E ). The values of elastic moduli and Debye temperature decreases with increasing Zn 2+ composition x . AC susceptibility measurement confirms the decrease in Curie temperature with increasing Zn 2+ content. Dielectric loss ( e ″) increases with increasing Zn 2+ content.

Series of the ferrite samples with a chemical formula Ni 0.7 Zn 0.3 Cr x Fe 2− x O 4 ( x = 0.0–0.5) were prepared by a sol–gel auto-combustion method and annealed at 600 °C for 4 h. The prepared samples have the cubic spinel structure with no impurity phase. As the Cr 3+ content x increases, the unit cell dimensions decrease with an increase in Cr 3+ content x . The crystallite size is decreases from 37 nm to 21 nm as the Cr 3+ content increases from x = 0.0 to 0.5. Resistivity increases whereas dielectric constant decreases with an increase in Cr 3+ content x . Maxima in the dielectric loss tangent versus frequency appear when the frequency of the hopping charge carriers coincides with the frequency of the applied alternating field. Dielectric constant and dielectric loss tangent increases with increase in temperature. Saturation magnetization of sintered samples showed higher values as compared to as-prepared sample. Curie temperature deduced from AC susceptibility data decreases with increasing x .

A series of ferrite samples with the chemical formula Ni 0.7 Zn 0.3 Cr x Fe 2− x O 4 ( x = 0.0–0.5) were prepared by a sol–gel auto-combustion method and annealed at 600 °C for 4 h. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and permeability studies. The prepared samples have a cubic spinel structure with no impurity phase. As the Cr 3+ content x increases, bulk density and crystallite size decrease, whereas porosity increases. The saturation magnetization decreases linearly from 58.31 to 42.90 emu/g with increasing Cr 3+ content. However, coercivity increases with increasing Cr 3+ substitution. The magnetic moments calculated from Neel's molecular-field model are in agreement in the experiment results. The initial permeability ( μ i ) decreases with increasing Cr 3+ substitution. The decrease in initial permeability ( μ i ) is attributed to decrease in magnetization on addition of Cr 3+ . The real part of the permeability decreases gradually with increasing frequency in accordance with Snoek's law. The Curie temperature decreases linearly with increasing Cr 3+ content.

The spinel ferrite systems NiFe2−xCrxO4 (with x = 0.0–1.0 in step of 0.2) were prepared by wet chemical co-precipitation method, using sulphates of respective metal ions. Formation of single phase spinel crystal structure of sample has been confirmed by X-ray diffraction method. Magnetic parameters such as coercivity and saturation magnetization are measured by employing vibrating samples magnetometer (VSM). The saturation magnetization and magneton number (nB) decreases with increase in Cr3+ content x. The Mossbauer spectra taken at 300 K and spectra are assigned to two strongly overlapping sextets corresponding to tetrahedral A sites and octahedral B sites. The cation distribution of ferrites system has been calculated by X-ray method, the behaviour of magnetization curve and Mossbauer spectra. The evaluation of cation distribution indicates that, in Cr-dominant region, few nickel ions occupies tetrahedral A sites which convert perfect inverse spinel to partially normal spinel structure.

CoCrxFe2–xO4 (0 ≤ x ≤ 1.0) was synthesized by sol–gel auto combustion method using nitrates of respective elements and by keeping 1:3 ratio of metal nitrate to citrate. Representative sample was in...

The physico-chemical, structural and electrical properties of zinc substituted copper ferrites having the general formula Cu 1-x Zn x Fe 2 O 4(x=0.0 to x=0.8) have been studied as a function of zinc ion concentration. The sample was prepared by co-precipitation method from corresponding metal sulphates. X-ray diffraction patterns were used to confirm the structure of synthesized samples. The calculated and theoretical values of average lattice constant, tetrahedral bond, tetrahedral edge and unshared octahedral edge were found to increase, while the shared octahedral edge and octahedral bond decrease as the Zn ion concentration increases. The dielectric constant (ε′) and dielectric loss tangent ( tan δ) were measured at a constant frequency 1 kHz as a function of temperature. The dielectric constant and loss tangent were found to increase with rise in temperature. The conduction mechanism in these ferrites is discussed on the basis of electron exchange between Fe 2+ and Fe 3+ ions. The temperature dependent dc resistivity was carried out in the temperature range 300 to 800 K. The plots of log ρ versus 103/ T are linear showing two regions, corresponding to ferrimagnetic and paramagnetic regions.

The effect of Ce4+ substitution in NiFe2O4, with the chemical formula Ni1−2xCexFe2O4 (0 ⩽ x ⩽ 0.25), is presented in this paper. The Ce4+ ion increases the DC resistivity of the NiFe2O4. The dielectric constant (e′) decreases with frequency as well as with composition. Infrared spectroscopy is employed to determine the local symmetry in crystalline solids and to shed light on the ordering phenomenon. The band frequency ν1 decreases slightly with increasing Ce4+ concentration, whereas ν2 remains almost constant.

Nickel-aluminum ferrite system NiAlxFe2−xO4 has been synthesized by wet chemical co-precipitation method. The samples were studied by means of X-ray diffraction, d.c. electrical resistivity, a.c. electrical resistivity, a.c. conductivity and switching properties. The XRD patterns confirm the cubic spinel structure for all the synthesized samples. The crystallite size calculated from XRD data which confirm the nano-size dimension of the prepared samples. Electrical properties such as a.c. and d.c. resistivities as function of temperature were studied for various Al substitution in nickel ferrite. The dielectric constant and dielectric loss tangent were also studied as a function of frequency. The dielectric constant follows the Maxwell–Wagner interfacial polarization. A.C. conductivity increases with increase in applied frequency. The d.c. resistivity decreases as temperature increases, which indicate that the sample have semi-conducting nature. Verwey hoping mechanism explains the observed variation in resistivity. The activation energy is derived from the temperature variation of resistivity. Electrical switching properties were studied as I–V measurements. The current controlled negative resistance type switching is observed in all the samples. The Al substitution in nickel ferrite decreases the required switching field.

The elastic behaviour of Cr3+ substituted Co–Zn ferrites system; Co0.7Zn0.3CrxFe2−xO4 where x=0.0 to 0.5 in step of 0.1 were prepared by ceramic method. Ultrasonic Pulse transmission method at room temperature was employed to determine the elastic properties of the presently investigated samples. Values of longitudinal and shear wave velocities were increased with an increases in Cr composition x. Using these value of velocities; Young's modulus, rigidity modulus, bulk modulus, longitudinal modulus and Poisson's ratio were calculated. The values of moduli were increased with an increases in Cr composition x. Debye temperature of all sample were calculate using the Anderson equation and it shows increasing trend with an increases in Cr composition x. The behaviour elastic moduli of Co0.7Zn0.3CrxFe2−xO4 are interpreted in terms of bonding force between atoms.

The ferrite samples of a chemical formula Ni 0.5− x Mn x Zn 0.5 Fe 2 O 4 (where x =0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized by sol–gel auto-combustion method. The synthesized samples were annealed at 600 °C for 4 h. An analysis of X-ray diffraction patterns reveals the formation of single phase cubic spinel structure. The lattice parameter increases linearly with increase in Mn content x . An initial increase followed by a subsequent decrease in saturation magnetization with increase in Mn content is observed showing inverse trend of coercivity ( H c ). Curie temperature decreases with increase in Mn content x . The initial permeability is observed to increase with increase in Mn content up to x =0.3 followed by a decrease, the maximum value being 362. Possible explanation for the observed structural, magnetic, and changes of permeability behavior with various Mn content are discussed.

The electric properties of Ni1−xZnxFe2O4 ferrites (with x=0.0–0.7, in steps of x=0.1) prepared by wet chemical co-precipitation method have been studied as a function of frequency, temperature and composition. The broad XRD peaks of green samples and TEM image confirm the nano-dimensional nature of the prepared samples. Dispersion in the dielectric constant with frequency is observed and dielectric loss also shows decrease in its value with increase in frequency. The dielectric loss tangent was found to increase with frequency. Dielectric constant (e′), dielectric loss (e″) and loss tangent (tan δ) show strong temperature dependence at all frequencies. The temperature dependent results indicate that the values of e′, e″ and tan δ increase with increase in temperature.

The soft spinel ferrite system having the general formula Co 1-x Zn x Fe 2 O 4 with x varying from 0.0 to 0.7 has been prepared by wet-chemical co-precipitation technique. The prepared samples were characterized by XRD technique. The analysis of XRD pattern revealed the formation of single-phase cubic spinel structure. The Bragg peaks in XRD pattern are broader indicating fine particle nature of the sample. XRD data have been used to study structural parameter and cationic distribution in Co – Zn ferrite. The particle size is of nanometer dimension. Cation distribution results suggest that Co 2+ occupy B-site, Zn 2+ occupy A-site, and Fe 3+ occupy both the A- and B-site.

Ultrafine crystals of chromium-substituted nickel ferrite were prepared by wet chemical co-precipitation method using sulphates of respective metal ions. Formation of these materials has been confirmed by X-ray powder diffraction method. The fine crystal nature of these materials is evidenced from scanning electron microscope (SEM). Cation distribution has been investigated using X-ray diffraction technique. Cation distribution indicates that chromium occupy octahedral site for all the values of composition x. The saturation magnetization and magneton number both are decreasing with increase of chromium concentration x. The decrease in saturation magnetization and magneton number is attributed to the substitution of the Cr3+ ions. Curie temperature (TC) from susceptibility plot is found to decrease with Cr concentration x. Curie temperature of all the compositions are also obtained theoretically and it agrees with observed Curie temperature.

Morphological and magnetic characteristics of cobalt ferrite nanoparticles synthesized by sol–gel auto combustion method using nitrates of respective metal ions have been studied. X-ray diffraction pattern was indexed by a Rietveld program to calculate accurate unit cell dimension. A Transmission Electron Microscope (TEM) confirmed the formation of single phase cobalt ferrite nanoparticles in the range 11–40 nm depending on the annealing temperature and time. The size of the particles increases with annealing temperature and time while the coercivity goes through a maximum, peaking at around 25 nm. A very large coercivity (10.2 kOe) is observed on cooling down to 77 K while typical blocking effects are observed below about 260 K. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles. Mossbauer spectra recorded at room temperature is a sextet indicating that there is a strong magnetic coupling and increase in sintering temperature from 570 ∘C to 800 ∘C do not affect Mossbauer parameters.