Dielectric properties and magnetoelectric effect of (x)NiFe2O4+(1−x)Ba0.8Sr0.2TiO3 composites

Magnetoelectric composites of NiFe₂O₄ and Ba_0.8Sr_0.2TiO₃ were prepared using conventional double-sintering ceramic method. The phase formation of magnetoelectric composites was confirmed by XRD technique. Variation of dielectric constant and loss tangent at room temperature with frequency in the range 100 Hz-1 MHz has been studied. Also the variation of dielectric constant and loss tangent with temperature and composition at fixed frequencies of 1 kHz, 10 kHz, 100 kHz and 1 MHz is reported. The static value of the magnetoelectric conversion factor was measured as a function of intensity of the magnetic field. The ME voltage coefficient of about 430 μV/cm Oe was observed for 15% NiFe₂O₄+85% Ba_0.8Sr_0.2TiO₃ composite. All the samples show linear variation of magnetoelectric conversion in the presence of static magnetic field.     

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.     

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.     

Electrical conduction and magnetoelectric effect of (x) BaTiO3 + (1−x) Ni0.92Co0.03Cu0.05Fe2O4 composites in ferroelectric rich region

Particulate composites with composition (x)BaTiO₃+(1−x)Ni_0.92Co_0.03Cu_0.05Fe₂O₄ in which x varies as 1, 0.85, 0.70, 0.55 and 0 (in mol%) were prepared by the conventional double sintering ceramic technique. The presence of two phases viz. ferromagnetic (Ni_0.92Co_0.03Cu_0.05Fe₂O₄) and ferroelectric (BaTiO₃) was confirmed by X-ray diffraction analysis. The dc resistivity and thermo-emf measurements were carried out with variation of temperature. The ac conductivity (σ_ac) measurements investigated in the frequency range 100 Hz to 1 MHz conclude that the conduction in these composites is due to small polarons. The variation of dielectric constant and loss tangent with frequency (20 Hz to 1 MHz) was studied. The static magnetoelectric conversion factor, i.e. dc (dE/dH)_H was measured as a function of intensity of applied magnetic field. The changes were observed in electrical properties as well as in magnetoelectric voltage coefficient as the molar ratio of the constituent phases was varied. A maximum value of magnetoelectric conversion factor of 536.06 μV/cm Oe was observed for the composite with 70% BaTiO₃+30% Ni_0.92Co_0.03Cu_0.05Fe₂O₄ at a dc magnetic field of 2.3 K Oe. The maximum magnetoelectric conversion output has been explained in terms of ferrite-ferroelectric content, applied static magnetic field and resistivity.     

Magnetic properties of xNi0.53Cu0.12Zn0.35Fe1.88O4+(1−x)BaTiO3 nanocomposites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe_1.88O₄ and BaTiO₃ powders were prepared using Microwave-Hydrothermal (M-H) method at 160 °C/45 min. The as synthesized powders were characterized using the X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The size of the powders that were synthesized using M-H system was found to be ∼30 and ∼50 nm for ferrite phase and ferroelectric phases, respectively. The powders were densified using microwave sintering method at 900 °C/30 min. The ferrite and ferroelectric phases were observed from XRD and morphology of the composites was observed with the Scanning Electron Microscope (SEM).The magnetic hysteresis loops were recorded using the Vibrating Sample Magnetometer (VSM).The frequency dependence of real (μ′) and imaginary (μ″) parts of permeability was measured in the range of 1 MHz-1.8 GHz. The permeability decreases with an increase of BaTiO₃ content at 1 MHz. The transition temperature (T_C) of ferrite was found to be 245 °C. The T_C of composite materials decreases with an increase in BaTiO₃ content.     

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.     

Magnetic properties and magneto electric effect in ferroelectric rich Ni0.5Zn0.5Fe2O4+BPZT ME composites

Magneto electric composites with composition (x) Ni_0.5Zn_0.5Fe₂O₄+(1-x) Ba_0.8Pb_0.2Zr_0.8Ti_0.2O₃ (BPZT—barium lead zirconate titanate) in which x varies as 0.0, 0.15, 0.30, 0.45 and 1.0 mol% were prepared by standard double sintering ceramic method. The presence of constituent phases such as ferrite and ferroelectric was confirmed by X-ray diffraction. The structural analyses were carried out by using X-ray diffraction pattern. Scanning electron micrographs (SEM) were taken to understand the microstructure of the samples. The calculated values of the porosity of the samples lie between 4.5% and 16.2%. The hysteresis measurements were made to determine saturation magnetization (M_s), magnetic moment (μ_B) and coercivity (H_c). From the AC susceptibility measurements the contribution of both the single domain (SD) particles of the ferrite phase and mixed domain (SD+MD) particles of the composites in ME output were explained. The static magneto electric voltage coefficients (dE/dH)_H were measured as a function of intensity of magnetic field and the maximum ME coefficient were observed for the composites with 30% of ferrite+70% of ferroelectric phase.     

Structural and magnetic properties of sol-gel Co2xNi0.5−x Zn0.5−xFe2O4 thin films

Nanocrystalline Co_2xNi_0.5−xZn_0.5−xFe₂O₄ (x=0−0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform without microcracks. The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size independent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe₂O₄. The maximum absorption band shifted from 13 to 11 GHz as cobalt content increased from x=0.1 to 0.2.     

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.     

Preparation and magnetic properties of the conductive Co(1−x)NixFe2O4/polyaniline microsphere composites

Core-shell Co_(1−x)Ni_xFe₂O₄/polyaniline nanoparticles, where the core was Co_(1−x)Ni_xFe₂O₄ and the shell was polyaniline, were prepared by the combination of sol-gel process and in-situ polymerization methods. Nanoparticles were investigated by Fourier transform spectrometer, X-ray diffraction diffractometer, Scanning electron microscope, Differential thermal analysis and Superconductor quantum interference device. The results showed that the saturation magnetization of pure Co_(1−x)Ni_xFe₂O₄ nanoparticles were 57.57 emu/g, but Co_(1−x)Ni_xFe₂O₄/polyaniline composites were 37.36 emu/g. It was attributed to the lower content (15 wt%), smaller size and their uneven distribution of Co_(1−x)Ni_xFe₂O₄ nanoparticles in the final microsphere composites. Both Co_(1−x)Ni_xFe₂O₄ and PANI/Co_(1−x)Ni_xFe₂O₄ showed superparamagnetism.     

(Ni0.8Zn0.2Fe2O4)epoxy-PZT双层膜中的磁电效应

讨论了Ni_0.8Zn_0.2Fe₂O₄ (NZFO)与锆钛酸铅(PZT)的双层膜结构样品的磁电(ME)效应.NZFO粉料由溶胶-凝胶法制成，再经900℃热压，并高温烧结.在该双层膜中测量到了很强的磁电相互作用.发现横向的磁电效应比纵向效应大一个数量级，并且随NZFO烧结温度的提高而增加.当烧结温度从950℃上升到1380℃时，横向ME电压系数(α_E)的最大值变化范围为25.6 mV Am^-2≤α_E≤199.6 mV Am^-2.理论分析显示NZFO-PZT双层膜样品中ME效应源于NZFO与PZT之间相对良好的磁电耦合. 关键词： 镍铁氧体 PZT 热压法 ME效应     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

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.     

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.     

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.     

The effect of rare earth substitution on the magnetic properties of Ni0.5Zn0.5MxFe2−xO4 (M: rare earth)

Variation of the complex permeability with frequency of Ni_1−xZn_xFe₂O₄ (x=0.5, 0.6 and 0.7) prepared by a combustion method has been measured over a wide range of frequency, up to 1.8 GHz. Zn content improves permeability but moves the onset of resonance to lower frequencies.

The influence on some properties of samples prepared by the insertion of small amounts of Ruthenium, Yttrium and rare-earth cations into the Ni_0.5Zn_0.5M_xFe_2−xO₄ ferrite has been also investigated. Relative losses and Curie temperature of all the samples have been measured. Ferrites substituted with Ru and Gd improve microwave behavior compared to non-substituted samples.     

Growth temperature dependence of the hysteretic behavior of Ni0.5Zn0.5Fe2O4 thin films

Herein, a discussion of the effect of deposition temperature on the magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ thin films. The thin films were grown by r.f. sputtering technique on (1 0 0) MgO single-crystal substrates at deposition temperatures ranging between 400 and 800 °C. The grain boundary microstructure was analyzed via atomic force microscopy (AFM). AFM images show that grain size (φ∼70-112 nm) increases with increasing deposition temperature, according to a diffusion growth model. From magneto-optical Kerr effect (MOKE) measurements at room temperature, coercive fields, H_c, between 37and 131 Oe were measured. The coercive field, H_c, as a function of grain size, reaches a maximum value of 131 Oe for φ ∼93 nm, while the relative saturation magnetization exhibits a minimum value at this grain size. The behaviors observed were interpreted as the existence of a critical size for the transition from single- to multi-domain regime. The saturation magnetization (21 emu/g<M_s<60 emu/g) was employed to quantify the critical magnetic intergranular correlation length (L_c≈166 nm), where a single-grain to coupled-grain behavior transition occurs. Experimental hysteresis loops were fitted by the Jiles-Atherton model (JAM). The value of the k-parameter of the JAM fitted by means of this model (k/μ_o∼50 A m²) was correlated to the domain size from the behavior of k, we observed a maximum in the density of defects for the sample with φ∼93 nm.     

Electromagnetic properties of low-temperature co-fired xPZT+(1−x)NiCuZn-ferrite composites

To adjust the dielectric constant and the initial permeability of NiCuZn-ferrite materials for suppressing electromagnetic interference, Pb_0.95Sr_0.05(Zr_0.52Ti_0.48)O₃ (PZT) was introduced. The effects of PZT additive on the phase composition, densification, microstructures and electromagnetic properties of the ceramics with composition of xPZT+(1−x)NiCuZn-ferrite (where x=0-25 wt%) were investigated. The results show that these composites are mainly of spinel phase, exhibiting relatively high dielectric constant and initial permeability due to the mutual effects of magnetization of NiCuZn-ferrite phase and polarization of PZT phase. Meanwhile, relatively high-quality factor (Q) and excellent frequency stability were obtained for the samples with PZT additive. These materials are expected to be used as embedded inductors or embedded capacitors.     

Synthesis and magnetic properties of ferrites spinels MgxCu1−xFe2O4

Polycrystalline Mg_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and magnetic measurements.Using mean field theory and high-temperature series expansions (HTSE), extrapolated with the padé approximants method, the magnetic properties of Mg_1−xCu_xFe₂O₄ have been studied. The nearest neighbor super-exchange interactions for intra-site and inter-site of the Mg_1−xCu_xFe₂O₄ ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_c is calculated as a function of Mg concentration. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.