Microwave absorption properties of a double-layer absorber based on nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers

The nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers with diameters of 1–5 μm, high aspect ratios and large specific areas are prepared by the citrate gel transformation and reduction process. The nanocomposite BaFe12O19/α-Fe microfibers show some exchange–coupling interactions largely arising from the magnetization hard(BaFe12O19) and soft(α-Fe) nanoparticles. For the microwave absorptions, the double-layer structures consisting of the nanocomposite BaFe12O19/α-Fe and α-Fe microfibers each exhibit a wide band and strong absorption behavior. When the nanocomposite BaFe12O19/α-Fe microfibers are used as a matching layer of 2.3 mm in thickness and α-Fe microfibers as an absorbing layer of 1.2 mm in thickness, the optimal reflection loss(RL) achieves-47 dB at 15.6 GHz, the absorption bandwidth is about 12.7 GHz ranging from 5.3 to 18 GHz, exceeding-20 dB, which covers 72.5% C-band(4.2–8.2 GHz)and whole X-band(8.2–12.4 GHz) and Ku-band(12.4–18 GHz). The enhanced absorption properties of these double-layer absorbers are mainly ascribed to the improvement in impedance matching ability and microwave multi-reflection largely resulting from the dipolar polarization, interfacial polarization, exchange–coupling interaction, and small size effect.     

Effective Anisotropy in Magnetically Nd2Fe14B/α-Fe Nanocomposite

Considering different contact situations of grains, we effective anisotropies of magnetically soft α-Fe grains, investigate the effects of exchange-coupling interaction on hard Nd2Fe14B grains and NdaFe14B/α-Fe nanocomposite. An expression of effective anisotropy suitable for different degrees of exchange-coupling between grains is presented. The calculation results show that the exchange-coupling interaction increases the average anisotropy of soft grains and decreases that of hard grains. The effective anisotropy of Nd2Fe14B/α-Fe nanocomposite decreases smoothly with decreasing grain size when the grain size is larger than 20 nm while it decreases dramaticaily with further decrease of the grain size. In order to maintain high coercivity in Nd2Fe14B/α-Fe nanoeomposite, the grain size should not be less than 20nm.     

Double-layer microwave absorber of nanocrystalline strontium ferrite and iron microfibers

Microwave absorption properties of the nanocrystalline strontium ferrite (SrFe₁₂O₁₉) and iron (α-Fe) microfibers for single-layer and double-layer structures are investigated in a frequency range of 2 GHz-18 GHz. For the single-layer absorbers, the nanocrystalline SrFe₁₂O₁₉ microfibers show some microwave absorptions at 6 GHz-18 GHz, with a minimum reflection loss (RL) value of -11.9 dB at 14.1 GHz for a specimen thickness of 3.0 mm, while for the nanocrystalline α-Fe microfibers, their absorptions largely take place at 15 GHz-18 GHz with the RL value exceeding -10 dB, with a minimum RL value of about -24 dB at 17.5 GHz for a specimen thickness of 0.7 mm. For the double-layer absorber with an absorbing layer of α-Fe microfibers with a thickness of 0.7 mm and matching layer of SrFe₁₂O₁₉ microfibers with a thickness of 1.3 mm, the minimum RL value is about -63 dB at 16.4 GHz and the absorption band width is about 6.7 GHz ranging from 11.3 GHz to 18 GHz with the RL value exceeding -10 dB which covers the whole K_u-band (12.4 GHz-18 GHz) and 27% of X-band (8.2 GHz-12.4 GHz).     

Evolution of Structural and Magnetic Properties of BaFe12O19 with B2O3 Addition

We investigate the effects of B2O3 addition on structural and magnetic properties of hard magnetic BaFe12O19 particles. The conventional solid state reaction method is used as the synthesis route. Single phase BaFe12O19 could be synthesized with very small amounts of B2O3 addition and with calcination at low temperatures （850°C） in short times （1 h）. B2O3 addition also improves the magnetic parameters significantly. Remanence magnetization and specific magnetization at 1.5 T increase by ～40% in magnitude although no significant variations on coercivity is observed.     

The influence of interface exchange coupling on the demagnetization process for perpendicularly oriented FePt/α-Fe/FePt trilayers

The influence of the interface exchange coupling on the magnetization reversal process for a FePt/α-Fe/FePt tri-layer structure has been studied through a micromagnetic approach.The analytical formula of the nucleation field has been derived.It is found that the nucleation field increases as the interface coupling constant rises.Especially when the thickness of the soft layer is small,the influence of the exchange coupling on the nucleation field is significant.The angular distributions of the magnetization for various exchange coupling constants have been obtained by numerical calculation.It is found that the angular distribution of the magnetization is discontinuous at the interface of the hard and soft layers.In the meantime,the pinning field decreases with the increase of the thickness of the soft layer and the exchange coupling constant.     

Electromagnetic wave absorbing properties and hyperfine interactions of Fe–Cu–Nb–Si–B nanocomposites

The Fe–Cu–Nb–Si–B alloy nanocomposite containing two ferromagnetic phases(amorphous phase and nanophase phase) is obtained by properly annealing the as-prepared alloys. High resolution transmission electron microscopy(HRTEM) images show the coexistence of these two phases. It is found that Fe–Si nanograins are surrounded by the retained amorphous ferromagnetic phase. M¨ossbauer spectroscopy measurements show that the nanophase is the D03-type Fe–Si phase, which is employed to find the atomic fractions of resonant57 Fe atoms in these two phases. The microwave permittivity and permeability spectra of Fe–Cu–Nb–Si–B nanocomposite are measured in the frequency range of 0.5 GHz–10 GHz. Large relative microwave permeability values are obtained. The results show that the absorber containing the nanocomposite flakes with a volume fraction of 28.59% exhibits good microwave absorption properties. The reflection loss of the absorber is less than-10 dB in a frequency band of 1.93 GHz–3.20 GHz.     

Micromagnetism simulation on effects of soft phase size on Nd_2Fe_(14)B/α-Fe nanocomposite magnet with soft phase imbedded in hard phase

In this study, micromagnetism simulation by using finite difference method is carried out on the Nd_2Fe_(14)B/α-Fe nanocomposite magnet with soft phase imbedded in hard phase. The effects of soft magnetic phase size(S) on the magnetic properties and magnetic reversal modes are systematically analyzed. As S increases from 1 nm to 48 nm,the remanence(J_r) increases, while the coercivity(H_(ci)) decreases, leading to the result that the magnetic energy product [(BH)_(max)] first increases slowly, and then decreases rapidly, peaking at S = 24 nm with the(BH)_(max) of 72.9 MGOe(1 MGOe = 7.95775 kJ·m~(-3)). Besides, with the increase of S, the coercivity mechanism of the nanocomposite magnet changes from nucleation to pinning. Furthermore, by observing the magnetic moment evolution in demagnetization process, the magnetic reversal of the soft phase in the nanocomposite magnet can be divided into three modes with the increase of S: coherent rotation(S 3 nm), quasi-coherent rotation(3 nm ≤S 36 nm), and the vortex-like rotation(S 36 nm).     

Spin-Wave Modes in Exchange-Coupled FePt/FeNi Bilayer Films

A simple magnetic modulation structure of the exchange-coupling FePt/FeNi bilayer film is fabricated and studied for its magnetization dynamics using time-resolved magneto-optical polar Kerr spectroscopy. It is found that two spin-wave modes can be excited. One is fixed at -3.2 GHz in frequency for any external field and may serve as a frequency-stabilized spin-wave filter, while the other is external field dependent. In contrast, only the external field-dependent mode is excited in single-layer FeNi, supporting the localized origin of the mode at -3.2 GHz, which is confined to a thin exchange-coupling region. The other external field-dependent mode in frequency is attributed to the Kittel mode.     

Magnetic field control of ferroelectric polarization and magnetization of LiCu_2O_2 compound

A spin model of LiCu2O2 compound with ground state of ellipsoidal helical structure is adopted. Taking into account the interchain coupling and exchange anisotropy, we investigate the magnetoelectric properties in a rotating magnetic field and perform the Monte Carlo simulation on a two-dimensional lattice. A prominent anisotropic response is observed in both the magnetization curve and the polarization curve, qualitatively coinciding with the behaviors that are detected in the experiment. In addition, the influences of the magnetic field with various magnitudes on polarization are also explored and analyzed in detail. As the magnetic field increases, a much smoother polarization of angle dependence is exhibited,indicating the strong correlation between the magnetic and ferroelectric orders.     

Remanence Characteristic of Nanostructure of Hard／Soft Magnetic Multilayered Systems

The relation between microscopic properties (e.e.,layer thickness,easy axis orientation) and the macroscopic magnetic properties such as remanent magnetization of the ferromagnetic multilayer system is investigated based on a simple micromagnet approach.We concentrate on a multilayer design with periodic boundary condition,where alternating soft/hard layers build a nanostructured multilayer.For any easy axis direction in the soft and hard layers a simple explicit expression of remanence of the system has been derived analytically.We find that the remanence clearly depends on the thickness of the soft magnetic layer and is nearly independent of the thickness of hard magnetic layer.On the other hand,the remanence increases upon reducing the angle enclosed by the saturation magnetization and the easy axis directions of soft magnetic layer.However,it is unsensitive to the easy axis direction of hard magnetic layer,but there exists a maximum remanence for a certain easy axis direction of hard magnetic layer.     

Magnetic properties and in compound magnetocaloric effect PrFe12B6

Magnetic properties and magnetocaloric effect of compound PrFe12B6 are investigated. The coexistence of hard phase PrFe12B6 and soft phase α-Fe causes interesting phenomena on the curves for the temperature dependence of magnetization. PrFe12B6 experiences a first order phase transition at the Curie temperature 206 K, accompanied by an obvious lattice contraction, which in turn results in a large magnetic entropy change. The Maxwell relation fails to give the correct information about magnetic entropy change due to the first order phase transition nature. The large magnetic entropy changes of PrFe12.3B4.7 obtained from heat capacity method are 11.7 and 16.2 J/kg.K for magnetic field changes of 0-2 T and 0-5 T respectively.     

Magnetic properties and magnetocaloric effect in compound PrFe12B6

Magnetic properties and magnetocaloric effect of compound PrFe 12 B 6 are investigated.The coexistence of hard phase PrFe 12 B 6 and soft phase α-Fe causes interesting phenomena on the curves for the temperature dependence of magnetization.PrFe 12 B 6 experiences a first order phase transition at the Curie temperature 200 K,accompanied by an obvious lattice contraction,which in turn results in a large magnetic entropy change.The Maxwell relation fails to give the correct information about magnetic entropy change due to the first order phase transition nature.The large magnetic entropy changes of PrFe 12.3 B 4.7 obtained from heat capacity method are 11.7 and 16.2 J/kg.K for magnetic field changes of 0-2 T and 0-5 T respectively.     

Magnetization and demagnetization behaviours of melt-spun Pr12Fe82B6 and Pr8Fe87B5 ribbons

Nanocrystalline Pr_{12}Fe_{82}B_6 and nanocomposite Pr_8Fe_{87}B_5 ribbons have been prepared using a melt spinning technique. Recoil loops have been measured at 20, 200 and 300K. Demagnetization curves are analysed by dividing it into reversible and irreversible portions. High recoil loop susceptibility at low applied field and large reversible change in the demagnetization curve have been found in Pr_8Fe_{87}B_5 ribbons, showing that the reversible behaviours in nanocomposite permanent magnets originate primarily from the magnetically soft phase. The hysteresis in recoil loops found in Pr_8Fe_{87}B_5 ribbons originates from the soft phase α-Fe that suffers a stress.     

Effect of deposition temperature on SrFe_(12)O_(19)@carbonyl iron core–shell composites as high-performance microwave absorbers

The SrFe_(12)O_(19)@carbonyl iron(CI) core–shell composites used in microwave absorption are prepared by the metal–organic chemical vapor deposition(MOCVD). The x-ray diffractometer, scanning electron microscope, energy dispersive spectrometer, and vector network analyzer are used to characterize the structural, electromagnetic, and absorption properties of the composites. The results show that the SrFe_(12)O_(19)@CI composites with a core–shell structure could be successfully prepared under the condition: deposition temperatures above 180℃, deposition time 30 min, and gas flow rate 30 m L/min.The electromagnetic properties of the composites change significantly, and their absorption capacities are improved. Of the obtained samples, those samples prepared at a deposition temperature of 180℃ exhibit the best absorption performance.The reflection loss of SrFe_(12)O_(19)@CI(180℃) with 1.5 mm–2.5 mm in thickness is less than-10 dB in a frequency range of 8 GHz–18 GHz, which covers the whole X band and Ku band.     

Electrical Properties and Raman Spectra of BaBi1-xPbxO3

Crystal structure, electrical properties and Raman spectra of BaBi1-xPbxO3 are reported. The result of x-ray diffraction shows that the specimen is pure, and the lattice parameters decrease continuously in the semiconducting range, whereas it vibrates similarly to a sine wave in the superconducting range, which is ascribed to the existence of oxygen vacancies and the function of breathing modes of Bi（Pb）O6. The temperature dependence of resistivity indicates that the electrical property of the samples is connected sensitively with the crystal structures. Raman spectra show that the specimen becomes disorder when x increases, and the critical temperature To depends not only on the deformation potential of the soft A19 mode derived from the Bi（Pb）O6 rigid rotation, but also on the energy shift of the mode.     

Double spin-glass-like behavior and antiferromagnetic superexchange interaction between Fe~(3+) ions in α-Ga_(2-x)Fe_xO_3(0≤x≤0.4)

Single phase of Fe3+-doped α-Ga2-xFexO3(α-GF x O, x = 0.1, 0.2, 0.3, 0.4) is synthesized by treating the β-Ga2-x Fe x O3(β-GF x O) precursors at high temperatures and high pressures. Rietveld refinements of the X-ray diffraction data show that the lattice constants increase monotonically with the increase of Fe3+content. Calorimetric measurements show that the temperature of the phase transition from α-GF x O to β-GF x O increases, while the associated enthalpy change decreases upon increasing Fe3+content. The optical energy gap deduced from the reflectance measurement is found to decrease monotonically with the increase in Fe3+content. From the measurements of magnetic field-dependent magnetization and temperature-dependent inverse molar susceptibility, we find that the superexchange interaction between Fe3+ions is antiferromagnetic. Remnant magnetization is observed in the Fe3+-doped α-GF x O and is attributed to the spin glass in the magnetic sublattice. At high Fe3+doping level(x = 0.4), two evident peaks are observed in the image part of the AC susceptibility χ ac. The frequency dependence in intensity of these two peaks as well as two spin freezing temperatures observed in the DC magnetization measurements of α-GF0.4O is suggested to be the behavior of two spin glasses.     

Effect of exchange coupling on magnetic property in Sm–Co/α-Fe layered system

The hysteresis loops as well as the spin distributions of Sm–Co/α-Fe bilayers have been investigated by both threedimensional(3D) and one-dimensional(1D) micromagnetic calculations, focusing on the effect of the interface exchange coupling under various soft layer thicknesses t~s. The exchange coupling coefficient Ahsbetween the hard and soft layers varies from 1.8 × 10~(-6)erg/cm to 0.45 × 10~(-6)erg/cm, while the soft layer thickness increases from 2 nm to 10 nm. As the exchange coupling decreases, the squareness of the loop gradually deteriorates, both pinning and coercive fields rise up monotonically, and the nucleation field goes down. On the other hand, an increment of the soft layer thickness leads to a significant drop of the nucleation field, the deterioration of the hysteresis loop squareness, and an increase of the remanence. The simulated loops based on the 3D and 1D methods are consistent with each other and in good agreement with the measured loops for Sm–Co/α-Fe multilayers.     

Microwave Magnetic Properties of Nd2Fe17N3-δ with Planar Anisotropy

Microwave magnetic properties are studied for rhombohedral structure Nd2Fe17N3-δ with planar magnetic anisotropy. Its resin composites show the permeability μ＇0 = 4.15 at low frequency, the natural resonance frequency fT = 1.71 GHz and the resonance bandwidth 6.66 GHz. The calculated static permeability of Nd2Fe17N3-δ reaches 133. The microwave magnetic properties are determined by the c-axis anisotropy field, basal plane anisotropy field and high saturation magnetization. Based on microwave measurement and theoretical fitting on complex permeability spectra, Nd2Fe17N3-δ may be a promising microwave absorber with bandwidth wider than traditional hexaferrites materials in GHz ranges.     

Microwave absorption properties of Ag naowires/carbon black composites

The composite that can absorb the high-performance electromagnetic(EM) wave is constructed into a sandwiched structure composed of carbon black(CB)/ethylene-vinyl acetate(EVA) and Ag naowires(Ag NWs). The Ag NWs sandwiched between two CB/EVA layers are used to improve the absorption properties of composite. The effects of EVA-to-CB weight ratio, concentration and diameter of Ag NWs with a thickness of 0.4 mm on microwave absorption are investigated.The results indicate that for an EVA-to-CB weight ratio of 1:3, Ag NW concentration of 1.0 mg/100 m L, and average diameter of 56 nm, the reflection loss(RL) of the composite is below-10 d B in a frequency range of 9.3 Ghz–18.0 GHz, with the minimum values of-40.0 d B and-25.6 d B at 13.5 GHz and 15.3 GHz, respectively. A finite element method(FEM)is used for calculating the RL of the composite. The calculated results are in agreement with the experimental data.     

Templated synthesis of highly ordered mesoporous cobalt ferrite and its microwave absorption properties

Based on the nanocasting strategy, highly ordered mesoporous CoFe2O4 is synthesized via the ‘two-solvent’ impregnation method using a mesoporous SBA-15 template. An ordered two-dimensional(P6mm) structure is preserved for the CoFe2O4/SBA-15 composite after the nanocasting. After the SBA-15 template is dissolved by NaOH solution, a mesoporous structure composed of aligned nanoparticles can be obtained, and the P6 mm structure of the parent SBA-15is preserved. With a high specific surface area(above 90 m2/g) and ferromagnetic behavior, the obtained material shows potential in light weight microwave absorption application. The minimum reflection loss(RL) can reach-18 dB at about16 GHz with a thickness of 2 mm and the corresponding absorption bandwidth is 4.5 GHz.