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.     

The Influence of Annealing Temperature on the Magnetic Properties of Mn0.5Co0.5Fe2O4 Nanoferrites Synthesized via Mechanical Milling Method

Mn_0.5Co_0.5Fe₂O₄ nanosized ferrites have been made directly from MnFe₂O₄ and CoFe₂O₄ ferrites and from metal oxides by using high-energy ball milling. Single-phase formation and microstructure of the as-milled samples and samples annealed at 100, 200, 300, 400 and 500 ^°C under argon atmosphere were studied using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average grain sizes were estimated from XRD measurements and found to be between 7 and 11 nm. The microstrain for each sample was relieved by annealing due to crystallite growth. Room temperature magnetic properties were investigated by zero-field ⁵⁷Fe Mössbauer spectroscopy and vibrating sample magnetometer (VSM). Saturation magnetizations of the samples were estimated using the empirical law of approach to saturation. The variation of coercive field, saturation magnetization, maximum magnetization and remanent magnetization for each sample was found to depend on the annealing temperature. The coercive fields are observed to increase with increased annealing temperature (from about 300 Oe for the as-milled samples to about 1000 Oe for samples annealed at 500 ^°C) which we attribute to increases in grain sizes.     

Synthesis of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite and study of its magnetic behavior at different temperatures

Ni_0.5Zn_0.5Fe₂O₄ ferrite nanocrystals with average diameter in the range of 1–2 nm have been synthesized by reverse microemulsion. X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) are used to characterize the structural, morphological and magnetic properties. X-ray analysis showed that the nanocrystals possess cubic spinel structure. The absence of hysteresis, negligible remanence and coercivity at 300 K indicate the superparamagnetic character and single domain in the nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ ferrite materials. The nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ ferrite were annealed at 600 °C. As a result of heat treatment the average particle size increases from 2 nm to 5 nm and the corresponding magnetization values have increased to 21.69 emu/g at 300 K. However, at low temperature of 100 K, the annealed samples show hysteresis loop which is the characteristic of a superparamagnetic to ferromagnetic transition. In addition, a comparative study of the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄ ferrite nanocrystals obtained from reverse microemulsion has been carried out with those obtained from the general chemical co-precipitation route.     

Structural and Magnetic Studies on Chromium substituted Ni-Zn Nano Ferrite Synthesized by Citrate Gel Auto Combustion Method

Nano crystalline Ni_0.5Zn_0.5Cr_xFe_2?xO₄ particles with x varying from 0.00 to 0.25 in steps of 0.05 have been synthesized through citrate gel autocombustion method. When Ni_0.5Zn_0.5Fe₂O₄ nano particles were annealed at 1000 °C, the crystallite size increased while the lattice constant decreased slightly. For, the Cr³⁺ substituted samples annealed at 1000 °C, the variation in lattice constant, bondlengths, M_e-M_e distances and other structural parameters have been attributed to the dissimilarity in the ionic radius of the displaced (Fe³⁺) ion and the substituted (Cr³⁺) ion. Thermal studies indicated the autocombustion process which is an exothermic reaction between the nitrates salt solutions and the citric acid took place at about a temperature of 400 °C for Ni_0.5Zn_0.5Fe₂O₄. The M-H loops for all samples indicated a soft ferrite nature for all samples. The non-saturated hysteresis loop and high coercivity for the as prepared Ni_0.5Zn_0.5Fe₂O₄ nano particles has been attributed to the core-shell structure of the fine particles. When annealed at 1000 °C the saturation magnetization of Ni_0.5Zn_0.5Fe₂O₄ nano particles increased and attained the bulk value (70emu/gm). The specific saturation magnetization has been observed to decrease with increasing Cr³⁺ substitution and is ascribed to the reduction in the predominant A-B exchange interaction mechanism. By considering the site preferences cations a suitable distribution of the cations among the A & B-sites has been proposed for Ni_0.5Zn_0.5Cr_xFe_2?xO₄ nano particles annealed at 1000 °C and has been verified using the X-ray diffraction line intensity calculations. The FT-IR spectra of the annealed ferrite powders showed two significant absorption bands in the wave numbers around 400 cm^?1 & 580 cm^?1 and an additional shoulder at 360cm^?1. The position and width of the bands have been observed to vary with Cr³⁺ substitution. The results of IR spectra are in support of the proposed cation distribution.     

Semiconductor to Metallic Phase Transition in Nickel Doped Co0.5Ni X Fe(0.5−X) Fe2O4

Co_0.5Ni _X Fe_(0.5?X) Fe₂O₄ of composition (X=0.1 to 0.5) has been prepared by sol gel auto combustion citrate nitrate method. The molar ratio of metal nitrates to citric acid was maintained as 1:3 to obtain the small crystallite size. A systematic study of XRD, impedance, ac conductivity and dielectric studies of the samples were carried out in the present work. A semiconducting to metallic changeover accompanied by the grain boundary effect gradually replaced by grain contribution has been found from the impedance analysis. Confirmation of phase transition is also obtained from ac conductivity and dielectric analysis of activation energy graph where the slope changes from negative to positive. Nickel doped cobalt ferrite exhibits a semiconducting nature up to 513 K for all the compositions and underwent to metallic phase above 513 K. It is attributed to cation distribution between A and B sites as a function of temperature.     

Synthesis and electromagnetic properties of microwave absorbing material: Cox(Cu0.5Zn0.5)1−xFe2O4 (0 < x < 1) nanoparticles

Spinel ferrite Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ over a compositional range 0 < x < 1 was prepared using a simple hydrothermal method. Particle sizes could be varied from 14 to 25 nm by changing the x value. X-ray diffraction results confirmed that all the as-prepared nanoparticles revealed typical spinel structure and transmission electron microscopy images showed that the particle size of the samples increased with increasing x value. The magnetic properties of the as-prepared Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ nanoparticles have been systematically examined. The maximum saturation magnetization existed at the highest Co content (x = 1). The electromagnetic properties of all the samples have been measured by an Agilent network analyzer and the results showed that Co_0.1(Cu_0.5Zn_0.5)_0.9Fe₂O₄ possessed the best microwave absorbing properties.     

Photoluminescence and photoconductivity studies of ZnO nanoparticles prepared by solid state reaction method

In the present work, the effect of annealing temperature on the luminescence and photoconductivity properties of ZnO nanoparticles (NPs) has been investigated. The ZnO NPs have been prepared at low temperature by a simple one step solid state reaction method using ZnSO₄·7H₂O as a starting precursor. X-ray diffraction results show, the prepared samples have a hexagonal wurtzite structure of ZnO NPs. FE-SEM reveals that the prepared ZnO nanoparticles have perfect spherical shape with little agglomeration. UV–visible absorption spectrum of as-prepared ZnO sample shows an absorbance peak at ~372 nm (~3.32 eV), which is blue shifted as compared to bulk ZnO (~386 nm). The annealed sample exhibits red shift of absorption peak. The photoluminescence spectra of as-prepared sample as well as annealed samples show one emission peak in UV region, and violet, blue, blue-green and green emissions in visible region. The sample annealed at 650 °C results in a significant reduction in luminescence as compared to that of the sample annealed at 450 °C. The photoconductivity properties such as voltage dependence of photocurrent, growth and decay of photocurrent as well as wavelength dependence of photocurrent have been studied in detail.     

Synthesis of Zn0.5Mn0.5Fe2O4 by low-temperature spray pyrolysis

The effect of synthesis temperature on the structural perfection of the Zn_0.5Mn_0.5Fe₂O₄ ferrite synthesized via spray pyrolysis of a solution of Zn(II), Mn(II), and Fe(III) nitrates has been studied using X-ray diffraction, scanning electron microscopy, and IR spectroscopy. The material obtained at 650°C is shown to have a nanocrystalline structure. IR spectroscopy results indicate that the synthesized Zn_0.5Mn_0.5Fe₂O₄ spinel ferrite is highly homogeneous in composition and structure.     

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.     

Non-stoichiometric NiZn ferrite by sol-gel processing

Non-stoichiometric Ni_0.5Zn_0.5Fe_2−xO_4−3/2x ferrites over a wide range of x = 0∼0.8 are synthesized by a sol-gel processing. Phase evolution, crystal structure and crystallite size of spinel ferrites are dependent on annealing temperature and the amount of Fe deficiency. The crystallite size of spinel increases with annealing temperature and grows faster in stoichiometric ferrites than that of non-stoichiometric. Fe deficiency results in the partial reduction of spinel ferrite to zincite ZnO. XRD indicates that the crystallization temperature of ZnO is increased to about 700 °C. Zincite reduces the number of ferrite crystallites and disfavors the growth of spinel ferrites. The lattice parameters decrease with Fe deficiency and are insensitive to the variation in composition in the samples annealed at lower temperature due to the segregation of ZnO and lattice expansion in the ultrafine crystallites.     

The Transition from Paramagnetic to Ferromagnetic States as Influenced by Evolving Microstructure of Ni0.5Zn0.5Fe2O4

In this paper the effect of sintering temperature on Ni_0.5Zn_0.5Fe₂O₄ is examined closely. The evolution of toward magnetically ordered materials was to be tracked with the parallel evolving microstructure subjected to sintering temperatures in an ascending order. The starting powder of Ni_0.5Zn_0.5Fe₂O₄ was prepared via mechanical alloying and later molded into toroidal samples. After each sintering, we observed the resulting changes in the materials. The XRD data showed a single phase being formed as early as 600 °C and the peak intensity was increasing with the sintering temperature indicating an increase in the degree of crystallinity. The BH hysteresis loops showed the evolution from paramagnetism to moderate ferromagnetism to strong ferromagnetism with microstructural changes. For lower sintering temperatures, the samples showed paramagnetic behavior dominating the samples. As sintering temperature increased, paramagnetic states decreased and, at 900 °C, a moderately ferromagnetic state appeared. Sintering at 1000 °C produced a strongly ferromagnetic state giving a well-formed sigmoid-shape hysteresis loop.     

Synthesis and Magnetic Properties of Bulk Ferrites Spinels Ni0.5Zn0.5Fe2O4: Experimental an Ab-Initio Study

Polycrystalline Ni_0.5Zn_0.5Fe₂O₄ ferrites have been prepared using the solid-state reaction technique. The structure of ferrite was measured using an X-ray diffractometer (XRD). It is shown that the structure of Ni_0.5Zn_0.5Fe₂O₄ ferrites is a single spinel structure. The magnetic properties of the samples were tested at room temperature by a superconducting quantum interference device (SQUID) to determine magnetic properties versus temperature and applied magnetic field. Based on first-principles spin-density functional calculations, using the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA), the ferromagnetic and half-metallic behaviors was observed with LDA (local density approximation) and LDA–SIC (local density approximation-self-interaction correction) approximation.     

Catalytic Activity of the Spinel Ferrite Nanocrystals on the Growth of Carbon Nanotubes

We prepared three ferrite nanocatalysts: (i) copper ferrite (CuFe₂O₄) (ii) ferrite where cobalt was substituted by nickel (Ni _x Co_1?x Fe₂O₄, with x=0, 0.2, 0.4, 0.6), and (iii) ferrite where nickel was substituted by zinc (Zn _y Ni_1?y Fe₂O₄ with y=1, 0.7, 0.5, 0.3), by the sol-gel method. The X-ray diffraction patterns show that the ferrite samples have been crystallized in the cubic spinel structural phase. We obtained the size of grains by field emission scanning electron microscopy images and their magnetic properties by vibrating sample magnetometer. Next, carbon nanotubes were grown on these nanocatalysts by the catalytic chemical vapor deposition method. We show that the catalytic activity of these nanocrystals on the carbon nanotube growth depend on cation distributions in the octahedral and tetrahedral sites, structural isotropy, and catalytic activity due to cations. Our study may have applications in finding a suitable candidate of doped ferrite nanocrystals as catalysts for carbon nanotube growth. More interestingly, the yield of fabrication of carbon nanotubes can be considered as an indirect tool to study catalytic activity of ferrites.     

Preparation of cobalt-zinc ferrite (Co0.8Zn0.2Fe2O4) nanopowder via combustion method and investigation of its magnetic properties

Cobalt-zinc ferrite (Co_0.8Zn_0.2Fe₂O₄) was prepared by combustion method, using cobalt, zinc and iron nitrates. The crystallinity of the as-burnt powder was developed by annealing at 700 °C. Crystalline phase was investigated by XRD. Using Williamson-Hall method, the average crystallite sizes for nanoparticles were determined to be about 27 nm before and 37 nm after annealing, and residual stresses for annealed particles were omitted. The morphology of the annealed sample was investigated by TEM and the mean particle size was determined to be about 30 nm. The final stoichiometry of the sample after annealing showed good agreement with the initial stoichiometry using atomic absorption spectrometry. Magnetic properties of the annealed sample such as saturation magnetization, remanence magnetization, and coercivity measured at room temperature were 70 emu/g, 14 emu/g, and 270 Oe, respectively. The Curie temperature of the sample was determined to be 350 °C using AC-susceptibility technique.     

Effect of Zn<Superscript>2+</Superscript> doping on the structural,magnetic and dielectric properties of MnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> prepared by the sol–gel method

Mn_1?xZn_xFe₂O₄ (x?=?0.2–0.8) ferrite samples were successfully prepared by the sol–gel method. X-ray diffraction study reveals that single cubic spinel phase was formed in Mn_1?xZn_xFe₂O₄ samples. The SEM micrographs revealed that the microstructures change significantly with different Zn²⁺ doping concentration and sintering temperature while the grain size grow up to 9.48 μm for Mn_0.6Zn_0.4Fe₂O₄ sample sintered at 1100 °C. Further, the dielectric and magnetic measurements indicated that both Zn²⁺ doping and sintering temperature could affect both electrical and magnetic parameters such as dielectric constant and saturation magnetization in a great manner. The Mn_0.6Zn_0.4Fe₂O₄ sample sintered at 1100 °C for 8 h is found to show the largest M _s value (77.30 emu/g) in this work. These results indicate that Zn²⁺ doping or sintering temperature can adjust the microstructures, dielectric and magnetic properties of Mn_1?xZn_xFe₂O₄ ferrites.     

Synthesis,phase formation study and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowder prepared by mechanical milling

In this work we report the phase formation and magnetic properties of CoFe₂O₄ nanopowder prepared by mechanical alloying technique using metallic cobalt and hematite powder (1:1 molar ratio) as the initial raw material in ambient air atmosphere. The formation of single phase cobalt ferrite of (Co _0.18 ²⁺ Fe _0.82 ³⁺ )[Co _0.82 ²⁺ Fe _1.18 ³⁺ ]O₄ stoichiometry was confirmed for the samples milled above 15 h without any heat-treatment by XRD and Mössbauer techniques. The average crystallite size of the sample milled for 30 h was ～13 nm. The highest room temperature value of the magnetization measured at 1.5 T was 51 e.m.u/g for the sample milled for 25 h which was much lower than the corresponding value of the bulk cobalt ferrite (80.8 e.m.u/g at 300 K) due to the size effect.     

The preparation and microwave properties of Ba3ZnZCo2-ZFe24O41/SiO2 microcrystalline glass ceramics by citrate sol-gel process

A series of Ba₃Zn_ZCo_2-ZFe₂₄O₄₁/SiO₂ microcrystalline glass ceramics with Z=0.0,0.4,0.8 and 1.2 were prepared by citrate sol-gel process. The result showed that Ba₃Zn_ZCo_2-ZFe₂₄O₄₁ hexferrite crystallites could be obtained by this process at 1200 °C in the system of BaO-Fe₂O₃-CoO-ZnO-SiO₂. The complex dielectric constant and complex permeability of Ba₃Zn_ZCo_2-ZFe₂₄O₄₁/SiO₂ microcrystalline glass ceramics calcined at different temperature were measured in the range of 200MHz-6 GHz by transmission/reflection coaxial line method. The complex dielectric constant and dielectric loss exhibited insignificant variety in the whole range of measuring frequencies for all samples. The real part of the permeability decreased as the measuring frequency increasing, and calcining temperature had a clear influence on the value of μ′ for Ba₃Zn_ZCo_2-ZFe₂₄O₄₁/SiO₂ microcrystalline glass ceramics, and so did the content of Zn²⁺ and Co²⁺. The natural resonance phenomenon was observed in μ′′ spectra for all the Ba₃Zn_ZCo_2-ZFe₂₄O₄₁/SiO₂ microcrystalline glass ceramics. The substitution of Zn²⁺ ion and annealing temperature closely affect the resonance frequency, the more the Zn²⁺ ion, the higher the annealing temperature and the lower the resonance frequency. Received: 23 July 2001 / Accepted: 24 July 2001     

Effect of annealing on the structure and magnetic property of Zn0.9Fe0.1S nanoparticles

Zn_0.9Fe_0.1S nanoparticles were synthesized in microemulsion and then annealed in nitrogen atmosphere at different temperatures. The XRD results show that the samples annealed below 300 °C are pure cubic ZnS. Higher annealing temperature introduces the Fe₃O₄ phase. The X-ray absorbing fine structure (XAFS) results indicate that annealing induces the Fe ions to change from divalence to trivalence and crystalline quality gradually improves with the increase of annealing temperature. In view of the structural characteristics indicated by XRD and XAFS, the origin of ferromagnetism at low annealing temperature may be attributed to trivalent Fe ions.     

Bulk magnetic properties of Co-Zn ferrites prepared by the co-precipitation method

The alternating current (a.c.) susceptibility versus temperature and magnetization measurements are reported for the disordered spinel ferrite system Zn _x Co_1-x Fe₂ O₄ prepared by a wet chemical method before and after high temperature annealing. The low field a.c. susceptibility measurements indicate that the low temperature synthesis of wet prepared Co-Zn ferrites aids the formation of spin-clusters and thereby increases the magnetic inhomogeneity. The X-ray analysis shows that the samples are single phase spinels and the variation of lattice constant with zinc concentration deviates from Vegard's law [1]. The high temperature annealing changes the wet prepared ferrites into the ordered magnetic structure of the ceramic ferrites.     

Cellulose-precursor synthesis of nanocrystalline Co0.5Cu0.5Fe2O4 spinel ferrites

Nanocrystalline Cu_0.5Co_0.5Fe₂O₄ powders were prepared via a metal-cellulose precursor synthetic route. Cellulose was used as a fuel and a dispersing agent. The resulting precursors were calcined in the temperature range of 450–600 °C. The phase development of the samples was determined by using Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD). The field-dependent magnetizations of the nanopowders were measured by vibrating sample magnetometer (VSM). All XRD patterns are of a spinel ferrite with cubic symmetry. Microstructure of the ferrites showed irregular shapes and uniform particles with agglomeration. From XRD data, the crystallite sizes are in range of 16–42 nm. Saturation magnetization and coercivity increased with increasing calcining temperature due to enhancement of crystallinity and reduction of oxygen vacancies.