Mixed Spinel Structure Research Articles

Structural, impedance and Mössbauer studies of magnesium ferrite synthesized via sol–gel auto-combustion process

Crystalline magnesium ferrite (MgFe2O4) spinel oxide powder was synthesized by nitrate–citrate sol–gel auto-combustion process with stoichiometric composition of metal nitrate salts, urea and citric acid. The study was focused on the modification of synthesis conditions and effect of these modified conditions on the structural and electrical properties of synthesized MgFe2O4 ceramic materials. Phase composition, crystallinity, structure and surface morphology were studied by X-ray diffraction, FTIR and SEM. Pure single phase MgFe2O4 spinel ferrite was obtained after calcination at 850°C. Rietveld refinement of XRD result confirmed the single cubic phase spinel oxide with the lattice constant of a=8.3931Å and Fd3m symmetry. UV–visible absorption study of calcined powder revealed an optical band gap of 2.17eV. SEM images of sintered specimens (1050–1450°C) showed that the grain size increased with the increase in sintering temperature. From the impedance results of the sintered MgFe2O4 specimens, it was found that the resistance of grain, grain boundary and electrode effect decreased with an increase in sintering temperature and associated grain growth. In the intermediate frequency region lowering of impedance and dielectric values was observed due to the decrease in grain boundary areas. Mössbauer studies indicated that magnesium ferrite had a mixed spinel structure in calcined and sintered samples, however, the well refined single phase MgFe2O4 was observed due to well developed high crystalline structure at 1350°C and 1450°C.

Effects of pH Value, Reaction Time, and Filling Pressure on the Hydrothermal Synthesis of ZnFe<sub>2</sub>O<sub>4</sub> Nanoparticles

We investigated the effect of the pH value, reaction time, and filling pressure on the preparation of ZnFe2O4 (ZFO) nanoparticles using a hydrothermal synthesis. X-ray diffraction showed that all three parameters should be controlled in order to prepare ZFO nanoparticles without impurities. Furthermore, the nanoparticle size reduced from 60 to 5 nm when the filling pressure was increased from 50% to 70%. The X-ray photoelectron spectroscopy showed that the ZFO nanoparticles actually possess a mixed spinel structure due to the coexistence of Fe2+ and Fe3+, and there were significant amounts of oxygen vacancies near the surface. The cation inversion and surface effect worked together to determine the magnetic properties of the ZFO nanoparticles. The ZFO nanoparticles of 60 nm sizes showed a much enhanced room temperature ferromagnetism together with a larger coercivity compared with that of the ZFO nanoparticles of 5 nm sizes. Our results provide valuable information regarding the optimization of the hydrothermal synthesis of ferrite nanoparticles for desired applications.

Structural phase change in Co2.25Fe0.75O4 spinel oxide by vacuum annealing and role of coexisting CoO phase on magnetic properties

The spinel oxide Co2.25Fe0.75O4 has been prepared by co-precipitation route. The as prepared material has been annealed at 800 °C under vacuum. Properties of the samples annealed under vacuum have been compared with the sample annealed in air. Synchrotron X-ray diffraction, Raman spectroscopy and X-ray absorption near edge structure spectroscopy have been used for structural phase characterization. The cubic spinel structure in air annealed sample has been splitted into Co and Fe rich phases. The Co rich spinel phase has transformed into Cobalt monoxide (CoO) after vacuum annealing. This has resulted an increase of Fe:Co ratio in the Fe rich spinel phase. Raman spectra indicated a transformation of the distribution of Co and Fe ions from normal spinel like structure for the sample annealed in air to mixed spinel structure for the samples annealed under vacuum. The existence tetravalent Fe4+ ion has been confirmed in all the samples and it is new information for Co doped spinel ferrite system. All the samples exhibited ferrimagnetic behavior and exchange bias effect. The focus point is that vacuum annealing has been used as an effective route of material processing to control the structural phase and also, enhancement of magnetic properties in Co rich ferrite.

Studies on structural and electrical properties of Li0.5−0.5xCoxFe2.5−0.5xO4 (0≤x≤0.6) spinel ferrite

In the present work, nanocrystalline Li0.5−0.5xCoxFe2.5−0.5xO4 (for x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ferrite systems were synthesized by solution combustion method. The Rietveld analysis of X-ray result confirms the formation of a single phase spinel cubic crystal structure of the ferrite sample. The lattice constant of the material increases from 8.33Å to 8.36Å with increasing cobalt content in lithium ferrite. The cation distribution study reveals that the Co–Li ferrite is in the mixed spinel structure of the composition. The DC electrical resistivity result confirms the semiconducting nature and the Curie temperature decreases with increase in Co2+ content. The dielectric constant, loss tangent and dielectric loss decrease with frequency and remain constant at higher frequencies are observed, showing the usual dielectric dispersion due to space charge polarization. The impedance spectroscopy analysis of samples reveals the grain interior contribution in the conduction process. The AC conductivity as a function of frequency verifies that the small polarons are responsible for conduction process.

Cation distribution and enhanced surface effects on the temperature-dependent magnetization of as-prepared NiFe2O4 nanoparticles

Nickel ferrite, i.e., NiFe2O4, nanoparticles are synthesized by sol–gel method using urea as a neutralizing agent. The formation of spinel phase and crystal structure of the as-prepared sample is analyzed by X-ray diffraction and transmission electron microscope. In order to confirm phase formation and cation arrangement, room temperature 57Fe Mossbauer spectroscopy is employed. The degree of inversion (i) estimated from the relative peak area is found to be 0.6, which confirms a mixed spinel structure of the as-prepared sample. Zero-field-cooled/field-cooled measurements showed evidence of superparamagnetic behavior associated with the nanosized particles. Hysteresis loop measurements revealed temperature-dependent magnetic properties: The coercive field (H C) decreases with increasing temperature and deviates from the Kneller’s law for ferromagnetic nanostructures; and the saturation magnetization (M s) follows modified Bloch’s law in the temperature range between 25 and 400 K. However, below 25 K, an abrupt increase in magnetization of nanoparticles is observed. In order to understand this behavior, an additional contribution has to be added to the core magnetization to properly fit the data. Hence, a surface correction term to the Bloch’s law is found to describe the temperature dependence of magnetization in the core–shell NiFe2O4 nanoparticles.

Accelerating the Discovery of Multivalent Cathode Materials Via High-Throughput First-Principles Calculations

Batteries that shuttle multi-valent ions such as Mg2+ and Ca2+ ions are promising candidates for achieving higher energy density than available with current Li-ion technology. Finding electrode materials that reversibly store and release these multi-valent cations is considered a major challenge for enabling such multi-valent battery technology. In this work, we employ the recent advances in high-throughput first-principles calculations1 to design and evaluate multivalent intercalation cathode materials so that we are able to quickly seek and assess the feasibility of multi-valent insertion cathodes from an in silico big data approach prior to experimental synthesis and characterization. It is our hope that our research provides general guidelines for multi-valent cathode material engineering and moreover, can greatly expedite the material discovery process. We use density functional theory calculations (VASP code) combined with the robust high-throughput infrastructure1 (pymatgen, pymatpro and FireWorks codes) to automatically calculate and analyze the properties and viabilities of possible multivalent cathode compounds. Based on the calculated results, we estimate the insertion voltage, capacity, thermodynamic stability and thermal stability of charged and discharged states. Therefore, the compounds with both good energy storage capability and superior structural stability can be identified as promising candidate materials for future experimental investigation and validation. To date, we have calculated and evaluated over 1800+ newly designed structures with five possible working ions (Mg, Ca, Zn, Y, Al). In general, we find that multivalent cathodes exhibit lower voltages compared to Li cathodes; the voltages of Ca coumpounds are ~0.2-0.5V higher than those of Mg compounds; Zn compounds usually have much lower voltage (Fig. 1). As a showcase, we systemically evaluate the theoretical performance of the spinel structure host with the general formula AB2O4 across a matrix of chemical compositions spanning A={Al, Y, Mg, Ca, Zn} and B={Ti, V, Cr, Mn, Fe, Co, Ni} for multivalent cathode applications.2 As shown in Fig 2, considering all computed properties, Mn2O4 spinels are particularly interesting due to their stability. Among the divalent cations, both Mg2+ and Ca2+ may potentially be mobile in the spinel structure2,3, warranting further experimental and computational investigation (particularly at small particle sizes). Mixed spinel structures provide a promising avenue, as the Ni4+/3+ and Co4+/3+ show higher voltage than the Mn4+/3+ redox couple. Acknowledgement: This work was supported by Joint Center for Energy Storage Research (JCESR) and the infrastructure and algorithmic was supported by the Materials Project (BES DOE Grant No. EDCBEE). We also thank NERSC for providing computation resources.

Synthesis and Characterization of CoxZn1−xAlFeO4 Nanoparticles

Nanocrystalline powders of cobalt and aluminum co-substituted zinc ferrites with general formula CoxZn1−xAlFeO4 (x = 0.0–1.0) have been synthesized for the first time. Using the citrate-microwave technique and the citric acid as combustion–complexion agent (fuel), materials with spinel mono-phase cubic spinel structure were successfully prepared. The characterization of products was done by XRD, SEM and VSM. The crystallite size estimated by Scherrer formula has been found in the range of 7.7–9.6 nm. The magnetic properties were studied by room temperature (RT) VSM magnetization measurements. The small remanent magnetization (Mr) and coercivity (Hc) values reveal the superparamagnetic nature of nanoparticles (NPs) at RT. The extrapolated saturation magnetization (Ms) is maximum for Co0.8Zn0.2AlFeO4 (17.15 emu/g) and minimum for ZnAlFeO4 particles (4.22 emu/g). This case is attributed to high or low amount of cation distribution change from normal to mixed spinel structure. The average magnetic diameters (D mag) were calculated from magnetic fit studies of M–H spectra. D mag values are between 8.17 and 8.46 nm and this range is in great accordance with the obtained diameters from XRD measurements. The small Mr/Ms ratios (maximum, 0.219) specify the uniaxial anisotropy according to Stoner–Wohlfarth model for CoxZn1−xAlFeO4 NPs. RT effective anisotropy constants (K eff ) were calculated by using Ms and Hc values. K eff constants increased with increasing Co content in the spinel NPs.

Structural, magnetic, dielectric and optical properties of nickel ferrite nanoparticles synthesized by co-precipitation method

Nickel ferrite nanoparticles were synthesized by wet chemical co-precipitation method and the corresponding temperature dependent structural, magnetic and optical properties of these nanoparticles have been investigated. X-ray diffraction patterns show the single phase cubic spinal crystal structure belonging to the space group Fd3m. The average crystallite size varies in the range 8–20nm with varying sintering temperature. Raman spectroscopy exhibits a doublet-like peak behaviour which indicates the presence of mixed spinel structure. The saturation magnetization, coercivity and remanence increase with increasing sintering temperature from 250 to 550°C. The non-saturation and low values of magnetization at high fields indicate the strong surface effects to magnetization in NiFe2O4 nanoparticles. The g-value calculated from electron spin resonance spectrum indicates the transfer of divalent metallic ion from octahedral to tetrahedral site (i.e. mixed spinel structure). The dielectric permittivity, loss tangent and ac conductivity measurements show strong temperature dependence at all frequencies. The observed ac conductivity response suggests that the conduction in ferrite nanoparticles is due to feeble polaron hopping between Fe3+/Fe2+ ions. Room temperature UV–vis diffuse spectra indicate that NiFe2O4 is an indirect band gap material with band gap ranges from 1.27 to 1.47eV with varying sintering temperature. The photoluminescence study clearly indicates that the Ni2+ ions occupy both octahedral and tetrahedral sites confirming mixed spinel structure.

Magnetic properties of mixed spinel BaTiO3-NiFe2O4 composites

Solid solution of nickel ferrite (NiFe2O4) and barium titanate (BaTiO3), (100-x)BaTiO3–(x) NiFe2O4 has been prepared by solid state reaction. Compressive strain is developed in NiFe2O4 due to mutual structural interaction across the interface of NiFe2O4 and BaTiO3 phases. Quantitative analysis of X-ray diffraction and X-ray photo electron spectrum suggest mixed spinel structure of NiFe2O4. A systematic study of composition dependence of composite indicates BaTiO3 causes a random distribution of Fe and Ni cations among octahedral and tetrahedral sites during non-equilibrium growth of NiFe2O4. The degree of inversion decreases monotonically from 0.97 to 0.75 with increase of BaTiO3 content. Temperature dependence of magnetization has been analyzed by four sublattice model to describe complex magnetic exchange interactions in mixed spinel phase. Curie temperature and saturation magnetization decrease with increase of BaTiO3 concentration. Enhancement of strain and larger occupancy of Ni2+ at tetrahedral site increase coercivity up to 200 Oe. Magnetostructual coupling induced by BaTiO3 improves coercivity in NiFe2O4. An increase in the demagnetization and homogeneity in magnetization process in NiFe2O4 is observed due to the interaction with diamagnetic BaTiO3.

Characterization of partially inverse spinel ZnFe2O4 with high saturation magnetization synthesized via soft mechanochemically assisted route

ZnFe2O4 was prepared by a soft mechanochemical route from two starting combinations of powders: (1) Zn(OH)2/α-Fe2O3 and (2) Zn(OH)2/Fe(OH)3 mixed in a planetary ball mill. The mechanochemical treatment provoked reaction leading to the formation of the ZnFe2O4 spinel phase that was monitored by XRD, TEM, IR and Raman spectroscopy. The spinel phase was first observed after 4h of milling and its formation was completed after 18h in both the cases of starting precursors. The synthesized ZnFe2O4 has a nanocrystalline structure with a crystallite size of about 20.3 and 17.6nm, for the cases (1) and (2), respectively. In the far-infrared reflectivity spectra are seen four active modes. Raman spectra suggest an existence of mixed spinel structure in the obtained nanosamples. In order to confirm phase formation and cation arrangement, Mössbauer measurements were done. Estimated degree of inversion is about 0.58 for both starting mixtures. The magnetic properties of the prepared ZnFe2O4 powders were also studied. The results show that the samples have a typical superparamagnetic-like behavior at room temperature. Higher values of magnetization in the case of samples obtained with starting mixture (2) suggest somewhat higher degree of cation inversion.

Study of structural, electrical and magnetic properties of Cr doped Ni–Mg ferrite nanoparticle

We have reported the effect of Cr3+ ion doping on the structural, ac transport and Mössbauer spectral studies of ferrite nanoparticles Ni0.5Mg0.5Fe2−xCrxO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) prepared by sol–gel auto combustion technique. The micro-structural studies of the samples were obtained by using scanning electron microscopy (SEM). The dependence of Mössbauer parameters, viz., line width, isomer shift, quadrupole splitting and hyperfine magnetic field on Cr3+ substitution have been analysed. Hyperfine magnetic field and isomer shift values decreases whereas line width increases with Cr3+ substitution. Rietveld refinement and Mössbauer spectroscopy studies reveal that the Ni0.5Mg0.5Fe2−xCrxO4 with various concentrations of x belongs to mixed spinel structure. Nèel’s molecular field model is used to explain the variation of hyperfine magnetic field with increasing Cr3+ substitution.

Exploring the structural, Mössbauer and dielectric properties of Co2+ incorporated Mg0.5Zn0.5−xCoxFe2O4 nanocrystalline ferrite

The nanocrystalline Mg0.5Zn0.5−xCoxFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) ferrites have been synthesized by sol–gel auto ignition method. Rietveld refinement of X-ray diffraction (XRD) patterns of all concentrations show mixed cubic spinel structure. The lattice constant values infer decreasing trend from 8.420 to 8.392Å with the substitution of Co2+. The crystallite size calculated from Scherer formula lies in the range between 28 and 37nm which confirms the nanocrystalline nature of synthesized samples. In order to study the morphology and phase structure of the synthesized samples, samples were examined by transmission electron microscopy (TEM). Scanning electron microscopy (SEM) confirmed the homogeneous and well defined surface morphology of the synthesized samples. The elemental analysis as obtained from energy dispersive X-ray (EDAX) is in close agreement with the expected composition from the stoichiometry of reactant solutions used. The valence states of the Co2+ ions have been confirmed with the help of X-ray photoelectron spectroscopy. Magnetization results obtained from the vibrating sample magnetometer (VSM) confirm that the substitution of Co2+ for Zn2+ caused an increase in the saturation magnetization and coercivity. The dependence of Mössbauer parameters, viz., line width, isomer shift, quadrupole splitting and hyperfine magnetic field on Co2+ substitution have been analyzed. Cation distribution estimated from XRD and Mössbauer spectroscopy are in good agreement with each other. The frequency dependent dielectric properties were studied by measuring dielectric constant (ε׳) and ac conductivity (σac) at 300K in the frequency range of 50Hz–5MHz. The highest value for ε׳ and σac is observed for x=0.5.

Structural, optical and magnetic studies of Ce doped NiFe2O4 nanoparticles

Structural, optical and magnetic properties of NiCexFe2−xO4 (x=0.0–0.10) nanoparticles prepared by the chemical route, have been studied. All the samples exhibit pure spinel phase except that with x=0.10. Crystallite size decreases gradually from 57 to 17nm as the Ce concentration increases from 0 to 0.10. Fourier transform infrared spectroscopy shows that in comparison to pure nickel ferrite, the bands corresponding to Fe–O vibration are shifted towards higher wave number after doping with Ce. The Mössbauer spectra for all the compositions show the presence of two sextets at room temperature showing the ferrimagnetic character of the samples. Magnetic hysteresis curves show that the saturation magnetisation changes from ∼41.8emu/g for pure nickel ferrite to ∼31.6emu/g after doping with Ce=0.10. However, the variation of magnetic moment with Ce concentration is irregular. The cation distribution in the samples shows the mixed spinel structure.

Effect of non-magnetic substitution on the structural and magnetic properties of spinel cobalt ferrite (CoFe2−xAlxO4) ceramics

One set of CoFe2−xAlxO4 (x = 0.1, 0.2, 0.3 and 0.4) ceramics in single domain range and another set in multi domain range were prepared by sintering at 400 and 800 °C respectively. Rietveld refinement of XRD patterns revealed that samples are mixed spinel structure and crystallize to $$ Fd\bar{3}m $$ space group with cubic symmetry. The lattice constant and crystallite size decrease with the increase in Al3+ concentration. The Fourier transform infrared spectroscopy study confirms the spinel structure of the samples. Particle size distribution for all samples has been studied by field emission scanning electron microscopy. Moreover, thermogravimetric analysis was performed to determine the sintering temperature. The saturation magnetization has been analyzed by the “law of approach ” technique. Saturation magnetization, coercivity and magnetocrystalline anisotropy constant are found to decrease with the increase in Al3+ ion concentration which is due to the non-magnetic impurity in the magnetic matrix.

Effect of geometry on the magnetic properties of CoFe2O4–PbTiO3 multiferroic composites

In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element- and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4–PbTiO3 (CFO–PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A- and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L3,2-edge XMCD measurements of the CFO–PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO–PTO composites.

Magnetic properties of nanocrystalline CoFe2O4 synthesized by thermal plasma in large scale

The paper reports the large scale synthesis of nanoparticles of CoFe2O4 using thermal plasma reactor by gas phase condensation method. The yield of formation was found to be around 15 g h−1. The magnetic properties of CoFe2O4, synthesized at different reactor powers, were investigated in view of studying the effect of operating parameters of plasma reactor on the structural reorganization leading to the different cation distribution. The values of saturation magnetization, coercivity and remanent magnetization were found to be influenced by input power in thermal plasma. Although the increase in saturation magnetization was marginal (61 emu g−1 to 70 emu g−1) with increasing plasma power; a significant increase in the coercivity (552 Oe to 849 Oe) and remanent magnetization (16 emu g−1 to 26 emu g−1) were also noticed. The Mössbauer spectra showed mixed spinel structure and canted spin order for the as synthesized nanoparticles. The detailed analysis of cation distribution using the Mössbauer spectroscopy and X-ray photoelectron spectroscopy leads to the conclusion that the sample synthesized at an optimized power shows the different site selective states.

MFe2O4 (M: Co2+, Ni2+) Nanoparticles: Mössbauer and X-ray Absorption Spectroscopies Studies and High-Temperature Superparamagnetic Behavior

MFe2O4 (M: Co2+, Ni2+) nanoparticles with different sizes and crystal-chemistry have been synthesized under solvothermal conditions. The inversion degree (x) of the mixed spinel structure (Fe1–xMx)[Fe1+xM1–x]O4 has been investigated by XAS and Mossbauer spectroscopies obtaining values between 0.20 and 0.30 in the case of Co-ferrite samples and between 0.00 and 0.16 for the Ni-ferrite ones. In order to have a better understanding of the superparamagnetic behavior of the nanosized samples, it was necessary to do high-temperature magnetic measurements. Although Co-ferrite samples present higher magnetization values, their effective superparamagnetic moment is similar to those found for the Ni-ferrite ones. This suggests that the dipolar interactions may be stronger in the Ni-ferrite system, probably due to a less-effective anisotropy that can be considered an impediment to the particle interactions. This fact is supported by the effective volume/single-particle volume ratio that presents similar values for b...

Microstructure and magnetic properties of substituted (Cr, Mn) - cobalt ferrite nanoparticles

Three mixed ferrite systems CoFe2O4, CoCr0.2Fe1.8O4 and CoMn0.2Fe1.8O4 were prepared by the co-precipitation method. X-ray diffraction (XRD) of the calcined powders at 900 °C confirms the accomplishment of the cubic spinel phase formation without any secondary phase. The cation distribution was determined from XRD data and suggested a mixed spinel structure. The higher value of the lattice parameter for Mn comparing with Cr substituted cobalt ferrite could be explained by migration of Co2+ cation from octahedral to tetrahedral sites. Infrared spectroscopy (FTIR) showed two absorption bands attributed to intrinsic vibrations of tetrahedral and octahedral complexes. The bands shifted toward lower frequency for doped samples comparing with corresponding Co-host ferrite frequencies. The microstructure was investigated by scanning electron microscopy (SEM) and suggested that the nanoparticles are agglomerated and have polygonal faced surfaces. The effect of Mn3+ and Cr3+ cation distribution among the tetrahedral (A) - and octahedral [B] - sites of Co substituted ferrite on magnetization and coercive field was investigated by vibrating sample magnetometer technique. Higher values of the coercive field of manganese and chromium substituted cobalt ferrite was explained by lower value of the average particle size. The small increase of saturation magnetization for doped ferrites was explained considering the oxidation state of the substituting ions.

Microstructure Characterization of Nanocrystalline Magnesium Ferrite Annealed at Elevated Temperatures by Rietveld Method

Nanocrystalline magnesium ferrite is synthesized at room temperature by high energy ball milling the MgO and α-Fe2O3 (1 : 1 mol fraction) powders. The Rietveld structure and microstructure refinements of X-ray powder diffraction data of 9 h milled sample reveal the presence of mixed and nearly inverse spinel nanocrystalline Mg-ferrite phases. Postannealing of nanocrystalline powder within 873–1473 K reveals continuous change in cation distribution among the tetrahedral and octahedral sites of mixed spinel lattice leading to nearly inverse spinel structure with increasing temperature. Mixed spinel structure finally transformed into inverse spinel structure after 1 h of annealing at 1073 K. The ferrite phase becomes completely stoichiometric by solid-state diffusion of unreacted (∼0.3 mol fraction) α-Fe2O3 into spinel lattice after 1 h of annealing at 1273 K. Interestingly, particle sizes of ferrite phases do not increase considerably up to 1073 K and increase suddenly after transformation of mixed spinel into nearly inverse spinel structure.

Metal-semiconductor transition in NiFe2O4 nanoparticles due to reverse cationic distribution by impedance spectroscopy

We have investigated the magnetic and electrical response of the sol-gel synthesized NiFe2O4 nanoparticles. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of the material. Thermally activated hopping carriers between Fe3+-Fe2+ and Ni2+-Ni3+ ions have been determined for a decrease in the resistance of the sample and a change in the conduction mechanism around 318 K. The mixed spinel structure and broken exchange bonds due to small size effects are due to the canted spin structure at the surface of the nanoparticles. The magnetization is found to be influenced by the surface spin canting and anisotropy. We have established the semiconducting to metallic transition (SMT) temperature to be around 358 K in terms of localized and delocalized eg electrons along with a transition from less conductive [Fe3+–O2−–Fe3+] and [Ni2+–O2−–Ni2+] linkage to more conductive [Fe3+–Fe2+] and [Ni2+–Ni3+] linkage at the octahedral B site. A decrease in the dielectric constant with temperature has been discussed in terms of the depletion of space charge layers due to the repulsion of delocalized eg electrons from the grain boundary planes. The anomalies in tangent loss and conductivity data around 358 K are discussed in the context of the SMT.