纳米结构SmCo/FeCo多层薄膜的磁性

研究了用磁控溅射方法制备的纳米结构Sm₂₂Co₇₈单层膜、Sm₂₂Co₇₈/Fe₆₅Co₃₅双层膜及Sm₂₂Co₇₈/Fe₆₅Co₃₅/Sm₂₂Co₇₈三层膜的磁性,特别是双层膜及三层膜系统中的Fe₆₅Co₃₅软磁层厚度d对薄膜剩磁比(M_r/M_s)和矫顽力(H_c)的影响.所有样品的磁滞回线测量表明,该系列薄膜的易磁化轴在膜面内.磁滞回线的单一硬磁相特征,说明SmCo硬磁层与FeCo软磁层之间的交换相互作用使两相很好地复合在一起.在双层膜和三层膜中,M_r/M_s随软磁层厚度d的增加单调上升,而矫顽力随d的变化出现一峰值.通过研究ΔM随磁场H的变化,发现随着软、硬磁相界面数的增加,ΔM曲线的正峰数量逐渐减少;负峰数量逐渐增强,负峰的半高宽逐渐减少. 关键词：     

Microwave reflection properties of planar anisotropy Fe50Ni50 powder/paraffin composites

The reflection properties of planar anisotropy Fe₅₀Ni₅₀ powder/paraffin composites have been studied in the microwave frequency range. The permeability of Fe₅₀Ni₅₀ powder/paraffin composites is greatly enhanced by introducing the planar anisotropy, and can be further enhanced by using a rotational orientation method. The complex permeability can be considered as the superposition of two types of magnetic resonance. The resonance peak at high frequency is attributed to the natural resonance, while the peak at low frequency is attributed to the domain-wall resonance. The simulated results of the microwave reflectivity show that the matching thickness, peak frequency, permeability, and permittivity are closely related to the quarter wavelength matching condition. The Fe₅₀Ni₅₀ powder/paraffin composites can be attractive candidates for thinner microwave absorbers in the L-band (1-2 GHz).     

Microstructure and microwave permeability of FeCo thin films with Co underlayer

Microstructure, static magnetic properties and microwave permeability of sputtered FeCo films were examined. Fe₆₀Co₄₀ films (100 nm in thickness) deposited on glass substrates exhibited in-plane isotropy and a large coercivity of 161.1 Oe. When same thickness films were deposited on 2.5 nm Co underlayer, well-defined in-plane anisotropy was formed with an anisotropy field of 65 Oe. The sample had a static initial permeability of about 285, maximum imaginary permeability of 1255 and ferromagnetic resonance frequency of 2.71 GHz. Cross-sectional TEM image revealed that the Co underlayer had induced a columnar grain structure with grain diameter of 10 nm in the FeCo films. In comparison, FeCo films without Co underlayer showed larger grains of 70 nm in diameter with fewer distinct vertical grain boundaries. In addition, the Co underlayer changed the preferred orientation of the FeCo from (1 0 0) to (1 1 0). The improvement in soft magnetic properties and microwave behavior originates from the modification of the film microstructure, which can be well understood by the random anisotropy theory.     

Preparation and characterization of the ZnO:Al/Fe65Co35/ZnO:Al multifunctional films

The ferromagnetic transparent conducting film is a multifunctional film which has high visible transmittance, low resistivity and room-temperature ferromagnetism, simultaneously. In this article, ferromagnetic transparent conducting ZnO:Al/Fe₆₅Co₃₅/ZnO:Al multilayer films were fabricated by inserting a middle magnetic Fe₆₅Co₃₅ layer into aluminum-doped zinc oxide (ZnO:Al) matrix using a magnetron sputtering apparatus at substrate temperature ranging from room temperature (RT) to 400^∘C. The total film thickness was about 400 nm and the middle Fe₆₅Co₃₅ alloy layer was 4 nm. The influences of substrate temperature (T _s ) on the structural, electrical, optical and magnetic properties of the multilayer films were systemically investigated. The results showed that the microstructure and performance of the composite multilayer films strongly depended on the substrate temperature. The present results also showed that the inserted middle Fe₆₅Co₃₅ alloy thin layer played an important role in providing the RT ferromagnetism and decreasing the resistivity of the multilayer films. Therefore, it is possible to obtain a multifunctional film material with the combination of good optical transparency, high electrical conductivity and RT ferromagnetism.     

高电阻率多层纳米颗粒膜软磁特性及微波磁导率

研究了应用于微波频段的多层纳米颗粒膜的电阻率、软磁特性和微波磁导率.采用多次顺序沉积Co₄₀Fe₄₀B₂₀和SiO₂薄层制备了薄膜.在100kA/m均匀面内磁场经过250℃真空退火2h,制备的Co₄₀Fe₄₀B₂₀/SiO₂多层膜具有难轴矫顽力为210A/m、饱和磁化强度为838.75kA/m、电阻率为2.06×10^{3关键词： 纳米颗粒膜 电阻率 软磁特性 微波磁导率}     

Synthesis, characterization and electromagnetic properties of Fe1−xCox alloy flower-like microparticles

Fe_1−xCo_x alloy microparticles with size 3-5 μm and novel flower-like shapes were prepared by a simple low temperature reduction method. The electromagnetic properties for the paraffin matrix composites containing Fe_1−xCo_x alloy microparticles were measured using a vector network analyzer in the 2-18 GHz frequency range. As a consequence of large surface- and shape-anisotropy energy for the flower-like shaped 3D microstructures, the strong natural resonance around 8-12 GHz and remarkable dielectric relaxation were observed in the complex permittivity and permeability spectrum, which are dominant in the enhanced electromagnetic wave absorption (EMA) performance. It was found that both the electromagnetic parameters of complex permittivity and permeability and the intensity and location of absorption band were remarkably dependent on the Co/Fe molar ratio. The enhanced EMA performance was obtained in these Fe_1−xCo_x-paraffin (x=0.4, 0.5, and 0.6) composites system. For the Fe_0.5Co_0.5 alloy, the reflection loss (RL) exceeding −20 dB was obtained in the broad frequency range of 5.4-18 GHz with a thin sample thickness of between 1.0 and 2.9 mm. In particular, an optimal RL of −59 dB was obtained at 3.61 GHz with a thin thickness of 3.6 mm for the Fe_0.4Co_0.6 sample. The Fe_1−xCo_x alloy microparticles may be attractive candidates for applications of microwave absorption materials with a wide frequency range and strong absorption in the high frequency region.     

Thickness-dependent magnetic properties of Ni81Fe19, Co90Fe10 and Ni65Fe15Co20 thin films

We present results of magnetization and magnetic anisotropy measurements in thin magnetic films of the alloys Ni₈₁Fe₁₉, Co₉₀Fe₁₀ and Ni₆₅Fe₁₅Co₂₀ that are commonly used in magnetoelectronic devices. The films were sandwiched between layers of Ta. At room temperature the critical thickness for all the films to become ferromagnetic is in the range 11–13 Å. In Co₉₀Fe₁₀ the coercivity and the anisotropy field both depend strongly on layer thickness.     

On the comparison of the polarisation behaviour of exchange-biased AF/F NiMn/Fe37Co48Hf15 bi-layer and multi-layer films with increased ferromagnetic cut-off frequencies

Antiferromagnetic/ferromagnetic (AF/F) NiMn/Fe₃₇Co₄₈Hf₁₅ films were investigated with respect to their exchange bias, in-plane unidirectional anisotropy, polarisation and high frequency behaviour. After deposition, carried out by r.f. magnetron sputtering, the films were post-annealed for 4 h at 300 °C in a static magnetic field, in order to induce exchange-bias, which results in a unidirectional anisotropy. Dependent on the presence of a bi-layer or multi-layer sandwich structure the films show a different exchange-bias field-ferromagnetic inter-layer thickness behaviour with exchange-bias fields μ₀?H_eb between 2 and 10 mT. The in-plane uniaxial (single film) or unidirectional anisotropy fields μ₀*H_UF were between 4 and 18 mT. This results in a significant increase of the cut-off frequency in the GHz range in comparison to a single Fe₃₇Co₄₈Hf₁₅ film, which is shown by frequency-dependent permeability plots. High damping in the imaginary part of the permeability, i.e., high resonance line broadening could be observed for films with high coercivity μ₀*H_c of around 7 mT in the easy axis of magnetisation.     

Effect of shape of Fe3Al particles on their microwave permeability and absorption properties

Flake-shaped and sphere-shaped Fe₃Al powder-paraffine composites have been studied in the microwave frequency range. Mössbauer results show that the flake-shaped Fe₃Al particles have easy magnetization plane, which indicates that it is planar-anisotropy. Enhanced permeability is achieved in flake-shaped Fe₃Al compared with the sphere-shaped Fe₃Al. The permeability is further enhanced by using a rotational orientation method. The complex permeability can be characterized by the superposition of two types of magnetic resonance. The resonance peak at high frequency is attributed to the natural resonance, while the peak at low frequency is attributed to the domain-wall resonance. By employing the shape effect and the rotational orientation, the peak frequency of reflection loss for the oriented sample was adjusted to L-band. The planar-anisotropy Fe₃Al powder-paraffine composite can be attractive candidates for thinner microwave absorbers in L-band (1-2 GHz).     

Fabrication, magnetism and high frequency application of exchange-coupled Fe65Co35±2-SiO1.7±0.2 granular films

A series of (Fe₆₅Co_35±2)_x-(SiO_1.7±0.2)_1−x nano-granular films with various metal volume fractions (x) were fabricated by rf sputtering. In a wide range, excellent soft magnetic properties have been achieved. In the x range from 0.7 to 0.48, the films exhibit small coercivity H_c not exceeding 4 Oe and high electrical resistivity ρ up to 1.15 × 10⁴ μΩ cm. And a minimum H_c value of 1.65 Oe was obtained for the sample of x = 0.57 with ρ = 2.86 × 10³ μΩ cm. At a frequency lower than 2.0 GHz, the real part μ′ of complex permeability of this sample is more than 170 and the FMR frequency is as high as 2.6 GHz, implying a high cut-off frequency for high frequency applications. With decreasing Fe₆₅Co_35±2 volume fraction, the resistivity of films increases remarkably and the grain size decreases obviously. At the same time, the coercivity H_c decreases with grain size decreasing, which is consistent with the conclusion resulted from random anisotropy model quoted by Herzer. Study on Henkel plots shows that intergranular ferromagnetic exchange coupling exists among grains and is important for realizing soft magnetic properties.     

Soft magnetic properties of FeCo films with Co underlayer

Bilayered Fe₆₅Co₃₅ (=FeCo)/Co films were prepared by facing targets sputtering with 4πM_s∼24 kg. The soft magnetic properties of FeCo films were induced by a Co underlayer. H_c decreased rapidly when the Co underlayer was 2 nm or more. The films showed well-defined in-plane uniaxial anisotropy with the typical values of H_ce=10 Oe and H_ch=3 Oe, respectively. High frequency characteristics of the films show the films can work at 0.8 GHz with real permeability as high as 250.     

Thickness dependence of microwave magnetic properties in electrodeposited Fe-Co soft magnetic films with in-plane anisotropy

In this work, the thickness effect of Fe₅₂Co₄₈ soft magnetic films with in-plane anisotropy on static and microwave magnetic properties was investigated. The hysteresis loop results indicated that the static in-plane uniaxial anisotropy field increased from almost 0-60 Oe with increasing film thickness from 100 to 540 nm and well-defined in-plane uniaxial magnetic anisotropy can be obtained as the thickness reached 540 nm or larger. Based on Landau-Lifshitz-Gilbert (LLG) equation, the microwave complex permeability spectra were analyzed and well fitted. The LLG curve-fitting results indicated that the initial permeability increased from 106 to 142 and the resonant frequency was shifted from 4.95 to 4.29 GHz as the film thickness was varied from 540 to 1500 nm. Moreover, it was found that there was a discrepancy between the static and the dynamically determined anisotropy field, which can be explained by introducing an additional effective isotropic ripple field. The decreased ripple field was suggested to result in a significant decrease of damping coefficient from 0.109 to 0.038.     

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.     

Determination of intrinsic FMR line broadening in ferromagnetic (Fe44Co56)77Hf12N11 nanocomposite films

Ferromagnetic nanocomposite (Fe₄₄Co₅₆)₇₇Hf₁₂N₁₁ films were deposited to investigate their intrinsic damping mechanisms due to scattering of itinerant electrons, which carry the magnetic moment of the ferromagnetic transition elements. The films were produced by reactive r.f. magnetron sputtering using a 6 in. Fe₃₇Co₄₆Hf₁₇ target. They were annealed at 400 °C in a static magnetic field, in order to induce in-plane uniaxial anisotropy. Subsequently, the films can be considered as uniformly magnetised. A ferromagnetic resonance frequency (FMR) of around 2.3 GHz could be attained, which was determined by measuring the real and imaginary parts of the frequency dependent permeability up to 5 GHz. The imaginary part, which represents a typical resonance curve, was utilised to obtain its full-width at half-maximum Δf_eff (FWHM) for the total damping behaviour characterisation. Thereby, it is possible to extract the intrinsic Gilbert damping parameter α_int, which in turn can be decomposed into two additional damping terms α_sf and α_os allocated to “spin-flip” and “ordinary scattering”, respectively. This result is correlated and discussed in terms of a verified theoretic model, to identify whether damping due to spin-flip scattering and/or ordinary scattering is dominant.     

Microwave dielectric and magnetic properties of superparamagnetic 8-nm Fe3O4 nanoparticles

The superparamagnetic 8-nm Fe₃O₄ nanoparticles were successfully prepared by chemical oxidation process. For the complex permittivity, the dual dielectric relaxation processes have been proved by two overlapped Cole–Cole semicircles, and the natural resonance frequency is 3.03 GHz for the complex permeability. The maximum reflection loss value reaches −55.5 dB at 6.11 GHz with 3.85 mm in the thickness of the absorbers for the superparamagnetic 8-nm Fe₃O₄ nanoparticles which is better than that of 150 nm and 30 nm Fe₃O₄ nanoparticles. It is believed that the superparamagnetic 8-nm Fe₃O₄ nanoparticles can be used as a kind of candidate for microwave absorber.     

Electromagnetic wave-absorption properties of rapidly quenched of Nd–Fe–B nanocomposites with low Nd content

The electromagnetic wave-absorption properties of Nd₃Fe_68−xMn_xCo₁₈B₁₁ (x=0, 1, 2) alloys obtained by rapid quenching from the melt was studied. The complex permittivity-frequency and permeability-frequency properties were determined in the microwave frequency regime of 2–18 GHz by vector network analysis. XRD spectra showed that only α-Fe diffraction peak was observed in the as-spun alloys. It is found that the acquired complex permittivity and permeability values match the microwave frequency when the 1 at% Mn content was doped. A minimum reflection loss of −6.9 dB is obtained at 2.7 GHz for composite Nd₃Fe₆₆Mn₂Co₁₈B₁₁ with absorber thickness of 1.5 mm. The exchange interaction was attributed to the microwave absorption properties. The results suggest a new design of microwave absorbers based on electromagnetic wave-absorbing materials.     

Glass additive influence on the sintering behavior, microstructure and microwave magnetic properties of Cu-Bi-Zn co-doped Co2Z ferrites

The Bi₂O₃-B₂O₃-ZnO-SiO₂ (BB35SZ) glass effects on the sintering behavior and microwave magnetic properties of Cu-Bi-Zn co-doped Co₂Z ferrites were investigated to develop low-temperature-fired ferrites. The glass wetting characteristics on the Co₂Z ferrite surface, X-ray diffractometer, scanning electron microscopy and a dilatometer were used to examine the BB35SZ glass effect on Co₂Z ferrite densification and the chemical reaction between the glass and Co₂Z ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Z ferrites from 1300 to 900 °C. 3(Ba_0.9Bi_0.1O)·2(Co_0.8Cu_0.2O)·12(Fe_1.975Zn_0.025O₃) ferrite with 2 wt% BB35SZ glass can be densified below 900 °C, exhibiting an initial permeability of 3.4. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

CoFeB/MgO不连续多层纳米软磁薄膜微波电磁特性

采用直流/射频磁控溅射方法和不连续多层交替沉积工艺,制备了CoFeB/MgO系列纳米不连续多层薄膜,研究了微波频段下的电磁性能.结果表明,通过调整CoFeB合金相和MgO介质相的相对含量可有效调控薄膜的微结构和电磁性能,且在［Co₆₄Fe₂₄B₁₂(0.7nm)/MgO(0.4nm)］₄₀薄膜中获得了优良的微波软磁性能和高电阻率,其饱和磁化强度1.3 T,难轴矫顽力130 A/m,电阻率3.4mΩ·cm,且共振频率高达2.1 GHz,磁导率实部μ′和虚部μ″在1.59 GHz处均高于240,并且在0.9—2 GHz宽频带范围同时大于100.该薄膜可用于微波吸收材料和电磁兼容的设计中. 关键词： 纳米磁性颗粒膜 磁导率频谱 电磁性能 吸波材料     

Structural and magnetic properties of free-standing Ni0.23Cu0.11Zn0.66Fe2O4 thick films prepared using a modified tape-casting method

Free-standing Ni_0.23Cu_0.11Zn_0.66Fe₂O₄ thick films have been prepared using a modified tape-casting method from a viscous paint. The surface morphology, saturation magnetization (M_s), coercivity (H_c) and complex permeability of the thick films were studied. It is demonstrated that the thick films have relatively larger complex permeability and higher resonance frequency as compared to their bulk material counterparts.     

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.