Microwave-absorbing properties of NiCoZn spinel ferrites

In this paper, the microwave-absorbing properties of (Ni_1−x−yCo_xZn_y)Fe₂O₄ spinel ferrites have been investigated within the frequency range of 0.5–14 GHz. There are two kinds of resonance peaks observed in the permeability spectra: domain-wall resonances at lower frequency and spin-rotation resonances at higher frequency. The reflection loss (RL) calculations show that the prepared NiCoZn spinel ferrites are good electromagnetic (EM) wave absorbers in microwave range. In terms of the absorbing frequency band (AFB) and the matching thickness (t_m), (Ni_0.407Co_0.207Zn_0.386)Fe₂O₄ shows the best performances: t_m=3.15 mm and the AFB is 8.64–11.2 GHz. Decreasing the weight ratio of NiCoZn ferrites in ferrites/wax composites, the matching thickness decreases and the AFB shifts to higher frequencies. Compared with the absorbers with single-layer ferrites, the absorbers with double-layers ferrites have better absorbing performances, such as a thinner matching thickness and a wider EM wave AFB.     

A new strip line broad-band measurement evaluation for determining the complex permeability of thin ferromagnetic films

In the present paper, a new method for determining the frequency dependent complex permeability of thin magnetic films, designed for measurements up to 5 GHz, is presented. The measurement technique described here was carried out by a one-port permeameter, which is based on a short-circuited strip line. The complex permeability was deduced by a new analytical approach from the measured reflection coefficient of a strip line (S₁₁) with and without a ferromagnetic film material inside. An adaptive error correction was applied in the measurement procedure. The spectral permeability of thin FeCoAlN films with an in-plane uniaxial anisotropy of μ₀^*H_a=3.2 mT induced by annealing at CMOS temperatures in a static magnetic field was investigated. The measurements were compared with a theoretical model taking the Landau–Lifshitz and eddy current theories into account. A resonant frequency of about 1.6 GHz was observed.     

Electromagnetic properties of manganese-zinc ferrite with lithium substitution

Polycrystalline manganese-zinc ferrite with lithium substitution of composition Li_0.5xMn_0.4Zn_0.6−xFe_2+0.5xO₄ (0.0≤x≤0.4) was prepared by the usual ceramic method. X-ray diffraction analysis confirmed that the samples have a spinel structure and are of single phase for some values of Li content. Lithium doping considerably modifies saturation magnetization since its value increases from 57.5 emu/g for x=0.0 to 82.9 emu/g for x=0.4. Lithium inclusion increases the real permeability (over 1 MHz) while the natural resonance frequency shifts to lower values as the fraction of Li increases. These ferrites show good electromagnetic properties as absorbers in the microwave range of 1 MHz - 1 GHz.     

Synthesis of nanocrystalline spinel CoFe2O4 via a polymer-pyrolysis route

Nanocrystalline CoFe₂O₄ spinel ferrites were synthesized via the pyrolysis of polyacrylate salt precursors prepared by in situ polymerization of metal salts and acrylic acid. The pyrolytic behaviors of the polymeric precursors were analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). The structural characteristics of the calcined products were obtained by powder X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscope (TEM). The results revealed that cobalt ferrites had nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average size of nanocrystalline cobalt ferrites ranged from 20 to 30 nm with a narrow size distribution, while the particle size increased with the increase of the calcination temperature. Magnetic properties were obtained at room temperature using a vibrating sample magnetometer. The samples exhibited hysteresis loop typical of magnetic behaviors, indicating that the presence of an ordered magnetic structure could exist in the mixed spinel system. The as-calcined cobalt ferrites at 500 °C exhibited the highest magnetization value of 77.4 emu/g at 10 kOe, while the highest remanence and coercivity of 35.6 emu/g and 1445 Oe, respectively, for those calcined at 700 °C were obtained.     

Impacts of neodymium on structural,spectral and dielectric properties of LiNi0.5Fe2O4 nanocrystalline ferrites fabricated via micro-emulsion technique

Soft ferrites are technologically advanced smart materials and their properties can be tailored by controlling the chemical composition and judicial choice of the metal elements. In this article we discussed the effect of rare earth neodymium (Nd³⁺) on various properties of LiNi_0.5Nd_xFe_2−xO₄ spinel ferrites. These ferrites have been synthesized by facile micro-emulsion route and characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), a.c. electrical conductivity and thermal analysis. The influence of Nd³⁺ doping on structural and electrical parameters has been investigated. XRD analysis revealed the formation of single cubic spinel structure for x≤0.07. Few traces of secondary phase (NdFeO₃) were found for x≥0.105. The secondary phase induced owing to the solubility limit of Nd³⁺ cations in these ferrites. The lattice parameter (a) and crystallite size (D) both exhibit non-linear relation. The values of “a” and “D” were found in the range 8.322–8.329 Å and 25–32 nm respectively. These variations were attributed to the larger ionic radius of Nd³⁺ cations as compared to the host cations and lattice strain produced in these ferrites. The dielectric parameters were studied in the range 1 MHz to 3 GHz and these parameters were damped by Nd³⁺ incorporation and also by increasing the frequency. The reduced dielectric parameters observed in wide frequency range proposed that these nanocrystalline ferrites are potential candidates for fabricating the devices which are required to operate at GHz frequencies.     

Improvement in electrical and magnetic properties of mixed Mg-Al-Mn ferrite system synthesized by citrate precursor technique

In the present work, mixed magnesium-manganese ferrites of composition Mg_0.9Mn_0.1Al_0.3Co_zFe_1.7−zO₄ where z=0.3, 0.5 and 0.7 have been synthesized by the citrate precursor technique. X-ray diffraction patterns of the samples confirmed the formation of single-phase spinel structure. The ferrites have been investigated for their electric and magnetic properties such as dc resistivity, Curie temperature, saturation magnetization, initial permeability and relative loss factor (RLF). Fairly constant value of initial permeability over a wide frequency range (0.1-20 MHz) and low values of the relative loss factor of the order of 10⁻⁴-10⁻⁵, in the frequency range 0.1-30 MHz, are the cardinal achievements of the present investigation. In addition to this, initial permeability was found to increase with an increase in temperature while RLF was observed to be low at these temperatures. The dc resistivity and Curie temperature were found to increase with an increase in cobalt content. The mechanisms contributing to these results are discussed in detail in this paper.     

A new perturbation method for determining the broadband complex permeability of magnetic thin films

In this paper, a new one-port microstrip line permeameter using the perturbation method is presented. A short-ended circuited microstrip line fixture was designed and implemented. Completely new analytical calculation formulas and a two-step measurement procedure were applied to deduce the complex permeability of the material in the frequency range from 100 MHz to 5 GHz. The measured results showed good agreement with the Landau–Lifchitz–Gilbert theory.     

Development of a new soft ferrite core for power applications

Manganese-substituted nickel–zinc ferrites have been investigated as power core materials for applications in switched-mode-power supplies. High frequency operation of these power supplies requires high performance cores with low power losses. The main contributors to the power loss are eddy current loss, hysteresis loss and residual loss. The ferrites have been synthesized by the citrate precursor technique and their electromagnetic properties such as resistivity, permeability, saturation magnetization and Curie temperature studied. A power loss of 500 mW/cc could be obtained at a frequency of 500 kHz, flux density of 50 mT and temperature 100 °C.     

Microwave absorption properties of Ce-substituted M-type barium ferrite

Ce-substituted barium ferrite with chemical composition BaCe_0.05Fe_11.95O₁₉ has been prepared by the citrate sol-gel method. The phase composition of BaCe_0.05Fe_11.95O₁₉ was characterized by X-ray powder diffraction analysis (XRD). The complex permittivity and complex permeability, microwave absorption properties of the resulting powder were measured by the transmission/reflection coaxial line method in the range of 8-13 GHz. The results show that the resulting powder has a minimum reflection loss value of - 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm.     

Frequency characterization of thin soft magnetic material layers used in spiral inductors

The paper details the characterization of thin magnetic materials layers, particularly soft materials, with respect to their behaviour in frequency (from 10 MHz to 1 GHz). The proposed method is suitable for any soft but insulating magnetic material; Yttrium Iron Garnet (YIG) is used as an example. The principle is based on a comparison between simulations for different values of the permeability and measurement values versus frequency of planar inductor structures; an experimental validation is proposed as well. Thin magnetic material is first deposited on an alumina substrate using RF sputtering technique; a planar spiral winding of copper is then deposited on the magnetic material by the same technique. The effective permeability versus frequency is obtained by comparing two samples of spiral windings with and without magnetic material. Network analyser measurements on samples of various geometrical dimensions and of different thicknesses are necessary to determine the effective magnetic permeability; we have obtained a relative effective permeability of about 30 for seven turns spiral inductor of a 17 μm YIG film.     

Preparation and magnetic properties of La-substituted Zn–Cu–Cr ferrites via a rheological phase reaction method

Nanocrystalline La-substituted Zn–Cu–Cr ferrites Zn_0.6Cu_0.4Cr_0.5La_xFe_1.5−xO₄ (x=0.00, 0.02, 0.04, 0.06) were prepared by a rheological phase reaction method. The obtained powders were characterized by X-ray diffractometer, transmission electron microscopy and vibrating sample magnetometer. Permeability of the samples was investigated using an impedance analyzer. The results indicated that ferrite samples had the single spinel phase at low La content. Lattice parameter increased with increasing La content, while particle size calculated from Scherrer's formula decreased with increasing La content in La-substituted ferrite samples. The magnetic properties of La-substituted ferrites were strongly affected by La content. The saturation magnetization decreased, while coercivity increased with increasing La content. The variation of real permeability with La content was investigated in the frequency range of 1 MHz–1 GHz.     

Effect of Pr doping on magnetic and dielectric properties of Ni-Zn ferrites by “one-step synthesis”

Pr³⁺-doped Ni-Zn ferrites with a nominal composition of Ni_0.5Zn_0.5Pr_xFe_2−xO₄ (where x=0-0.08) were prepared by a one-step synthesis. The magnetic and dielectric properties of the as-prepared Ni-Zn ferrites were investigated. X-ray diffraction data indicated that, after doping, all samples consisted of the main spinel phase in combination of a small amount of a foreign PrFeO₃ phase. The lattice constants of the ferrites initially increased after Pr³⁺ doping, but then became smaller with additional Pr³⁺ doping. The addition of Pr³⁺ resulted in a reduction of grain size and an increase of density and densification of the as-prepared samples. Magnetic measurement revealed that the saturation magnetization of the as-prepared ferrites, M_s, decreased, while the coercivity, H_c, increased with increasing substitution level, x, and the Curie temperature, T_c, kept a rather high value, fluctuating between 308 and 320 °C. Both the real and imaginary parts of permeability of the ferrites decreased slightly after Pr³⁺ doping. However, the natural resonance frequency shifted towards higher frequency from 13.07 to 36.17 MHz after the addition of Pr³⁺, driving the magnetic permeability to much higher frequency, reaching the highest value (36.17 MHz) when x=0.04. Introduction of Pr³⁺ ions into the Ni-Zn ferrite reduced the values of the dielectric loss tangent, especially in the frequency range of 1-400 MHz. However, the magnitude of dielectric loss of the samples doped with different amounts of Pr³⁺ raised little.     

Structural and magnetic properties of nano-crystalline Ni–Zn ferrites synthesized using egg-white precursor

Nano-crystalline nickel–zinc ferrites of different compositions; Ni_1−xZn_xFe₂O₄ (x=0.0–1.0) were prepared by a precursor method involving egg-white and metal nitrates. An appropriate mechanism for the egg-white-metal complexation was suggested. Differential thermal analysis-thermogravimetry, X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometer and AC-magnetic susceptibility measurements were carried out to investigate chemical, structural and magnetic aspects of Ni–Zn ferrites. XRD confirmed the formation of spinel cubic structure. The average crystallite size was calculated using line broadening in XRD patterns. Structural parameters like lattice constant, X-ray density, bond lengths and inter-cationic distance were determined from XRD data. TEM showed agglomerated particles with average size agreed well with that estimated using XRD. FT-IR spectra confirm the formation of spinel structure and further lends support to the proposed cation distribution. Zn-content was found to have a significant influence on the magnetic properties of the system. The changes in the magnetic properties can be attributed to the influence of the cationic stoichiometry and their occupancy in the specific sites.     

The study on microstructure and microwave-absorbing properties of lithium zinc ferrites doped with magnesium and copper

Lithium zinc ferrites doped with magnesium and copper were prepared by means of a combination of sol-gel method and subsequent calcination. The crystalline phase and microstructure of different doped lithium zinc ferrites were measured by X-ray powder diffraction and scanning electronic microscopy analysis. The results indicate that there are no remarkable differences in phase composition between pure lithium zinc ferrite and the as-doped lithium zinc ferrites. The effects of magnesium and copper dopants on microwave absorption in low-frequency region were investigated by the transmission/reflection coaxial line method. It was found from the present work that doping with copper improved microwave-absorbing properties, while doping with magnesium had little effect on microwave absorption of pure lithium zinc ferrite.     

Structural, magnetic and electrical properties of NiCuZn ferrites prepared by microwave sintering method suitable for MLCI applications

Polycrystalline NiCuZn soft ferrites with stoichiometric iron were prepared by a novel microwave sintering method. The powders were calcined, compacted and sintered at 950 °C for 30 min in a microwave sintering furnace. X-ray diffraction patterns confirm the formation of single phase cubic spinel structure. The grain size was estimated using SEM micrographs. The lattice constant is found to increase with increase in zinc concentration. The sintered ferrites have been investigated for their physical, magnetic and electrical properties such as bulk density, X-ray density, porosity, anisotropy constant, initial permeability, saturation magnetization, DC resistivity, dielectric constant and dielectric loss as a function of zinc concentration. Permeability, saturation magnetization, dielectric constant and dielectric loss were found to increase while DC resistivity was found to decrease with the replacement of Zn with Ni. The present series of ferrites are found to posses properties that are suitable for the core materials in multilayer chip inductors.     

Tailoring the microwave permittivity and permeability of composite materials

The microwave permittivity (ɛ_r) and permeability (μ_r) of composite materials are tailored by adding various loading agents to a host plastic and are subsequently modeled using the Maxwell Garnett theory and second order polynomials. With the addition of manganese zinc ferrite, strontium ferrite, nickel zinc ferrite, barium tetratitanate and graphite powders, materials with values of ɛ′, e″, μ′, μ″ as high as 22, 5, 2.5 and 1.7 have been obtained. Permittivity and permeability data are calculated at 2.0245 GHz from reflection and transmission measurements performed in a 7 mm coaxial test line. The Maxwell Garnett (MG) theory successfully models ɛ_r if the filling factor is less than 0.30 and ratio |ɛ₁| (host)/ |ɛ₂| (powder) is greater than 0.04. As this ratio decreases, the MG theory is shown to be independent of ɛ₂ and second order polynomials are used to effectively model the dielectric constant. Polynomials are also used for the ferrite composites because it was determined that the MG theory was unable to model μ_r. This deficiency is attributed to the difference of domain structures that exist in powdered and sintered ferrites.     

Microwave absorption properties of the Ni nanofibers fabricated by electrospinning

Ni nanofibers with an average diameter of about 100 nm were synthesized by a simple and cost-effective electrospinning technology. The nanofibers have a polycrystalline structure and each nanofiber is composed of fine particles. The complex permittivity and permeability properties of Ni nanofibers composite have been measured in the frequency range of 1–15 GHz. The double-resonance behavior of microwave magnetic permeability is observed. Natural resonance peak happens at 4.0 GHz with the contribution of shape anisotropy. The second resonance peak around 12.5 GHz originates from exchange resonance effect. The permeability spectra were fitted with the Landau–Lifshitz–Gibert equation. The minimum reflection loss of the Ni nanofibers composite reaches ?35.4 dB at 1.3 GHz with a matching thickness of 8.4 mm, which shows promising application of the Ni nanofibers composites in microwave absorber.     

A new method to measure permittivity and permeability in nanopowder materials in microwave range

A new method for measuring the electromagnetic properties (permittivity and permeability) of nanopowder materials in a wide microwave region is presented. Unlike previously developed systems, our experimental setup is based on reflection measurements over a short-circuited transmission line combined with the application of a uniform magnetostatic field. When this field is sufficiently high to saturate the material, the effective permeability of the sample equals the permeability of free space, without modifying its electrical properties. Hence, for each frequency, the permittivity can be obtained through the measurement of a single scattering parameter, such as the reflection coefficient. After this first measurement, and once the external field is removed, the reflection coefficient can be used again to obtain the permeability by means of the permittivity value obtained before. As the major advantage, this procedure allows the recording of the experimental data in just one sweep, using one-port measurements, and without modifying the geometrical characteristics of the sample holder. Hence, the measurement process can be easily automated.     

Mössbauer study of chromium-substituted nickel ferrites

The structural and magnetic properties of the Cr substituted NiCr_xFe_1−xO₄ (0x1.4) spinel ferrites have been investigated by means of X-ray diffraction and Mössbauer spectroscopy techniques. Their crystal structures are found to be pure cubic phases. The Mössbauer spectra at 295 and 78 K of all samples showed two well-resolved magnetic patterns corresponding to the tetrahedral A-sites and octahedral B-sites. The cation distributions driven from the area under resonance curve of each site revealed that the compounds are gradually transferred from perfect inverse spinel to partially normal spinel structure. The behavior of the magnetic properties as a function of Cr³⁺ concentration has been explained on the basis of the driven cation distribution and it showed that the chrome-rich compositions can be explained in terms of the non-collinear spin model.     

Preparation and microwave absorption properties of Ce-substituted lithium ferrite

Ce-substituted lithium ferrite, Li_0.5Ce_xFe_2.5−xO₄ (x=0, 0.015 and 0.15), was prepared from metal nitrates and citric acid by the citrate sol-gel method. The thermal decomposition process was investigated by TG-DSC. The phase composition and microstructure of Li_0.5Ce_xFe_2.5−xO₄ was characterized by X-ray powder diffraction analysis (XRD) and a transmission electron microscope (TEM). The complex permittivity and complex permeability and microwave absorption properties of Li_0.5Ce_xFe_2.5−xO₄-paraffin wax composite were measured by the transmission/reflection coaxial line method in the range of 2-18 GHz. It is shown that the substitution of cerium ion had a close effect on the properties of Li_0.5Ce_xFe_2.5−xO₄ ferrites. Also, the present investigation demonstrates that microwave absorbers for applications over 15 GHz, with satisfactory reflection loss, of more than −20 dB for specific frequencies, could be obtained by controlling the substituted Ce element.