Temperature dependence of excitonic luminescence from nanocrystalline ZnO films

The properties of the excitonic luminescence for nanocrystalline ZnO thin films are investigated by using the dependence of excitonic photoluminescence (PL) spectra on temperature. The ZnO thin films are prepared by thermal oxidation of ZnS films prepared by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The X-ray diffraction (XRD) indicates that ZnO thin films have a polycrystalline hexagonal wurtzite structure with a preferred (0 0 2) orientation. A strong ultraviolet (UV) emission peak at 3.26 eV is observed, while the deep-level emission band is barely observable at room temperature. The strength of the exciton-longitudinal-optical (LO) phonon coupling is deduced from the temperature dependence of the full-width at half-maximum (FWHM) of the fundamental excitonic peak, decrease in exciton-longitudinal-optical (LO) phonon coupling strength is due to the quantum confinement effect.     

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.     

Recovery of soft magnetic properties of FeNiSm films by Ta interlayer

The magnetic properties of FeNiSm thin films with different thicknesses, different Ta interlayer thicknesses and different numbers of Ta interlayers were investigated. The single layer FeNiSm shows in-plane uniaxial anisotropy at a thickness below critical value, but shows weak perpendicular anisotropy with a stripe domain structure at thickness above the critical value. Experiments indicate that one or more Ta interlayers inserted into thick FeNiSm films with weak perpendicular anisotropy were effective not only in canceling the perpendicular anisotropy, but also in recovering the in-plane uniaxial anisotropy. Blocking of the columnar growth of FeNi grains by the Ta interlayer is considered to be responsible for this spin reorientation phenomenon. Moreover, the magnetization reversal mechanism in FeNiSm films with uniaxial anisotropy can be ascribed to coherent rotation when the applied field is close to the hard axis and to domain-wall unpinning when the applied field is close to the easy axis. The dynamic magnetic properties of FeNiSm films with uniaxial anisotropy were investigated in the frequency range 0.1-5 GHz. The degradation of the soft magnetic properties of magnetic thin films due to the growth of columnar grains can be avoided by insertion of a Ta interlayer.     

一种软启停正负双直流电源的设计

变形镜驱动器正负电源加电或者断电不同步,导致其在加电瞬间或者断电瞬间,输出端会输出一个-38.0V和86.0V的冲击电压,这个冲击电压使得变形镜在未开始工作时就产生了较为严重的面形畸变,为此研制了基于微处理器的软启停直流电源,它输出两路按一定时序变化的输出电压作为后续两个继电器的控制端,再通过这两个继电器分别控制变形镜驱动器正、负电源的导断,从而使得变形镜驱动器的正负电源同时加载或者卸载。在硬件平台了进行了实验验证,结果表明,使用软启停直流电源后,变形镜驱动器加电瞬间或断电瞬间,其产生的脉冲电压的峰-峰不超过0.45V,远远小于未使用软启停直流电源前的冲击电压,满足了系统的设计要求。     

Nanocrystals magnetic contribution to FINEMET-type soft magnetic materials with Ge addition

Over the last years several works have been published in which magnetic and structural properties of soft magnetic nanocrystalline alloys were reported. Among these, there are a series of articles where the nanocrystals composition of FINEMET-type alloys with Ge addition was obtained by Mössbauer spectroscopy (MS) and X-ray diffraction (XRD). By considering a linear relationship between the magnetic moments of the nanocrystals and the composition of various elements in these crystallites, the magnetic moment of the nanocrystals was calculated. This paper reviews results obtained by different authors since 1980 and they are compared with ours. In turn, we revised some elements not previously considered for the calculus of the nanocrystals composition that allowed us to obtain the magnetic moment of the crystallites in the alloy. In particular, we analyzed FINEMET-type alloys with replacement of B for Ge: Fe_73.5Si_13.5Ge₂B₇Nb₃Cu₁ and Fe_73.5Si_13.5Ge₄B₅Nb₃Cu₁. The nanocrystalline structure was obtained by isothermal annealing of melt-spun ribbons at 823 K for 1 h. From MS and XRD we obtained the atomic composition of the nanocrystals in the magnetic material. The magnetic contribution of the nanocrystals to the alloy was calculated using a linear model and the results were compared with experimental measurements of the samples.     

A novel method determining longitudinally induced magnetic anisotropy in amorphous and nanocrystalline soft materials

In ferromagnetic amorphous and nanocrystalline soft magnetic alloys the induced magnetic anisotropy plays a fundamental role in the hysteresis behavior but, due to the elongated shape, it can be measured only if K_U is perpendicular to the sample long axis. In order to measure the longitudinal induced anisotropy, an original method derived from known thin layers measurement techniques was used. Hysteresis loops shifted by perpendicular bias field were recorded for this purpose. Direct measurement of the longitudinal induced anisotropy in amorphous and nanocrystalline ribbons or wire without needing sample preparation is reported for the first time. Evidence of self-induced anisotropy is brought in a Fe–Co-based nanocrystalline alloy.     

Evolution of structural and magnetic properties of sputtered nanocrystalline Co thin films with thermal annealing

Ultrafine grain films of cobalt prepared using ion-beam sputtering have been studied using X-ray diffraction (XRD), X-ray reflectivity (XRR), atomic force microscopy (AFM) and magneto-optical Kerr effect (MOKE) measurements. As-prepared films have very smooth surface owing to the ultrafine nature of the grains. Evolution of the structure and morphology of the film with thermal annealing has been studied and the same is correlated with the magnetic properties. Above an annealing temperature of 300 °C, the film gradually transforms from HCP to FCC phase that remains stable at room temperature. A significant contribution of the surface energy, due to small grain size, results in stabilisation of the FCC phase at room temperature. It is found that other processes like stress relaxation, grain texturing and growth also exhibit an enhanced rate above 300 °C, and may be associated with an enhanced mobility of the atoms above this temperature. Films possess a uniaxial anisotropy, which exhibits a non-monotonous behaviour with thermal annealing. The observed variation in the anisotropy and coercivity with annealing can be understood in terms of variations in the internal stresses, surface roughness, and grain structure.     

Influence of the nitrogen contents on the permeability characteristics and soft magnetic properties of FeHfN films

High permeability magnetic films can enhance the inductance of thin-film inductors in DC-DC converters. In order to obtain high permeability, the uniaxial anisotropy and coercivity should be as low as possible. This study employed dc reactive magnetron sputtering to fabricate nanocrystalline FeHfN thin films. The influence of the nitrogen flow on the composition, microstructure, and permeability characteristics, as well as magnetic properties was investigated. Increasing the nitrogen content can alter FeHfN films from amorphous-like to crystalline phases. The magnetic properties and permeability depend on variations in the microstructure. With the optimum N₂/Ar flow ratio of 4.8% (N₂ flow: 1.2 sccm), low anisotropy (H_K = 18 Oe), low coercivity (H_C = 1.1 Oe) and high permeability (μ′ > 600 at 50 MHz) were obtained for fabrication of a nanocrystalline FeHfN film with a thickness of around 700 nm. Such as-fabricated FeHfN films with a permeability of over 600 should be a promising candidate for high-permeability ferromagnetic material applications.     

射频溅射FeTaN纳米晶软磁薄膜结构和磁性

用射频反应溅射制备了FeTaN纳米晶软磁薄膜.研究了薄膜结构和磁性与制备条件的依赖关系.研究发现,当Ta的含量较高时,在N2+Ar混合气氛中易形成沉积态薄膜的非晶结构.适当的热处理后,αFe纳米晶从中晶化生成.薄膜显示出优良的软磁特性 关键词： 纳米晶 软磁性 非晶态     

Composition dependence of microstructure,magnetic and microwave properties in ball-milled FeSiB nanocrystalline flakes

The soft magnetic nanocrystalline/amorphous FeSiB flakes were fabricated by the ball-milling method and evaluations were made of the composition, microstructure, magnetic and microwave properties in the milling process. An investigation of the relationship between microstructure and magnetic/microwave properties showed that the electromagnetic characteristics were attributed to the changes of nanograin size, crystal and amorphous content corresponding to the composition variation. The replacing of Fe atoms by Si in α-Fe crystal caused the decrease of grain size, saturation magnetization and coercivity, while B content devoted to amorphous phase and decreased the permittivity. Consequently, it was observed that the optimum composition for microwave performance is Fe₈₂Si₅B₁₃.     

High-frequency magnetic properties of soft magnetic cores based on nanocrystalline alloy powder prepared by thermal oxidation

FeSiBNbCu nanocrystalline alloy powder was thermally oxidized in an air atmosphere to enhance an oxide layer formation on the surface of the powder and subsequently toroidal shape FeSiBNbCu nanocrystalline alloy powder cores were prepared by compaction at room temperature. The phase change on the surface of FeSiBNbCu nanocrystalline alloy powder by thermal oxidation was analyzed and its effect on the high frequency magnetic properties of the compacted cores was investigated. By thermal oxidation, the formation of the oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the surface of FeSiBNbCu nanocrystalline alloy powder was enhanced and the thickness of oxide layer could be controlled by changing the thermal oxidation time. FeSiBNbCu nanocrystalline alloy powder core prepared from the powder treated by thermal oxidation exhibits a stable permeability up to high frequency range over 10 MHz. The core loss could be reduced remarkably and the dc-bias property could be improved significantly, which were due to the formation of oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the FeSiBNbCu nanocrystalline alloy powder. The improvement in high-frequency magnetic properties of the FeSiBNbCu nanocrystalline alloy powder cores could be attributed to the effective electrical insulation by oxide layer between the FeSiBNbCu nanocrystalline alloy powders.     

The effect of anion on the magnetic properties of nanocrystalline NiO synthesized by homogeneous precipitation

The effect of using different anions (nitrate, chloride, sulfate, and acetate) during the precursor synthesis, by homogeneous precipitation, on the magnetic properties of the final product (nanocrystalline NiO), has been studied. The precursors and the oxide were characterized by various analytical techniques including powder X-ray diffraction, FT-IR spectroscopy, thermal gravimetry (TGA), and magnetic measurements. The synthesized NiO samples possess crystallite size in the range, ∼2−6 nm, depending on the anion of the nickel salt. The nickel oxide nanoparticles exhibit superparamagnetic behavior. Acetate and sulfate anions lead to NiO with higher saturation magnetization (∼1.2−1.8 emu/g), while chloride and nitrate anions lead to NiO nanoparticles with lower saturation magnetization (∼0.1-0.4 emu/g) values. The observed magnetic behavior has been attributed to the size effect.     

Microstructure and soft magnetic properties of Finemet-type ribbons obtained by twin-roller melt-spinning

Soft magnetic ribbons of Finemet-type (Fe_73.5Cu₁Nb₃Si_13.5B₉) alloys are synthesized by the twin-roller melt-spinning technique directly from the melt, at tangential wheel speeds of 15, 18, 19 and 20 m/s. The microstructure and the magnetic properties are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA), thermo-gravimetric analysis (TGA) and hysteresis loops measurements. Samples cooled at 20 m/s are amorphous, while those quenched at lower wheel speeds are partially crystalline. All samples studied present saturation magnetization values (150-160 A m²/kg) higher than the commercial Finemet alloys (∼135 A m²/kg), obtained by controlled crystallization of amorphous single-roller melt-spun alloys. Optimal soft magnetic properties - σ_S=(154±8) A m²/kg and H_C=(6.9±0.9) A/m - are found in samples quenched at 19 m/s, consisting of size-distributed bcc Fe-Si nanograins (∼18 nm in average) embedded in an amorphous residual matrix. A minority nanocrystalline magnetic phase (≤10 nm) is also detected.     

Effect of nanoparticle size on magnetic damping parameter in Co92Zr8 soft magnetic films

Co₉₂Zr₈(50 nm)/Ag(x) soft magnetic films have been prepared on Si (111) substrates by oblique sputtering at 45°. Nanoparticle size of Co₉₂Zr₈ soft magnetic films can be tuned by thickening Ag buffer layer from 9 nm to 96 nm. The static and dynamic magnetic properties show great dependence on Ag buffer layer thickness. The coercivity and effective damping parameter of Co₉₂Zr₈ films increase with thickening Ag buffer layer. The intrinsic and extrinsic parts of damping were extracted from the effective damping parameter. For x=96 nm film, the extrinsic damping parameter is 0.028, which is significantly larger than 0.004 for x=9 nm film. The origin of the enhancement of extrinsic damping can be explained by increased inhomogeneity of anisotropy. Therefore, it is an effective method to tailor magnetic damping parameter of thin magnetic films, which is desirable for high frequency application.     

Influence of environment and grain size on magnetic properties of nanocrystalline Mn–Zn ferrite

Nanocrystalline Mn_1−xZn_xFe₂O₄ (0.2?x?0.9) was prepared by mechanical alloying of the concerned oxide precursors and subsequent annealing in air and Ar atmosphere, respectively. Milling and annealing in air produces Zn-ferrites (ZnFe₂O₄) instead of Mn–Zn ferrites as MnO converts to higher oxides at higher oxygen partial pressure and fails to dissolve in the spinel phase. This is confirmed by careful quantitative X-ray diffraction analysis using Rietvelt profile matching and also by the non-saturating paramagnetic nature of the magnetization response with very low saturation level of these spinels milled and annealed in air. On the other hand, single-phase Mn–Zn ferrite results from the identical precursor oxide blend when milling and annealing are carried out under controlled (Ar) atmosphere. The average grain size of the as-milled and annealed powders, measured by Rietvelt refinement, varies between 6–8 and 14–18 nm, respectively. Further investigations performed with Mn_0.6Zn_0.4Fe₂O₄ reveal that a careful selection of annealing parameters may lead to an early superparamagnetic relaxation. Therefore, the blocking temperature can be significantly reduced through proper heat treatment schedule to ensure superparamagnetism and negligible hysteresis at low temperature.     

Optical properties of nanocrystalline silicon thin films produced by size-selected cluster beam deposition

Molecular beams of size-selected silicon clusters were used to grow nanocrystalline thin films. This technique allows the control of both average size and size dispersion of Si nanocrystals, and is then very useful to provide model materials for the study of the luminescence in silicon. We report results obtained by high-resolution electron microscopy, Raman spectrometry and photoluminescence spectroscopy.     

Controlled synthesis, characterization and optical properties of CdS nanocrystalline thin films via chemical bath deposition (CBD) route

In this study, the CdS nanocrystalline thin films obtained from an ammonia-free chemical bath deposition process. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structure and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin films deposition. The obtained samples were characterized by the techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), atomic force microscopy (AFM) and fluorescence spectroscopy. Also, the effect of two parameters such as pH and temperature of reaction on the synthesis of CdS nanocrystals was studied. Finally, it was found the CdS nanocrystals showed sharp excitation features and strong “band-edge” emission.     

Magnetic behavior of nanocrystalline powders of Co-doped ZnO diluted magnetic semiconductors synthesized by polymerizable precursor method

This paper reports the first synthesis of nanocrystalline powders of Co-doped ZnO (i.e. Zn_0.9Co_0.1O) diluted magnetic semiconductor by a polymerizable precursor method using nitrate salts of Zn and Co and a mixed solution of citric acid and ethylene glycol as a chelating agent and reaction medium, respectively. The polymeric precursors were characterized by TG-DTA to determine the thermal decomposition and crystallization temperature which was found to be at 723 K. The precursors were calcined at different temperatures of 773, 873, 973, and 1073 K for 1 h to obtain nanocrystalline powders. The morphology and crystalline size of the calcined particles were evaluated by SEM, TEM and Scherrer's equation. The average particle sizes calcined at 773, 873, 973, and 1073 K for 1 h were, respectively, 20, 60, 80, 150 nm, obtained from TEM. The XRD and Fourier transmission infrared (FT-IR) results indicated that the synthesized Zn_0.9Co_0.1O powders have the pure wurtzite structure without any significant change in the structure affected by Co substitution. Optical absorption measurements showed absorption bands indicating the presence of Co²⁺ in substitution of Zn²⁺. Room temperature magnetization results revealed a ferromagnetic behavior for the Zn_0.9Co_0.1O powders. Although the specific magnetization seemed to change with the particle size but there was no clear dependency since the largest magnetization was observed in the powders calcined at 873 K (60 nm). Instead, the specific magnetization appeared to show a trend of dependency on the lattice constant c of the wurtzite unit cell.     

High magnetostriction coefficient of Mn substituted cobalt ferrite sintered from nanocrystalline powders and after magnetic field annealing

Magnetostriction characteristics of Mn substituted cobalt ferrite, CoFe_2?xMn_xO₄ (0 ≤ x ≤ 0.3), sintered from nanocrystalline powders of average particle size of ～4 nm have been studied. Larger value of magnetostriction at lower magnetic field is achieved after substitution of Mn for Fe. The maximum value of magnetostriction coefficient is not much affected and the slope of the magnetostriction is increased with increasing Mn content. Higher maximum value of magnetostriction coefficient (λ) of 234 ppm comparable to that of the unsubstituted composition with larger strain derivative (dλ/dH) is obtained for x = 0.2 in CoFe_2?xMn_xO₄. The magnetostriction coefficient is increased to 262 ppm with further enhancement in the strain derivative after annealing the sintered compact at 300 °C in a magnetic field of 400 kA/m for 30 min.