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.     

晶粒易轴取向度对纳米晶永磁Pr2Fe14B磁性的影响

构造了立方和不规则形状晶粒的各向异性纳米晶单相Pr₂Fe₁₄B磁体 .利用微磁学的有限元法，模拟计算了样品的磁滞回线.计算结果表明，随着磁体晶粒易轴取向度的变差， 磁体的剩磁、矫顽力均随之下降.不同晶粒尺寸的纳米晶单相Pr₂Fe_{14B磁体，其磁 性能随取向度的变化快慢不同，原因在于磁体中的晶间交换作用 (IGEC) 的强弱不同.随着 晶粒取向度的提高，纳米晶单相磁体的矫顽力逐渐增加，这完全不同于烧结磁体. 关键词： 纳米晶磁体 矫顽力 剩磁}     

Synthesis and magnetic properties of Co-Cr2O3 nanocomposites

Co:Cr₂O₃ nanocomposites were prepared with phase separated metallic Co clusters in a wide range of concentrations namely 10, 20, 30, 40 and 50 wt% Co. Samples were annealed at different temperatures to study the effects of crystallization of Cr₂O₃ and the growth of Co metal clusters on the magnetic behavior of nanocomposites. Enhanced crystallinity of antiferromagnetic (Cr₂O₃) matrix and growth of Co clusters with higher annealing temperatures strongly affects the coercivity, saturation and magnetic viscosity in these hybrid materials. Amorphous Cr₂O₃ acts as a paramagnetic matrix for Co particles. Exchange anisotropy stabilizes magnetic moments of Co embedded in Cr₂O₃ only if Cr₂O₃ is crystalline. This exchange anisotropy leads to the enhancement of coercivity. Relaxation measurements confirm that exchange anisotropy is higher for samples with lower Co content.     

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.     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.     

Fabrication and magnetic properties of Ni0.50.5Zn0.5Fe2O4 nanofibres by electrospinning

NiZn ferrite/polyvinylpyrrolidone composite fibres were prepared by sol–gel assisted electrospinning.Ni0.5Zn0.5Fe2O4 nanofibres with a pure cubic spinel structure were obtained subsequently by calcination of the composite fibres at high temperatures.This paper investigates the thermal decomposition process,structures and morphologies of the electrospun composite fibres and the calcined Ni0.5Zn0.5Fe2O4 nanofibres at different temperatures by thermogravimetric and differential thermal analysis,x-ray diffraction,Fourier transform infrared spectroscopy and field emission scanning electron microscopy.The magnetic behaviour of the resultant nanofibres was studied by a vibrating sample magnetometer.It is found that the grain sizes of the nanofibres increase significantly and the nanofibre morphology gradually transforms from a porous structure to a necklace-like nanostructure with the increase of calcination temperature.The Ni0.5Zn0.5Fe2O4 nanofibres obtained at 1000 C for 2 h are characterized by a necklace-like morphology and diameters of 100–200 nm.The saturation magnetization of the random Ni0.5Zn0.5Fe2O4 nanofibres increases from 46.5 to 90.2 emu/g when the calcination temperature increases from 450 to 1000 C.The coercivity reaches a maximum value of 11.0 kA/m at a calcination temperature of 600 C.Due to the shape anisotropy,the aligned Ni0.5Zn0.5Fe2O4 nanofibres exhibit an obvious magnetic anisotropy and the ease magnetizing direction is parallel to the nanofibre axis.     

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.     

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.     

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.     

Synthesis and magnetic characterization of Zn0.7Ni0.3Fe2O4 nanoparticles via microwave-assisted combustion route

We report on the synthesis of Zn_0.7Ni_0.3Fe₂O₄ nanoparticles via microwave assisted combustion route by using urea as fuel. XRD and FT-IR analyses confirm the composition and structure as spinel ferrite. The crystallite size estimated from XRD (16.4 nm) and the magnetic core size (15.04 nm) estimated from VSM agree well, while a slightly smaller magnetic diameter reflects a very thin magnetically dead layer on the surface of the nanoparticles. Morphological investigation of the products was done by TEM which revealed the existence of irregular shapes such spherical, spherodial and polygon. Magnetization measurements performed on Zn_0.7Ni_0.3Fe₂O₄ nanoparticles showed that saturation was not attained at even in the high magnetic field. The sample shows superparamagnetic behavior at around the room temperature and ferromagnetic behavior below the blocking temperature which is measured as 284 K.     

The effect of B2O3 addition to the microstructure and magnetic properties of Ni0.4Zn0.6Fe2O4 ferrite

The effects of 0.01 and 0.1 mol B₂O₃ addition to the microstructure and magnetic properties of a Ni–Zn ferrite composition expressed by a molecular formula of Ni_0.4Zn_0.6Fe₂O₄ were investigated. The toroid-shaped samples prepared by pressing the milled raw materials used in the preparation of the composition were sintered in the range of 1000–1300 °C. The addition of 0.01 mol B₂O₃ increased the grain growth and densification giving rise to reduced intergranular and intragranular porosity due to liquid-phase sintering. The sintered toroid sample at 1300 °C gave the optimum magnetic properties of B_r=170 mT, H_c=0.025 kA/m and a high initial permeability value of μ_i=4000. The increment of the B₂O₃ content to 0.1 mol resulted in a pronounced grain growth and also gave rise to large porosity due to the evaporation of B₂O₃ at higher sintering temperatures. Hence, it resulted in an air-gap effect in the hysteresis curves of these samples.     

Magnetic properties of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite at low temperatures

Mössbauer spectra and magnetic measurement of Ni_0.7Mn_0.3Gd_0.1Fe_1.9O₄ ferrite were investigated by Oxford MS-500 Mössbauer spectrometer and superconducting quantum interference device (SQUID) magnetometer with a field 5 T. Ni_0.7Mn_0.3Gd_0.1Fe_1.9O₄ nanoparticles have a considerable coercivity of 1040 Oe when the test temperature is reduced to 2 K. Mössbauer spectra show that Ni_0.7Mn_0.3Gd_0.1Fe_1.9O₄ nanoparticles exhibit superparamagnetism at room temperature and ferrimagnetism at 77 K.     

Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite

In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe₂O₄ and ferrimagnetic oxide/ferromagnetic metal CoFe₂O₄/CoFe₂ nanocomposite. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupled. Two steps were followed to synthesize the bimagnetic CoFe₂O₄/CoFe₂ nanocomposite: (i) first, preparation of CoFe₂O₄ nanoparticles using a simple hydrothermal method, and (ii) second, reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe₂O₄ nanoparticles were investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50 K. The mean diameter of CoFe₂O₄ particles is about 16 nm. Mossbauer spectra revealed two sites for Fe³⁺. One site is related to Fe in an octahedral coordination and the other one to the Fe³⁺ in a tetrahedral coordination, as expected for a spinel crystal structure of CoFe₂O₄. TEM measurements of nanocomposite showed the formation of a thin shell of CoFe₂ on the cobalt ferrite and indicate that the nanoparticles increase to about 100 nm. The magnetization of the nanocomposite showed a hysteresis loop that is characteristic of exchange coupled systems. A maximum energy product (BH)_max of 1.22 MGOe was achieved at room temperature for CoFe₂O₄/CoFe₂ nanocomposites, which is about 115% higher than the value obtained for CoFe₂O₄ precursor. The exchange coupling interaction and the enhancement of product (BH)_max in nanocomposite CoFe₂O₄/CoFe₂ are discussed.     

Microwave anneal effect on magnetic properties of Ni0.6Zn0.4Fe2O4 nano-particles prepared by conventional hydrothermal method

Ni_0.6Zn_0.4Fe₂O₄ ferrite nano-particles with a crystallite size of about 20 nm were prepared by the conventional hydrothermal method, followed by annealing in a microwave oven for 7.5-15 min. The microstructure and magnetic properties of the samples were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and vibrating sample magnetometry. The microwave annealing process has slight effect on the morphology and size of Ni_0.6Zn_0.4Fe₂O₄ ferrite nano-particles. However it reduces the lattice parameter and enhances the densification of the particles, and then greatly increases the saturation magnetization (50-56 emu/g) and coercive force of the samples as compared to the non-annealing condition. The microwave annealing process is an effective way to rapidly synthesize high performance ferrite nano-particle.     

纳米晶复合Pr2Fe14B/α-Fe永磁材料磁性的研究

采用熔体快淬的方法制备Pr₂Fe₁₄B/α-Fe纳米晶复合永磁材料.使用振动样品磁强计(VSM)测量样品的室温磁性能.实验合金成分为(Pr_xFe_94.3-xB_5.7)_0.99Zr₁(其中x＝8.2，8.6，9.0，9.4，9.8，10.2，10.6，11.0，11.4(原子分数，%)).系统地研究了辊速及合金成分对快淬带磁性能的影响，当Pr原子分数由8. 关键词： 纳米复合永磁材料 熔体快淬 2Fe₁₄B/α-Fe')" href="#">Pr₂Fe₁₄B/α-Fe 磁性     

Structural and magnetic studies on spark plasma sintered SmCo5/Fe bulk nanocomposite magnets

SmCo₅+x wt% Fe (x=0$x=0$, 5 and 10) nanocomposite powders were synthesized by mechanical milling and were consolidated into bulk shape by spark plasma sintering (SPS) technique. The evolution of structure and magnetic properties were systematically investigated in milled powders as well as in SPS samples. A maximum coercivity of 8.9 kOe was achieved in spark plasma sintered SmCo₅+5 wt% Fe sample. The exchange spring interaction between the hard and soft magnetic phases was evaluated using δM–H measurements and the analysis revealed that the SPS sample containing 5 wt% Fe had a stronger exchange coupling between the magnetic phases than that of the sample with10 wt% Fe.     

Dielectric properties of BaTiO3-CoFe2O4 nanocomposite thin films

We report on the dielectric characterization of CoFe₂O₄-BaTiO₃ nanocomposites grown by rf sputtering. Dielectric properties have been analyzed for samples grown at different deposition temperatures and with different thicknesses. Impedance spectroscopy data has been analyzed by fitting to an equivalent circuit and different contributions have been identified. Correlations between dielectric properties and deposition temperature and thickness have been established.     

Improvement of the magnetic properties of Nd9.5Fe81Zr3B6.5 nanocomposite magnets by semi-melt spinning

Nd_9.5Fe₈₁Zr₃B_6.5 ribbons are prepared by single roller melt-spinning technique at 1150 °C which is in the solid and liquid coexistence zone. The phase evolution and magnetic properties were studied by X-ray diffraction, differential scanning calorimetry, transmission electron microscopy observations, and magnetization measurements. The experimental results show that in comparison to the ribbons quenching at higher temperature, the thickness of ribbons prepared at 1150 °C are insensitive to the wheel speed and an uniform nanoscale structure with fine grains can be obtained directly from the semi-melt and the exchange coupling interaction between the grains was enhanced for the nanocomposite permanent alloy which can contributed to excellent magnetic properties.     

Yafet-Kittel-type magnetic ordering in Ni0.35Zn0.65Fe2O4 ferrite detected by magnetosensitive microwave absorption measurements

Magnetosensitive microwave absorption measurements of polycrystalline ferrite Ni_0.35Zn_0.65Fe₂O₄ was carried out at 9.4 GHz (X-band) as a function of temperature. Temperature dependence of the total linewidth (ΔH_pp) deduced from the resonance spectra showed the passage through the Curie point (T_c~430 K). Additionally, the plot ΔH_pp vs. T also indicated the existence of another magnetic phase transition at ~240 K, which can be associated with a Yafet-Kittel-type canting of the magnetic moments. Low-field microwave absorption (LFMA) and the magnetically modulated microwave absorption spectroscopy (MAMMAS) were used to give a further knowledge on this material. For low temperature, these techniques give evidence of a Yafet-Kittel-type canting of the magnetic moments.     

X-ray Debye temperature of mechanically milled Ni0.5Zn0.5Fe2O4 spinel ferrite

The X-ray Debye temperatures, θ _M , of spinel ferrite composition Ni_0.5Zn_0.5Fe₂O₄, mechanically milled upto 9 hrs, were determined from integrated intensities of selected Bragg reflections. The θ _M was found to increase with milling time. The results are explained in the light of milling induced grain orientation and surface effect. The values of θ _M were found to be lower as compared to the Debye temperatures obtained from infrared spectral data analysis. The difference can be explained on the basis of increase in an excess free volume in the form of vacancies and vacancy clusters associated with grain boundaries.