Solvothermal one-step synthesis of MWCNTs/Ni0.5Zn0.5Fe2O4 magnetic composites

A magnetic multi-walled carbon nanotubes-based (MWCNTs-based) composite, MWCNTs/Ni_0.5Zn_0.5Fe₂O₄, was synthesized via a facile solvothermal approach. The composites were characterized by X-ray diffraction analysis, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and vibrating sample magnetometry. The results confirmed that MWCNTs and Ni_0.5Zn_0.5Fe₂O₄ coexisted in the composites. The TEM and HRTEM results showed a thick layer of Ni_0.5Zn_0.5Fe₂O₄ was intimately connected to the surface of MWCNTs. The saturation magnetization value of the composites was 45.8 emu/g. Furthermore, the probable synthesis mechanism of the magnetic composites was also investigated based on the experimental results.     

一维Ni0.5Zn0.5Fe2O4/SiO2复合纳米结构的制备及其磁性能

采用溶胶-凝胶法结合静电纺丝技术制备了Ni_0.5Zn_0.5Fe₂O₄/SiO₂复合纳米纤维.利用热重-差热分析、X射线衍射、场发射扫描电镜、高分辨透射电镜和振动样品磁强计研究了前驱体纤维的热分解及相转化过程以及焙烧温度和SiO₂含量对目标纳米纤维的相组成、微观结构、形貌及磁性能的影响.结果表明,在450 ℃焙烧时,立方尖晶石结构已基本形成.随着焙烧温度由450 ℃升高到100     

Dependence of magnetic properties and microstructure of mechanically alloyed Ni0.5Zn0.5Fe2O4 on soaking time

Possible soaking-time effects on the magnetic and microstructural properties of polycrystalline samples of Ni_0.5Zn_0.5Fe₂O₄ have been studied. Nanosize powder produced by mechanical alloying was sintered at 800 °C with various soaking times. All samples showed the signature peak of Ni_0.5Zn_0.5Fe₂O₄ even with one hour of soaking time. The size distributions show a slow growth of microstructural evolution related to density, porosity and also to the magnetic hysteresis loops. Within these distributions it is observed that the formation of multi-domains is not possible and probably there are the regions of superparamagnetic and single-domain grains. From the permeability studies, it is believed that the rise of the magnetic moment on the B sites give rise to the total saturation magnetization with increase of soaking time. The hysteresis loop of one-hour soaking time showed paramagnetic behavior dominating while longer soaking times showed ferromagnetic behavior starting to dominate. The coercivity was observed to increase with soaking time, signaling the increase of the anisotropy fields which was attributed to the shape anisotropy and also to the magnetocrystalline anisotropy. By correlating the morphology, phase analysis, permeability and hysteresis loops results, it is believed that there was an increase in number of crystalline-growth regions which together formed a total mass of mixed superparamagnetic and ferromagnetic grains with the latter starting to dominate the samples.     

Low-temperature synthesis and growth of superparamagnetic Zn0.5Ni0.5Fe2O4 nanosized particles

Mono-disperse spinel Ni_0.5Zn_0.5Fe₂O₄ nanosized particles have been synthesized via a hydrothermal method at low temperature. X-ray diffraction (XRD), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) analysis indicated that the synthesized nanocrystals were of pure cubic spinel structure with the size about 6-20 nm. The activation energy of grain growth is 35.06 kJ/mol experimented by the Arrhenius equation. A primary experimental model was put forward to shed light on the growth mechanism of crystallined spinel Ni-Zn ferrite nanosized particles under hydrothermal conditions. The magnetic measurements shows that the prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticle possess good superparamagnetic behavior.     

Surface magnetic disorder in nanostructured Ni0.5Zn0.5Fe2O4 particles

Nanostructured ferroxide particles with initial formula Ni_0.5Zn_0.5Fe₂O₄ are investigated. The aim was to explore the monodomain and the superparamagnetic states of the ferrospinel and the impact of the surface magnetic disorder on the magnetization processes. Mössbauer spectroscopy (MöS) demonstrated that the ion distribution follows the general formula (Zn_0.5Fe_0.5)_A[Ni_0.5Fe_1.5]_BO₄, where A is the tetrahedral and B, the octahedral sublattice. MöS in an external magnetic field (5 T) at 4.2 K shows non-collinearity of the sublattices’ magnetic moments and deviations in the hyperfine magnetic field that could be related to a canting effect. Magnetic measurements were applied to characterize the temperature behavior of the magnetic properties and the a.c. complex magnetic susceptibility.     

Maxwell-Wagner characterization of dielectric relaxation in Ni0.8Zn0.2Fe2O4/Sr0.5Ba0.5Nb2O6 composite

Dense composites were prepared through incorporating the dispersed Ni_0.8Zn_0.2Fe₂O₄ ferromagnetic particles into Sr_0.5Ba_0.5Nb₂O₆ ferroelectric matrix. Extrinsic dielectric relaxation and associated high permittivities of the materials are reported in the composites. We used an ideal equivalent circuit to explain electrical responses in impedance formalism. A Debye-like relaxation in the permittivity formalism was also found. Interestingly, real permittivity (ε′) of the sample containing 30% Ni_0.8Zn_0.2Fe₂O₄ shows obvious independence of the temperature at 100 kHz. Dielectric relaxation and high-ε′ properties of the composites are explained in terms of the Maxwell-Wagner (MW) polarization model.     

Structural,magnetic and dielectric properties of (Li1+, Al3+) co-doped Ni0.5Zn0.5Fe2O4 ferrite ceramics prepared by the sol-gel auto-combustion method

(Li¹⁺, Al³⁺) co-doped Ni_0.5Zn_0.5Fe₂O₄ ferrites, Ni_0.5-xZn_0.5-xLi_xAl_xFe₂O₄ (x = 0.000, 0.025, 0.050 and 0.100), were synthesized by the sol-gel auto-combustion method. X-ray diffraction (XRD), field emission scanning electronic microscope (FESEM), vibrating sample magnetometer (VSM) and LCR meter were used to investigate the structural, magnetic and dielectric properties. Results of XRD and SEM indicate that both doping amount and calcination temperature play significant roles in crystal structure and grain growth. Also, it can be observed that the saturation magnetization and the coercivity change in a noticeable manner. The Ni_0.475Zn_0.475Li_0.025Al_0.025Fe₂O₄ ferrite sintered at 1200 °C has a relatively low coercivity value (62.93 Oe) and the largest saturation magnetization (110.95 emu/g). Besides, dielectric behavior is also improved by Li¹⁺ and Al³⁺ co-doping.     

Ferroelectricity and ferromagnetism in fine-grained multiferroic BaTiO3/(Ni0.5Zn0.5)Fe2O4 composites prepared by a novel hybrid process

Dense, homogeneous, and fine-grained multiferroic BaTiO₃/(Ni_0.5Zn_0.5)Fe₂O₄ composite ceramics are synthesized by a novel powder-in-sol precursor hybrid processing route. This route includes the dispersion of nanosized BaTiO₃ ferroelectric powders prepared via conventional sold-state ceramic process into (Ni_0.5Zn_0.5)Fe₂O₄ ferromagnetic sol-gel precursor prepared via a sol-gel wet chemistry process. The composite ceramics show coexistence of obvious ferroelectric and ferromagnetic hysteresis loops at room temperature. Very low dielectric loss of about 0.02–0.0067 in the range of 10 kHz–10 MHz can be achieved, which is about an order of magnitude lower than the results of many reports using conventional processes at room temperature. The combination of high permeability and permittivity with low losses in the ceramics enables significant miniaturization of electronic devices based on the ceramics.     

Preparation and magnetic properties of BaFe12O19/Ni0.8Zn0.2Fe2O4 nanocomposite ferrite

Nanocomposite of hard (BaFe₁₂O₁₉)/soft ferrite (Ni_0.8Zn_0.2Fe₂O₄) have been prepared by the sol–gel process. The nanocomposite ferrite are formed when the calcining temperature is above 800 °C. It is found that the magnetic properties strongly depend on the presintering treatment and calcining temperature. The “bee waist” type hysteresis loops for samples disappear when the presintering temperature is 400 °C and the calcination temperature reaches 1100 °C owing to the exchange-coupling interaction. The remanence of BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite with the mass ratio of 5:1 is higher than a single phase ferrite. The specific saturation magnetization, remanence magnetization and coercivity are 63 emu/g, 36 emu/g and 2750 G, respectively. The exchange-coupling interaction in the BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite is discussed.     

Magnetic properties of the novel nanocomposite (Zn0.15Ni0.85Fe2O4)0.15/(SiO2)0.85 at room temperature

The value of the effective magnetic anisotropy constant of the ferrimagnetic nanoparticles Zn_0.15Ni_0.85Fe₂O₄ embedded in a SiO₂ silica matrix, determined through ferromagnetic resonance (FMR), is much higher than the magnetocrystalline anisotropy constant. The higher value of the anisotropy constant is due to the existence of surface anisotropy. However, even if the magnetic anisotropy is high, the ferrimagnetic nanoparticles with a 15% concentration, which are isolated in a SiO₂ matrix, display a superparamagnetic (SPM) behavior at room temperature and at a frequency of the magnetization field equal to 50 Hz. The FMR spectrum of the novel nanocomposite (Zn_0.15Ni_0.85Fe₂O₄)_0.15/(SiO₂)_0.85, recorded at room temperature and a frequency of 9.060 GHz, is observed at a resonance field (B_0r) of 0.2285 T, which is substantially lower than the field corresponding to free electron resonance (ESR) (0.3236 T). Apart from the line corresponding to the resonance of the nanoparticle system, the spectrum also contains an additional weaker line, identified for a resonance field of ∼0.12 T, which is appreciably lower than B_0r. This line was attributed to magnetic ions complex that is in a disordered structure in the layer that has an average thickness of 1.4 nm, this layer being situated on the surface of the Zn_0.15Ni_0.85Fe₂O₄ nanoparticles that have a mean magnetic diameter of 8.9 nm.     

Effect of calcination temperature on the structure and hydroxylation activity of Ni0.5Cu0.5Fe2O4 nanoparticles

Nanocrystalline Ni_0.5Cu_0.5Fe₂O₄ was synthesized by sol-gel method with varying calcination temperature over the range of 500-1000. The powders obtained were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In addition, thermal analysis (TG-DTG-DTA) of the precursor was carried out. The study reveals the simultaneous decomposition and ferritization process at rather low temperature (280-350). For the crystalline structure investigated, single cubic spinel is gained when the precursor was decomposed at 800-1000, whereas separated crystal CuO formed when calcination temperature is below 800. The increase of calcination temperature favors the appearance of Fe_B³⁺, Cu_A²⁺ and O on the spinel surface. The hydroxylation activity is relative to the amount of Cu_B²⁺ species on the spinel surface. The lattice oxygen species on the spinel surface are favorable for the deep oxidation of phenol.     

Synthesis and magnetic properties of hard/soft SrFe12O19/Ni0.7Zn0.3Fe2O4 nanocomposite magnets

Magnetic nanocomposite SrFe₁₂O₁₉/Ni_0.7Zn_0.3Fe₂O₄ powders with different weight fractions of the Ni_0.7Zn_0.3Fe₂O₄ soft ferrite were synthesized by a combination of the sol–gel self-propagation and glyoxilate precursor methods. The results of magnetic measurements revealed the higher Mr/Ms ratio for the nanocomposites than that for the single phase SrFe₁₂O₁₉ which proves the existence of the intergrain exchange coupling between hard and soft magnetic phases with the exchange spring behavior. The highest Mr/Ms ratio of 0.63 was obtained in the composite consisting of 80 wt% of SrFe₁₂O₁₉ and 20 wt% Ni_0.7Zn_0.3Fe₂O₄. The microstructural studies of this sample exhibited the average dimensions of hard and soft phases about 20 nm and 15 nm, respectively which are small enough for strong exchange coupling according to the theoretical studies. The variations of the reduced remanence (Mr/Ms) with increasing the weight fraction of the soft phase could be also explained by the role of the exchange and dipolar interactions in tuning the magnetic properties of the nanocomposites.     

Adjustable microwave absorption properties of flake shaped (Ni0.5Zn0.5)Fe2O4/Co nanocomposites with stress induced orientation

Flake shaped (Ni_0.5Zn_0.5)Fe₂O₄/Co nanocomposites were successfully fabricated by co-precipitating of Ni-Zn ferrite on the surface of cobalt nanoflakes. The electromagnetic characteristics of the samples were studied at the frequency of 0.1–14 GHz. The results showed that the cobalt nanoflakes in compacted nanocomposites were well orientated, and the nanocomposites were characterized with low optimal reflection loss (RL) of −33.8 dB at 11.5 GHz and broad RL bandwidth for <−20 dB in the frequency range of 7.6–12.1 GHz. At the same time, the position of the absorptive band can be adjusted by changing the mass ratio of ferrite to cobalt in the nanocomposites. It is proposed that the excellent microwave absorption properties are related to the combination of strong shape anisotropy of cobalt nanoflakes and adjustable dielectric loss.     

The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

Effect of grain size on the magnetic properties of superparamagnetic Ni0.5Zn0.5Fe2O4 nanoparticles by co-precipitation process

Ni_0.5Zn_0.5Fe₂O₄ (NZFO) spinel-type nanoparticles were directly fabricated by the chemical co-precipitation process using metal nitrate and acetate as precursors since nitrogen and carbon would be taken away in the forms of oxynitride and oxycarbide, respectively, after the precursors were annealed and then investigated in detail by employing X-ray diffraction (XRD), magnetic measurement and Raman spectroscopy. XRD analysis indicates that the as-prepared nanocrystals are all of a pure cubic spinel structure with their sizes ranging from 20.8 to 53.3 nm, as well as peaks of some samples shifting to lower angles due to lattice expansion. Calculations from the derived XRD data indicate that the activation energy is 30.83 kJ/mol. The magnetic measurements show that these samples are superparamagnetic. The saturation magnetization increases with annealing temperature, which may be explained by super-exchange interactions of Fe ions occurring at A- and B-sites. The variation of coercivity with particle size is interpreted on the basis of domain structure and crystal anisotropy. Furthermore, these nanoparticles exhibit a redshift phenomenon at lower temperatures seen in the Raman spectra, which could be related to ionic substitution.     

Reentrant spin glass behavior and large initial permeability of Co0.5−xMnxZn0.5Fe2O4

A series of polycrystalline ferrites having nominal chemical composition Co_0.50−xMn_xZn_0.5Fe₂O₄ (0<x<0.4) have been synthesized by the solid-state reaction technique. The XRD analysis confirms single phase cubic spinel structure for all compositions. Lattice constant increases from 0.84195 to 0.84429 nm with the increasing Mn content and obeys Vegard's law. The average grain size increases by increasing both Mn content and sintering temperatures. Room temperature saturation magnetization increases for x=0.1 and decreases for increasing Mn content. The coercivity decreases with increasing Mn content due to the decrease of anisotropy constant. A reentrant spin glass behavior of these samples is observed from the zero field cooled magnetization measurements. The real part of the initial permeability increases by increasing both Mn content and sintering temperatures. This is due to the homogeneous grain growth and densification of the ferrites. The highest initial permeability 137 is observed for x=0.4 sintered at 1573 K on the other hand, the highest relative quality factor (2522) is obtained for the sample Co_0.2Mn_0.3Zn_0.5Fe₂O₄ sintered at 1523 K. The Mn substituted Co_0.50−xMn_xZn_0.5Fe₂O₄ ferrites showed improved magnetic properties.     

Milling time and BPR dependence on permeability and losses of Ni0.5Zn0.5Fe2O4 synthesized via mechanical alloying process

Ni_0.5Zn_0.5Fe₂O₄ has been synthesized using mechanical alloying method with two variables (milling time and ball-to-powder weight ratio (BPR)) were varied in order to study its effect on the magnetic properties of the material. The effects of these two variables were studied using XRD, SEM, TEM and later by impedance analyzer with the frequency range from 1 MHz to 1.8 GHz. The results obtained however show that there are no significant trends to relate the milling time and BPR with the permeability and losses of the material studied. After being sintered at 1150 °C, all the effects of alloying process seem to diminish.     

Estimating the cation distributions in the spinel ferrites Cu0.5−xNi0.5ZnxFe2O4 (0.0≤x≤0.5)

Ferrite samples of the composition Cu_0.5−xNi_0.5Zn_xFe₂O₄ (0.0≤x≤0.5) were synthesized by chemical co-precipitation. The samples exhibited a single phase cubic spinel structure, and the saturation magnetization of the samples was found to increase with increasing Zn content. Using a quantum mechanical method proposed by our group, the cation distributions in the samples were estimated. Estimated cation distributions obtained by fitting the magnetic moments of the samples were then used to perform Rietveld fitting for X-ray diffraction patterns. The acceptable error parameters in the Rietveld fitting indicate that the estimated cation distributions in the samples are reasonable.     

Microemulsion-mediated in-situ synthesis and magnetic characterization of polyaniline/Zn<Subscript>0.5</Subscript>Cu<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite

Polyaniline/Zn_0.5Cu_0.5Fe₂O₄ nanocomposite was synthesized by a simple, general and inexpensive in-situ polymerization method in w/o microemulsion. The effects of polyaniline coating on the magnetic properties of Zn_0.5Cu_0.5Fe₂O₄ nanoparticles were investigated. The structural, morphological and magnetic properties of as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM) and magnetic measurements. The morphology analysis confirmed that polyaniline was deposited on the porous surface of magnetic Zn_0.5Cu_0.5Fe₂O₄. It was shown that the saturation magnetization and coercivity of Zn_0.5Cu_0.5Fe₂O₄ decreased after polyaniline coating, which can be interpreted by the interparticle dipole–dipole interactions that contributed to magnetic anisotropy and changed the magnetic properties of the nanoparticles. PACS  74.25.Ha; 81.05.-t; 81.05.Lg     

Effects of sintering temperature on grain growth and the complex permeability of Co0.2Ni0.3Zn0.5Fe2O4 material prepared using mechanically alloyed nanoparticles

Nanoparticle-sized Co_0.2Ni_0.3Zn_0.5Fe₂O₄ was prepared using mechanical alloying and sintering. The starting raw materials were milled in air and subsequently sintered at various temperatures from 600 to 1300 °C. The effects of sintering temperature on physical, magnetic and electrical characteristics were studied. The complex permittivity and permeability were investigated in the frequency range 10 MHz to 1.0 GHz. The results show that single phase Co_0.2Ni_0.3Zn_0.5Fe₂O₄ could not be formed during milling alone and therefore requires sintering. The crystallization of the ferrite sample increases with increasing sintering temperature; which decrease the porosity and increase the density, crystallite size and the shrinkage of the material. The maximum magnetization value of 83.1 emu/g was obtained for a sample sintered at 1200 °C, while both the retentivity and the coercivity decrease with increasing the sintering temperature. The permeability values vary with both the sintering temperature and the frequency and the absolute value of the permeability decreased after the natural resonance frequency. The real part of the permittivity was constant within the measured frequency, while the loss tangent values decreased gradually with increasing frequency.