Electromagnetic and microwave absorption properties of Fe3O4 magnetic films plated on hollow glass spheres

Magnetic hollow spheres of low density were prepared by plating Fe₃O₄ magnetic films on hollow glass spheres using ferrite plating. The complex permeability and permittivity of spheres–wax composites were measured in the range of 2–18 GHz. The complex permeability and permittivity increased, and the dielectric and magnetic losses were improved as the volume fraction of the magnetic spheres in the composites increased from 60% to 80%, which also resulted in a great improvement of microwave absorption properties. For composites with volume fraction 80%, its magnetic resonance frequency was at about 13 GHz and it appeared three loss peaks in the calculated reflection loss curves; the bandwidth less than −10 dB was almost 4 GHz which was just in the Ku-band frequencies (12–18 GHz) and a minimum reflection loss of −20 dB was obtained when the thickness was 2.6 mm; the microwave absorbing properties were mainly due to the magnetic loss. The results showed that the magnetic spheres composites were good and light microwave absorbers in the Ku-band frequencies.     

Electromagnetic wave absorption properties of carbonyl iron-ferrite/PMMA composites fabricated by hybridization method

The surface of carbonyl iron powder or a mixture of carbonyl iron and ferrite was coated with polymethylmethacrylate (PMMA) microspheres by a hybridization method to make hybrid powders, and then electromagnetic wave absorption properties of the hybrid composites prepared with these hybrid powders have been investigated. As for the carbonyl iron/PMMA hybrid composite, the reflection loss less than −20 dB could be achieved in a frequency range of 1.7–5.0 GHz when the composite thickness was below 5.00 mm. In the case of the carbonyl iron-ferrite/PMMA hybrid composite, a similar reflection loss was observed in a frequency range of 4.3–13.0 GHz. Thus, the addition of ferrite was found to be useful for achieving a large absorption in a wide frequency range, especially for higher frequency values. Simulated values for the minimum reflection loss are well agreed with actually measured ones, because of homogeneous distribution of carbonyl iron and/or ferrite in these hybrid composites.     

Iron-based soft magnetic composites with Mn–Zn ferrite nanoparticles coating obtained by sol–gel method

This paper focuses on iron-based soft magnetic composites which were synthesized by utilizing Mn–Zn ferrite nanoparticles to coat iron powder. The nanocrystalline iron powders, with an average particle diameter of 20 nm, were obtained via the sol–gel method. Scanning electron microscopy, energy dispersive X-ray spectroscopy and distribution maps show that the iron particle surface is covered with a thin layer of Mn–Zn ferrites. Mn–Zn ferrite uniformly coated the surface of the powder particles, resulting in a reduced imaginary permeability, increased electrical resistivity and a higher operating frequency of the synthesized magnets. Mn–Zn ferrite coated samples have higher permeability and lower magnetic loss when compared with the non-magnetic epoxy resin coated compacts. The real part of permeability increases by 33.5% when compared with the epoxy resin coated samples at 10 kHz. The effects of heat treatment temperature on crystalline phase formation and on the magnetic properties of the Mn–Zn ferrite were investigated via X-ray diffraction and a vibrating sample magnetometer. Ferrites decomposed to FeO and MnO after annealing above 400 °C in nitrogen; thus it is the optimum annealing temperature to attain the desired permeability.     

Preparation of the SrFe12O19-based magnetic composites via boron oxide glass devitrification

Magnetic composites were obtained in the system SrO–Fe₂O₃–B₂O₃ by oxide glass heat treatment at 600–950 °C. Samples of the composites were investigated using XRD analysis, magnetic measurements, electron microcopy, and thermal analysis. It was shown that chemical composition of the precursor oxide glass and thermal treatment conditions influenced on the SrFe₁₂O₁₉ particles morphology and magnetic properties. The composites and powders were obtained containing hexaferrite as single domain platelet crystals or polycrystalline aggregates with a coercive force up to 6300 Oe in the former case and 4200 Oe in the latter case.     

Magnetic losses of the soft magnetic composites consisting of iron and Ni–Zn ferrite

Ferromagnetic powders which are surrounded by an electrically insulating film (soft magnetic composites (SMCs)) exhibit unique magnetic properties, such as relatively low magnetic losses and 3D isotropic magnetic behavior. In some electromagnetic applications, including microwave frequency range applications, it is necessary to increase electrical resistivity without any noticeable reduction in magnetic properties. To achieve this purpose, electrically resistant materials, for example, ferrites with acceptable magnetic properties, are suitable candidates. This paper focuses on the effects of the synthesized Ni–Zn ferrite addition on the magnetic properties of the SMCs containing Ni–Zn ferrite within iron particles. The structure was studied by means of X-ray diffraction (XRD). The microstructure and the powder morphology were examined by the use of scanning electron microscopy (SEM). The magnetic measurements on powders and samples were carried out using a vibrating sample magnetometer (VSM) and an LCR meter, respectively. The results indicate that the lowest magnetic loss and the highest magnetic permeability are related to the composites with 20 wt% ferrite and 2 wt% ferrite, respectively. Also, the composites with 10 wt% ferrite show a good combination of magnetic loss and magnetic permeability in the range 0–500 kHz.     

Magnetic properties of spray-formed Fe–6.5%Si and Fe–6.5%Si–1.0%Al after rolling and heat treatment

The maximum silicon content in commercial Fe–Si steels is limited to about 3.5 wt%Si, since the ductility declines sharply as this maximum is exceeded, hindering the production of thin sheets by cold/hot rolling. However, the best magnetic properties are attained at about 6.5 wt%Si, a silicon content that renders magnetostriction practically null and minimizes magnetic losses. Using spray-forming, our research group has successfully produced this type of high silicon alloy in thin sheet form by carefully controlling the many variables of the process and subsequent rolling operations. In the present study, we investigated the magnetic properties and the microstructure of spray-formed Fe–6.5 wt%Si and Fe–6.5 wt%Si–1.0 wt%Al alloys after warm rolling and heat treatment. The main cause for the brittleness of Fe–6.5 wt%Si alloy has been attributed to the B2 phase long-range ordering, which leads to premature fractures. The presence of aluminum could avoid B2 formation and improve the alloy's ductility. The binary Fe–6.5 wt% Si alloy showed the best magnetic properties, which were ascribed to a recrystallized, coarse grain size (∼500 μm; and 340 μm for the Al-containing alloy). TEM analysis showed that a well-developed B2 domain structure (about 50–300 nm in size) was formed in the binary alloy when low cooling rates are prevailing after heat treatment. This structure contributed to improve additionally the magnetic properties, but its effect was not so strong as that of the grain size. The addition of Al to the binary alloy suppressed B2 formation, as indicated by Mossbauer spectroscopy, and apparently hindered excessive grain growth, which may explain the slightly poorer magnetic properties when compared with the binary alloy.     

Preparation of nanocrystalline Fe–Ni powders by mechanical alloying used in soft magnetic composites

This paper focuses on the preparation of nanocrystalline Fe–Ni powders by mechanical alloying method, which can be used in soft magnetic composites. Fe–10 wt% Ni and Fe–20  wt % Ni alloys were prepared using a high-energy ball mill. The magnetic properties of samples were measured by a B–H curve analyzer and microstructures of the samples were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The bcc Fe(Ni) phase formation was identified by XRD and completed after 45 h of milling. It was found that higher milling time resulted in, larger lattice parameter, higher microstrains and smaller crystallite sizes. Also, results showed that with increasing the milling time, coercivity increased and saturation intrinsic flux density firstly increased noticeably and then decreased in higher milling times (>70 h).     

The influence of the dielectric on the properties of dielectromagnetic soft magnetic composites. Investigations with silica and silica hybrid sol–gel derived model dielectric

Dielectromagnetics made from organic–inorganic hybrid silica-coated iron powders were characterised by determination of their physical, mechanical and magnetic properties. The influence of three main factors, dielectric composition, addition level and heat treatment conditions were investigated. Results showed that these factors have significant effects on the performance of the dielectromagnetics. Increase in the organic phase content in these dielectric coatings tends to increase both the electrical resistivity and magnetic permeability of dielectromagnetics, although the strength and density are slightly impaired. Increasing the coating thickness leads to improvements in resistivity and thus reduced eddy current losses, but these are offset by reductions in density, strength and particularly magnetic permeability. A hybrid organic–inorganic coating formulation based on 40 mol% MTMS and 60 mol% TEOS precursors was found to be the optimum composition investigated. Addition levels between 0.1% and 0.3% were found to offer a good compromise between maximum permeability (μ_max>400) and minimum loss (typically <8 W/kg) for operation at 50 Hz/1 T, and the system can be optimised within this range for the desired performance.     

Magnetic,magnetoelectric and dielectric behavior of CoFe2O4–Pb(Fe1/2Nb1/2)O3 particulate and layered composites

Magnetic, magnetoelectric and dielectric properties of multiferroic CoFe₂O₄–Pb(Fe_1/2Nb_1/2)O₃ composites prepared as bulk ceramics were compared with those of tape cast and cofired laminates consisting of alternate ferrite and relaxor layers. X-ray diffraction analysis and Scanning Electron Microscope observations of ceramic samples revealed two-phase composition and fine grained microstructure with uniformly distributed ferrite and relaxor phases. High and broad maxima of dielectric permittivity attributed to dielectric relaxation were found for ceramic samples measured in a temperature range from −55 to 500 °C at frequencies 10 Hz–2 MHz. Magnetic hysteresis, zero-field cooled (ZFC) and field cooled (FC) curves, and dependencies of magnetization on temperature for both magnetoelectric composites were measured with a vibrating sample magnetometer in an applied magnetic field up to 80 kOe at 4–400 K. The hysteresis loops obtained for composites are typical of a mixture of the hard magnetic material with a significant amount of the paramagnet. The bifurcation of ZFC–FC magnetizations observed for both composites implies spin-glass behavior. Magnetoelectric properties at room temperature were investigated as a function of dc magnetic field (0.3–7.2 kOe) and frequency (10 Hz–10 kHz) of ac magnetic field. Both types of composites exhibit a distinct magnetoelectric effect. Maximum values of magnetoelectric coefficient attained for the layered composites exceed 200 mV/(cm Oe) and are almost three times higher than those for particulate composites.     

Microwave-absorbing properties of shape-optimized carbonyl iron particles with maximum microwave permeability

The microwave-absorbing properties for different shapes of carbonyl-iron particles prepared by the high-energy planetary ball milling with 40 vol% in epoxy resin matrix have been investigated. Higher value of magnetic permeability and permittivity can be obtained in the composites for thin flake carbonyl iron than spherical powders. The results are attributed to reduction of eddy current loss, orientation of magnetic moment and space-charge polarization with the shape change from spherical powders to thin flake particles. As the iron flakes with 0.4 μm in thickness as the absorbent fillers, the minimum RL value of −6.20 dB was observed at 4.57 GHz with thickness of 1 mm. The minimum reflection loss (RL) shifts to lower frequency and the value declines with change from spherical powders to thin flakes. It results from the considerable dielectric loss in the absorbing materials.     

Influence of particle size and compaction pressure on the magnetic properties of iron-phenolic soft magnetic composites

This paper investigates the effect of particle size and compaction pressure on the magnetic properties of iron-phenolic soft magnetic composites (50 Hz-1000 kHz). The results showed that the optimum amount of phenolic resin to attain maximum permeability and minimum loss factor at 10 kHz is 0.7 wt% for samples containing iron powder with average particle size ∼150 μm compacted at 800 MPa. In accordance with this resin content, at high frequencies (>300 kHz), the sample with lower particle size ∼10 μm exhibits higher magnetic permeability, higher operating frequencies and lower imaginary part of permeability. With increase in the compaction pressure, specific resistivity decreases and imaginary and real parts of permeability increase at low frequencies.     

Low field magneto-transport in LBSMO–PMMA composite

The low field magneto-transport has been measured as a function of temperature in the range 77–300 K and magnetic field; H?3.6 kOe for La_0.7Ba_0.2Sr_0.1MnO₃ (LBSMO)–x wt% PMMA composites where x=0, 2, 6 and 10. The X-ray diffraction (XRD) study reveals that no structural modification has occurred in the LBSMO in the composite. Scanning electron microscopy (SEM) investigation shows PMMA getting dispersed through the sample volume and some LBSMO grains appear to be coated with the polymer. The metal-like transition observed at ∼150 K in the virgin LBSMO sample disappears in the composite samples and the resistance shows an increase of about three orders of magnitude as the polymer concentration is increased to 10 wt%. Despite this huge increase in the resistance, the low field magneto-resistance (LFMR) shows an enhancement although smaller than the values commonly observed for other manganite-polymer composite systems. Spin polarized tunneling that causes LFMR seems to be enhanced in the composites.     

Magnetic properties of HITPERM-type alloys at high temperature

(Fe,Co)–Zr,Hf)–Cu–B (HITPERM-type) alloys with variable Hf, Zr and Co content were isothermally crystallised at 500–650 °C for 1 h, and the optimum nanocrystallisation temperature was selected on the basis of the minimum coercive field at room temperature. The quasistatic hysteresis loops were measured at temperature from 20 to 650 °C. Subsequently, the optimally annealed alloys were subjected to long-term annealing at 500, 550 and 600 °C. Working temperature of 600°C is too high for the investigated alloys to maintain stable magnetic properties. Temperature of 550 or 500 °C permits the material to be magnetically stable for a long period. The magnetic hysteresis loops recorded for the nanocrystalline alloys, where Fe:Co ratio is close to 1 and refractory metals content is 7 at.%, prove that coercive field increases slightly with temperature, but remains in the range of 20–40 A/m (depending on the alloy composition) from 20 to 550 °C. This proves that the investigated alloys, after optimisation of chemical composition, may be suitable for high temperature use.     

Evolution of magnetic order in mechanically alloyed Al–1 at%Fe

The evolution of ferromagnetic order in high-energy ball-milled Al–1 at% Fe before the onset of a considerable Fe–Al solid solution phase has been investigated using ⁵⁷Fe Mössbauer and bulk magnetization studies. The unmilled sample does not exhibit bulk magnetic properties and an onset of bulk magnetization is observed only after 30 min of milling, when the grain size becomes comparable to the ferromagnetic exchange length. The Curie temperatures of all the samples are less than that of pure iron. The reduction in grain size is accompanied by an increase in coercivity and reduced remanence and a decrease in T_C. The effective magnetic moment per iron atom decreases with the development of a non-magnetic, Al-rich Fe–Al solution on longer milling. The clustering of Fe at grain boundaries is responsible for the observed bulk magnetic ordering. The systematic variation of the magnetic properties has been qualitatively correlated with the evolution of microstructure, reduction in grain size and enhanced inter-granular exchange coupling.     

Preparation and characterization of carbonyl iron/glass composite absorber as matched load for isolator

Composite absorbers made from 66 wt% carbonyl iron and 34 wt% low melting point glass powder were prepared by a pressureless sintering technique in a nitrogen atmosphere. Apparent porosity and bending strength of the as-prepared composites were investigated. The microstructure, heat resisting properties and electromagnetic properties were characterized by scanning electron microscopy, thermal gravimetric analysis–differential scanning calorimetry and vector network analyzer. The results show that the carbonyl iron/glass composite absorbers were difficult to densify. As the sintering temperature and soaking time increased, the apparent porosity first decreased and then increased, whereas the bending strength showed the opposite change. The composite absorber sintered at 520 °C for 40 min achieved the minimum apparent porosity of 13.08% and the highest bending strength of 52 MPa. Compared to the carbonyl iron/silicone rubber absorber, the carbonyl iron/glass composite absorber exhibited better heat resisting properties, and the initial oxidation temperature was increased about 200 °C. The composite absorber with a thickness of 1.25 mm showed a good microwave absorbing property in 8–12 GHz.     

High frequency properties of composite membrane with in-plane aligned Sendust flake prepared by infiltration method

Composite membranes (with thickness around 100–200 μm) containing highly in-plane aligned Sendust flakes embedded in polyvinyl alcohol matrix were prepared with a novel infiltration method. As compared with tape-casting method, infiltration method results in enhanced magnetic permeability, which could be caused by better alignment and less porosity. Annealing process could modify the grain size, improve saturation magnetization and coercivity of Sendust flakes. Hence, the radio and quasi-microwave frequency permeability (between 10 MHz to 3 GHz) of composites membranes with annealed Sendust flakes could be enhanced significantly as compared with that of the as-prepared flakes. Infiltration method is especially suitable for composites with high concentration of flaky fillers. The composite membranes prepared have potential applications, including electromagnetic shielding, noise reduction and wave absorption.     

Magnetodielectric effect in composites of nanodimensional glass and CuO nanoparticles

Nanocomposites comprising CuO particles of average diameter 21 nm coated with 5 nm silica glass containing iron ions were synthesized by a chemical route. An ion exchange reaction at the nanoglass/CuO interface produced iron-doped CuO with copper ion vacancies within the nanoparticles. Room temperature ferromagnetic-like behavior was observed in the nanocomposites. This was ascribed to uncompensated spins contributed by Fe ions with associated copper ion vacancies. A rather high value of magnetodielectric parameter in the range 16–26% depending on the measuring frequency was exhibited by these nanocomposites at a magnetic field of 10 KOe. This was caused by a magnetoresistance of 33% in the iron doped CuO nanoparticles. The experimental results were fitted to the Maxwell–Wagner Capacitor model developed by Catalan. These materials will be suited for magnetic sensor applications.     

Evolution of modulated structure in Fe–Cr–Co alloy during isothermal ageing with different external magnetic field conditions

The evolution of modulated structure in Fe–Cr–Co alloy during isothermal ageing with and without external magnetic field was investigated by using transmission electron microscopy (TEM) and phase-field simulation. The isotropic modulated structure in Fe–Cr–Co alloy formed during isothermal ageing without external magnetic field could be converted to be anisotropy during further isothermal ageing under an 8 kOe external magnetic (thermo-magnetic treatment), and the formation of anisotropy was slower than the direct formation from spinodal decomposition under external magnetic field and is time dependent. The anisotropy characteristic of modulated structure in Fe–Cr–Co alloy subjected to thermo-magnetic treatment for 1 h remained during further isothermal ageing without external magnetic field for up to 20 h. Novel modulated structure could be obtained through specific thermo-magnetic treatment processes, which was confirmed by phase-field simulation.     

Effect of composition on phase formation,microstructure and magnetic properties of Nd–Fe–B thin films

Nd–Fe–B hard magnetic thin films were deposited on a combined Cr/Ta buffer layer on heated MgO(1 0 0) substrates by pulsed laser deposition. The effect of composition on phase formation, morphology and magnetic properties was investigated. For a quantification of phase formation, hysteresis measurements along the hard axis are used. Optimum phase formation is obtained at a Nd/Fe ratio around 0.3. For this Nd content an excellent magnetic texture and a coercivity up to 1 T can be obtained; however, remanence is reduced and the films have a granular microstructure with high roughness. Also a certain B surplus is of benefit. However, with a too-high B or Nd surplus phase formation is more difficult.     

Magnetic properties of iron-based soft magnetic composites with MgO coating obtained by sol-gel method

Soft magnetic composites with a thin MgO insulating layer were produced by a sol-gel method. Energy dispersive X-ray spectroscopy, X-ray analysis, Fourier transform infrared spectroscopy, density measurement and compositional maps confirmed that thin layers of MgO covered the iron powders. Coercivity measurement showed that the stress relaxation and reduction of hysteresis loss efficiently occurred at 600 °C. At this temperature, the phosphate insulation of commercial SOMALOY^TM samples degrade and their electrical resistivity, magnetic permeability and operating frequency decreases noticeably. The results show that the MgO insulation has a greater heat resistance than conventional phosphate insulation, which enables stress-relief at higher temperatures (600 °C) without a large increase in eddy current loss. The results of annealing at 600 °C show that the electrical resistivity and ferromagnetic resonance frequency increased from 11 μΩ m and 1 kHz for SOMALOY^TM samples to 145 μΩ m and 100 kHz for the MgO insulated composites produced in this work.