Fe/[NiFe/Cu]30多层膜线列阵的铁磁共振研究

用铁磁共振(FMR)技术研究Fe/［NiFe/Cu］₃₀多层膜线列阵的特性,线列阵采用激光全息光刻和离子束刻蚀技术加工,线密度为250线/mm和500线/mm.线列阵样品的FMR谱在稍高于主峰（声学峰）的共振场下有光学峰出现,相当层间交换耦合为反铁磁型.光学峰与声学峰的峰位间距沿易轴小于沿难轴.又,来自非磁Cu夹层的顺磁共振峰的幅度远大于原始的连续膜,说明线列阵加工致使层结构退化. 关键词：     

FeCuCrVSiB单层和多层膜的横向巨磁阻抗效应

用射频溅射法制备了Fe_71.5Cu₁Cr_2.5V₄Si₁₂B₉单层膜和结构为(F/S)₃/M/(F/S)₃的多层膜，在制备过程中加72kA/m的纵向磁场.研究表明在制备过程中加磁场明显改善了材料的软磁性能，降低了材料的矫顽力.将样品经不同温度退火热处理后，发现经230℃退火1.5h的单层膜和多层膜具有最佳的软磁性能和最大的磁阻抗效应，单层膜最大横向磁阻抗比为37.5%，多层膜最大横向磁阻抗比高达277%.通过比较单层和多层膜磁阻抗效应随频率和磁场的变化，发现多层膜具有较低的磁阻抗效应的临界频率和峰值特征频率，和较大的磁阻抗变化率，而且有较低的横向磁阻抗效应的饱和场. 关键词： 铁基合金 多层膜 巨磁阻抗效应     

81NiFe/Cr多层膜磁电阻单向各向异性与交换耦合

采用电子束蒸发法制备的８１ＮｉＦｅ／Ｃｒ多层膜具有单向各向异性，磁滞回线非轴对称，相当存在１Ｏｅ数量级的交换偏场。磁电阻回线的上升和下降两枝差别明显，其中以横向磁电阻回线最为显著。有的试样上升枝与下降枝相应的最大横向磁电阻率之差已高达０．７８％，其最大各向异性磁电阻率（Ｒ_ｍａｘ－Ｒ_ｍｉｎ）／Ｒ为４．７２％，甚高于Ｍｉｙａｚａｋｉ等^[１］的优质８２ＮｉＦｅ合金单层膜的报道值３％。可以认为这类多层膜的磁电阻效应除主要来源于自旋－轨道耦合机制外， 关键词：     

FeCuCrVSiB多层膜巨磁阻抗效应的研究

研究了结构为 (FM/SiO₂)₃/Ag/(SiO₂/FM)₃ 多层膜的巨磁阻抗(GMI)效应（这里的F M≡FeCuCrVSiB）．多层膜采用射频溅射法沉积在单晶Si衬底上，沉积过程中，沿膜面长方 向施加约72kA/m的磁场，然后在不同的温度下对样品进行了退火处理．结果表明，该多层膜 样品即使在沉积态便具有相当好的软磁性能和GMI效应，在7MHz的频率下，最大纵向和横向 巨磁阻抗比分别为45%和44%．在230℃下经90min退火处理后的样品具有最佳的GMI效应，在85MHz的频率下，最大纵向和横向巨磁阻抗比分别达到251%和277%．与磁性层总厚度相同的FeCuCrVSiB/Ag/FeCuCrVSiB三层膜相比较，在这种多层结构中出现的GMI效应更强． 关键词： 多层膜 巨磁阻抗效应 趋肤效应 有效磁导率     

基于拖曳倾斜线列阵的海底反射损失提取方法

在浅海环境中,海底声学参数对水下声场的精确预报十分重要.现有的海底声学参数反演方法大多数是采用固定垂直阵进行的,其缺点是不能实施大面积、高分辨的走航式反演.在已提出的垂直阵海底反射损失反演方法的基础上,研究了基于三种线列阵阵形的海底反射损失提取方法及其特点,提出了基于拖曳倾斜线列阵与三个声源组合的走航式海底参数快速获取方法,并对该方法进行了误差分析.研究结果表明：覆盖同样的掠射角范围,垂直线列阵需与多个距离的声源组合,拖曳水平线列阵只需一个声源组合但需要采用较大的物理孔径,拖曳倾斜线列阵综合了垂直阵和水平 关键词： 水下声场预报 海底声学参数 海底反射损失 拖曳倾斜线列阵     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

磁控溅射Fe/Mo多层膜的巨磁电阻及层间耦合

研究了DC/RF磁控溅射Fe/Mo多层膜的磁电阻特性.在反铁磁性耦合的样品中,观察到了磁电阻比率约为12%的巨磁电阻效应.当Mo层厚度改变时,还观察到了周期约为1.0—1.2nm的层间耦合振荡.另外,F/Mo多层膜的磁电阻比率不仅随Mo层厚度改变出现振荡,而且与Fe层厚度的改变也有着很强的依赖关系. 关键词：     

Nd-Fe-B/FeCo多层纳米复合膜的结构和磁性

制备了Nd₂₈Fe₆₆B₆/Fe₅₀Co₅₀多层纳米复合磁性薄膜，对溅射态和650℃退火处理15 min试样的相成分分析和微结构的观察显示，溅射态薄膜呈非晶态，经650℃退火处理15 min后，薄膜主要相成分为硬磁性Nd₂Fe₁₄B相和软磁性相FeCo(110)相.Nd₂Fe₁₄B相呈柱状，其易磁化c轴垂直于膜面，尺寸约10 nm.在硬磁性相和软磁性相之间存在少量富Nd相和非晶态，富Nd相大小约7 nm.磁性测量和分析表明，1)该系列薄膜退火态具有垂直于膜面的磁晶各向异性.2)对于固定厚度(10 nm)层Nd-Fe-B和不同厚度(t_FeCo=1—100 nm)层FeCo多层纳米复合膜,剩磁随软磁相FeCo 厚度的增加快速增加，而矫顽力则减小.当t_FeCo=5 nm时，最大磁能积达到200 kJ/m³. 3)硬磁相Nd-Fe-B层和软磁相FeCo层之间交换耦合导致剩磁和磁能积增强. 关键词： Nd-Fe-B/FeCo多层纳米复合膜 交换耦合 磁各向异性     

FeCuNbSiB单层膜和三明治膜的磁特性与巨磁阻抗效应

用射频溅射法制备了Fe_73.5Cu₁Nb₃Si_13.5B₉单层膜和 Cu或Ag作为中间层的三明治膜.溅态膜为非晶态结构.磁畴观察结果表明,单层膜在380℃退火后,呈现均匀磁化的纳米晶结构,该样品软磁特性最佳,其巨磁阻抗效应最大,在13MHz,最大磁阻抗比纵向为18％,横向为14％.溅态三明治膜具有较大的巨磁阻抗效应,在13MHz,Cu夹层三明治膜的最大磁阻抗比纵向为32％,横向为11％,Ag夹层三明治膜的最大磁阻抗比纵向为47％,横向为57％.Cu夹层三明治膜经250℃退火后,在低频下表现为巨磁电感效应,在100kHz,最大磁电感比为1733％. 关键词：     

帽层对极紫外多层膜反射特性影响分析

介绍了在极紫外波段，利用帽层材料来减少多层膜反射镜因外部环境干扰而造成的反射率降低，使多层膜光学元件能够长时间稳定工作.计算了在139nm波长处Mo/Si极紫外多层膜反射镜在表面镀制不同帽层材料时的理论最大反射率，利用单纯形调优法，对帽层和多层膜的周期厚度进行优化，同时把分层理论用于多层膜帽层优化,可使多层膜的反射率得到进一步提高.分析了在加入帽层前后多层膜外层电场强度的分布变化情况. 关键词： 多层膜 反射率 帽层 极紫外     

Magnetic characterization of FeCo nanowire arrays by first-order reversal curves

FeCo nanowire arrays have been obtained by current pulse electrodeposition into nanoporous alumina templates. First-order reversal curve (FORC) diagrams have been used to investigate magnetostatic interaction and average coercivity of individual FeCo nanowires embedded in porous alumina templates. The FeCo nanowires with a wires length up to 3 μm and wires diameter ranging from 25 to 50 nm showed interacting single-domain behavior. Using FORC diagrams, the spread of coercivity distribution was seen to be almost independent of the wires diameter, but with increase in diameter the inter-wire magnetostatic interaction was increased. It was found that for arrays with higher diameter, the coercivity of the arrays is lower than the average coercivity of the individual wires. It was detected that an increase in wire diameter results in a considerable increase in the spread of the distribution in the H_u direction of FORC distribution. Curve fitting on the experimental data proved a relatively linear relation between interaction field and square diameter of the nanowires.     

Temperature dependence of the effective anisotropy in Ni nanowire arrays

Magnetic hysteresis in Ni nanowire arrays grown by electrodeposition inside the pores of anodic alumina templates is studied as a function of temperature in the range between 5 K and 300 K. Nanowires with different diameters, aspect ratios, inter-wire distance in the array and surface condition (smooth and rough) are synthesized. These microstructure parameters are linked to the different free magnetic energy contributions determining coercivity and the controlling magnetization reversal mechanisms. Coercivity increases with temperature in arrays of nanowires with rough surfaces and small diameters ─33 nm and 65 nm─ when measured without removing the alumina template and/or the Al substrate. For thicker wires ─200 nm in diameter and relatively smooth surfaces─ measured without the Al substrate, coercivity decreases as temperature rises. These temperature dependences of magnetic hysteresis are described in terms of an effective magnetic anisotropy K_a, resulting from the interplay of magnetocrystalline, magnetoelastic and shape anisotropies, together with the magnetostatic interaction energy density between nanowires in the array. The experimentally determined coercive fields are compared with results of micromagnetic calculations, performed considering the magnetization reversal mode acting in each studied array and microstructure parameters. A method is proposed to roughly estimate the value of K_a experimentally, from the hysteresis loops measured at different temperatures. These measured values are in agreement with theoretical calculations. The observed temperature dependence of coercivity does not arise from an intrinsic property of pure Ni but from the nanowires surface roughness and the way the array is measured, with or without the alumina template and/or the aluminum support.     

Hybrid Monopole/Loop Coil Array for Human Head MR Imaging at 7 T

The monopole coil and loop coil have orthogonal radiofrequency (RF) fields and thus are intrinsically decoupled electromagnetically if they are laid out appropriately. In this study, we proposed a hybrid monopole/loop technique which could combine the advantages of both loop arrays and monopole arrays. To investigate this technique, a hybrid RF coil array containing four monopole channels and four loop channels was developed for human head magnetic resonance (MR) imaging at 7 T. In vivo MR imaging and g-factor results using monopole-only channels, loop-only channels and all channels of the hybrid array were acquired and evaluated. Compared with the monopole-only and loop-only channels, the proposed hybrid array has the higher signal-to-noise ratio (SNR) and better parallel imaging performance. Sufficient electromagnetic decoupling and diverse RF magnetic field (B₁) distributions of monopole channels and loop channels may contribute to this performance improvement. From experimental results, the hybrid monopole/loop array has low g-factor and excellent SNR at both periphery and center of the brain, which is valuable for human head imaging at ultrahigh fields.     

Thermally activated magnetization reversal process of self-assembled Fe55Co45 nanowire arrays

We have investigated the temperature dependence of the magnetic properties and the magnetic relaxation of the Fe₅₅Co₄₅ nanowire arrays electrodeposited into self-assembled porous alumina templates with the diameter about 10 nm. X-ray diffraction (XRD) pattern indicates that the nanowire arrays are BCC structure with [1 1 0] orientation along the nanowire axes. Owing to the strong shape anisotropy, the nanowire arrays exhibit uniaxial magnetic anisotropy with the easy magnetization direction along the nanowire axes. The coercivity at 5 K can be explained by the sphere chains of the symmetric fanning mechanism. The temperature dependence of coercivity can be interpreted by thermally activated reversal mechanism as being the localized nucleation reversal mechanism with the activation volume much smaller than the wire volume. Strong field and temperature-dependent magnetic viscosity effects were also observed.     

Synthesis and characteristics of Fe<Subscript>96-x</Subscript>Zr<Subscript>x</Subscript>B<Subscript>4</Subscript> nanowire arrays

Fe-rich Fe_96-xZr_xB₄ (0≤x≤7) nanowires were first prepared by electrodepositing into anodic aluminum oxide templates. Transmission electron microscope analysis shows that the nanowires are uniform and are about 100 nm in diameter with an aspect ratio of around 75. The broad peaks of X-ray diffraction and the Mössbauer spectrum indicate that the Fe_96-xZr_xB₄ nanowires are composed of α-Fe-like and Zr-rich FeZrB phases. Selected area electro diffraction results also indicate that the structure of Fe₈₉Zr₇B₄ nanowires is amorphous. A vibrating sample magnetometer is employed to study the magnetic properties of nanowire arrays at room temperature. The coercivity of nanowire arrays in parallel to the wire axis decreases with increasing Zr content.     

Angular dependence of coercivity and remanence of Ni nanowire arrays and its relevance to magnetoviscosity

Ni nanowire arrays with varying wire dimensions (diameter d, length l) and center-to-center distances d_CC were synthesized by pulsed electrodeposition of Ni in porous Al templates. The magnetization-reversal behavior of the arrays was investigated by means of magnetometry for different angles θ between the wire axes and the applied magnetic field. The functional dependences of the characteristic parameters coercivity H_C(θ) and reduced remanence m_R/m_S(θ) exhibit a strong dependence on the wire dimensions and the center-to-center distance. For instance, for nanowire arrays with d=40 nm, d_CC=100 nm, and for θ=0°, the coercivity takes on a rather large value of μ₀H_C=85 mT and m_R/m_S≅94%; reducing d_CC to 30 nm and d to 17 nm results in μ₀H_C=49 mT and m_R/m_S≅57%, an observation which suggests an increasing magnetostatic interwire interaction at increased (d/d_CC)-ratio. The potential application of nanowires as the constituents of ferrofluids is discussed.     

Contribution of magnetostatic interaction to magnetization reversal of Fe3Pt nanowires arrays: A micromagnetic simulation

Using the micromagnetic simulations, we have investigated the magnetization reversal and magnetostatic interaction of Fe₃Pt nanowires arrays with wire diameters lower than 40 nm. By changing the number of interacting nanowires, N, interwire distance, a, and wire diameter, D, the effects of magnetostatic interaction on coercivity and remanence are investigated in detail. According to the simulated results, the contribution to the stray field induced by surface perpendicular magnetization at the end of wires is established.     

Understanding magnetic properties of arrays of small FePt dots with perpendicular anisotropy

FePt dot arrays with dot size down to 15 nm are fabricated by film annealing and patterning. The array coercivity shows an increase with dot size decreasing from 100 to 30 nm, and a slight reduction for the 15 nm dot sample. Annealing these dot arrays at higher temperatures results in large enhancements in the coercivities, except the 15 nm dot array where the coercivity increases a little. Micromagnetic models of a 15 nm FePt dot with uniform and nonuniform edges of soft magnetic defects and with inside defects are calculated to reveal the microstructure origins of the dot magnetic properties. It is found that the volume fraction of the L1₀-phase FePt with perpendicular c-axis orientation is about 50% in the dot and the switching field distribution of the dot array can be influenced significantly by the defect arrangement in the dots.     

X-ray observations of tungsten wire array Z-pinch implosions on QiangGuang-1 facility

Z-pinch experiments with two arrays consisting, respectively, of 32 4-μm- and 6-μm-diameter tungsten wires have been carried out on QiangGuang-1 facility with a current rising up to 1.5MA in 80ns. At early time of implosion, x-ray framing images show that the initial emission comes from the central part of arrays, and double clear emission rings, drifting to the anode and the cathode at 5×10⁶cm/s and 2.4×10⁷cm/s respectively, are often produced near the electrodes. Later, in a 4-μm-diameter tungsten wire array, filamentation caused by ohmic heating is prominent, and more than ten filaments have been observed. A radial inward shift of arrays starts at about 30\,ns earlier than the occurrence of the x-ray peak power for both kinds of arrays, and the shrinkage rate of emission region is as high as 1.7×10⁷cm/s in a 4-μm-diameter tungsten wire array, which is two times higher than that in a 6-μm one. Emission from precursor plasmas is observed in implosion of 6-μm-diameter tungsten wire arrays, but not in implosion of a 4-μm-diameter tungsten wire array. Whereas, in a 4-μm-diameter tungsten wire array, the soft x-ray emission shows the growth of m=1 instability in the plasma column, which is caused by current. The reasons for the discrepancy between implosions of 4-μm- and 6-μm-diameter tungsten wire arrays are explained.