The effect of domain wall motion on the nondiagonal impedance component in amorphous wires with circular magnetic anisotropy

The effect of the domain wall motion on the nondiagonal impedance component in amorphous ferromagnetic wires with circular anisotropy was theoretically studied. The frequency spectrum of the electromotive force (emf) induced in the pick-up coil wound around the wire is analyzed. For sufficiently small amplitudes of the ac current passing in the wire, the emf frequency equals doubled frequency of the current. For the ac current amplitudes exceeding certain threshold value, all even harmonics appear in the frequency spectrum of the signal. The emf strongly depends on the longitudinal component of the applied magnetic field at any value of the ac current amplitude. These results can be important for developing frequency transducers controlled by magnetic field.     

Frequency spectrum of the nonlinear magnetoimpedance of multilayer film structures

Magnetization reversal by high-frequency current in FeCuNbSiB/Al/FeCuNbSiB three-layer film structures is studied. The frequency spectrum of the voltage arising in a coil wound on the sample as a function of a permanent magnetic field (nonlinear magnetoimpedance) is taken. It is shown that the frequency spectra of the voltage are qualitatively different for the longitudinal and transverse orientations of the field with respect to the direction of the current. Frequency spectrum harmonics are demonstrated to be highly sensitive to a magnetic field. A simple electrodynamic model to describe experimental data is suggested.     

Current distribution and giant magnetoimpedance in composite wires with helical magnetic anisotropy

The giant magnetoimpedance effect in composite wires consisting of a non-magnetic inner core and soft magnetic shell is studied theoretically. It is assumed that the magnetic shell has a helical anisotropy. The current and field distributions in the composite wire are found by means of a simultaneous solution of Maxwell equations and the Landau–Lifshitz equation. The expressions for the diagonal and off-diagonal impedance are obtained for low and high frequencies. The dependences of the impedance on the anisotropy axis angle and the shell thickness are analyzed. Maximum field sensitivity is shown to correspond to the case of the circular anisotropy in the magnetic shell. It is demonstrated that the optimum shell thickness to obtain maximum impedance ratio is equal to the effective skin depth in the magnetic material.     

Detection of NMR signals with a radio-frequency atomic magnetometer

We demonstrate detection of proton NMR signals with a radio-frequency (rf) atomic magnetometer tuned to the NMR frequency of 62 kHz. High-frequency operation of the atomic magnetometer makes it relatively insensitive to ambient magnetic field noise. We obtain magnetic field sensitivity of 7 fT/Hz1/2 using only a thin aluminum shield. We also derive an expression for the fundamental sensitivity limit of a surface inductive pick-up coil as a function of frequency and find that an atomic rf magnetometer is intrinsically more sensitive than a coil of comparable size for frequencies below about 50 MHz.     

On some consequences of an external magnetic field applied to HTS coils

A computational model which enables to evaluate the distribution of the critical currents, electric fields and the voltage in the winding of a solenoidal high temperature superconducting (HTS) magnets subjected to an external magnetic field parallel with the magnet axis, was developed. The model comes out from the well-known power law between the electric field and the transport current of the HTS tape short sample. It allows to predict the voltage–current V(I) characteristics of both the pancake coils and the complete magnet. The model was applied to the magnet system consisting of 22 pancake coils made of multifilamentary Bi(2223)/Ag tape at 20 K, which is subjected to an external uniform magnetic field parallel with the coil axis. A rather unexpected behavior of the magnet at different operating conditions (operating current and external magnetic field strength) is predicted, analyzed and reported together with a theoretical explanation. On one hand, the external uniform magnetic field parallel with the coil axis increases the resulting magnetic field strength, however, on the other hand it simultaneously decreases the angle between the resulting magnetic field and the tape surface. Thus, the effect of higher magnetic loading caused by the presence of an external magnetic field strength which is acting on individual turns located close to the coil’s flanges is compensated by more favorable orientation of the tape with respect to the resulting magnetic field. As a result, increase in the critical currents of these turns is expected. Further, the results indicate, that in case of the high field HTS insert coils the anisotropy in the I_c(B) characteristic does not play a substantial role. As a consequence, the technology of the production of the tapes for high field insert HTS coils should concentrate rather on the tapes having the current carrying capacity as high as possible, than on the attempt how to decrease the anisotropy in the I_c(B) by changing the architecture of the filaments in the tape.     

Co基金属纤维不对称巨磁阻抗效应

以直径32 μm的熔体抽拉Co基非晶金属纤维为研究对象, 分析了该纤维不同激励条件下的巨磁阻抗(giant magneto impedance, GMI)效应. 实验结果表明: 这类纤维的GMI效应具有不对称性特点, 即 AGMI (asymmetric GMI)效应. 同时, 发现AGMI效应随激励条件不同而变化, 随交流频率或者激励幅值升高而逐渐增强; 当存在一定偏置电压时, AGMI效应大幅增强. 通过研究纤维的磁化过程, 分析了Co基金属纤维的AGMI效应. 由于Co基熔体抽拉纤维具有螺旋各向异性以及磁滞的存在使得GMI效应具有不对称性, 频率升高或者激励电流幅值增加有利于壳层畴环向磁化, AGMI增强. 当在纤维两端施加偏置电压时, 偏置电流诱发环向磁场增强了环向磁化, AGMI效应提高; 偏置电压较低时磁场响应灵敏度提高, 同时磁化翻转向高场移动, 阻抗线性变化对应的直流磁场区间增大. 这一方面拓宽了GMI传感器工作区间及灵敏度, 另一方面不利于获得更大的磁场响应灵敏度. 10 MHz (5 mA)激励时, 施加1 V强度的偏置电压后, 对应的磁场灵敏度从616 V/T 提高至5687 V/T; 偏置电压为2 V时, 灵敏度降低到4525 V/T. 因此, 可以通过适当提高环向磁场的方法获得大的磁场响应灵敏度及阻抗变化线性区域.     

导电导磁管道外任意放置线圈激励下脉冲涡流场时域解析解

利用二阶矢量位和洛伦兹互易定理,解析求解了导电、导磁金属管道外任意放置线圈激励下非轴对称涡流场的频域解.利用求频域式极点处留数的方法求解拉普拉斯反变换,得到了脉冲电流激励下检测线圈两端感应电压以及管壁内脉冲涡流分布的时域解析式.分析比较了不同线圈放置方式下管壁内脉冲涡流的分布和扩散过程,以及感应电压对管壁的灵敏度.研究结果表明:当线圈轴线沿管道径向法线方向放置时,得到的感应电压时域信号最强,对壁厚的检测灵敏度最高.     

Influence of current amplitude on the nonlinear asymmetric ac volt-ampere characteristics in amorphous ribbons with GMI effect

Asymmetric giant magnetoimpedance (GMI) effect in amorphous ribbons is very promising due to its possible application in developing the highly sensitive linear magnetic-field sensors. In this paper, the model of the nonlinear asymmetric volt-ampere characteristics in field-biased Co-based amorphous ribbon has been established through the time derivative of the longitudinal magnetization component in the ribbon. Its harmonics are also developed by Fourier analysis. The influence of the current amplitude on the nonlinear asymmetric effect has been studied. It is found that when the current amplitude is 8.54 mA, the sensitivity of the first harmonic voltage on the external field is equal to that of the second harmonic voltage. The results obtained are useful for developing the high-sensitive magnetic-field sensors.     

AC loss in a high-temperature superconducting coil

In a typical superconducting coil made of BSCCO/Ag tape, both amplitude and direction of the magnetic field determine the critical current, resistive voltage and AC loss. The distribution of the magnetic field along and across the superconducting tape in a coil is rather complex. This gives rise to the question: how accurate can one predict the critical current, V–I characteristic and AC loss of the AC coil from results of short sample measurements? To answer this question, we have measured and compared the characteristics of a short sample and a small coil employing 14 m of the same tape at 77 K. The comparison is performed as follows. First, a short sample is characterised with regard to the field dependence of the critical current, V–I characteristic and the AC loss. Second, the distribution of the magnetic field along the tape in a coil is accurately calculated. From the data, the voltage along the tape and the loss of the tape in the coil are found. Finally, the resistive voltage and the AC loss of the complete coil are calculated and compared to measured AC losses in the frequency range of 0 to 160 Hz, typical for power applications.     

磁场退火对Co基熔体抽拉丝巨磁阻抗效应的影响

对Co基熔体抽拉非晶丝进行了普通炉内退火和不同磁场强度（500 Oe,1600 Oe,4000 Oe）下的横向和纵向磁场退火,利用HP4192阻抗分析仪和Lakeshore7407VSM分析了退火态样品的巨磁阻抗（GMI）效应和软磁性能.研究结果表明,纵向磁场降低了环向各向异性,纵向磁场退火样品GMI效应降低且GMI曲线为单峰,最大阻抗变化率ΔZ/Z为131%,磁场响应灵敏度为7%/Oe;而横向磁场提高了环向畴体积,增加了环向各向异性场,导致退火样品GMI曲线随频率升高由单峰转 关键词： 非晶丝 巨磁阻抗效应 退火     

Effect of direct current on higher harmonic generation in the frequency spectrum of magnetoimpedance of amorphous wires with circular anisotropy

The effect of direct current on the nonlinear response of magnetoimpedance in circularly anisotropic amorphous wires is investigated. A model is proposed for calculating the voltage across the ends of the wire in the case of a weak skin effect in the sample. It is shown that a direct current passing through the wire can substantially increase the amplitude of even harmonics of the voltage. The conditions in which the sensitivity of the second harmonic to the applied magnetic field is maximal are determined.     

具有倾斜极化层的自旋阀结构中磁翻转以及磁振荡模式的微磁模拟

基于自由层与钉扎层均为垂直磁各向异性的自旋阀结构,采用微磁学模拟与傅里叶分析相结合的技术,研究了极化层磁矩小角度倾斜情形下自由层磁矩的进动翻转特性.通过沿样品垂直膜面方向同时施加电流与磁场,观察到自由层磁矩垂直膜面方向分量的平均值随磁场的演化翻转曲线中出现了多个凹槽.模拟研究结果表明:在一定的电流范围内,凹槽出现的位置与电流大小无关;而在固定的应用电流下,凹槽出现的位置将会受到样品厚度的影响;在凹槽区域内,非一致进动模式、自旋驻波模式、局域自旋波模式等多种磁振荡模式被激发.通过傅里叶分析,得到了各种磁振荡模式的频谱,频谱中的频率分布体现出了倍频以及间谐波的频率特性.     

A model for torsion-stress effect on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction

A model to describe the influence of torsional stress on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction is proposed. The nonlinear voltage response is found in the framework of the low-frequency approximation taking into account the spatial distribution of the circular magnetic field and the magnetoelastic anisotropy induced by the torsional stress. It is demonstrated that the application of torsional stress results in an increase of the second harmonic amplitude in voltage due to a reinforcement of helical anisotropy in the wire. The second harmonic amplitude is analyzed as a function of external field, torsional stress and current amplitude. The ranges of torsional stress and current amplitude to achieve maximal field sensitivity of the second harmonic are found.     

Structural and magnetic properties of electrodeposited Cobalt nanowire arrays

Ordered magnetic nanowires have tremendous potential in future magnetic storage and high frequency magnetic logic devices. Here, we present the fabrication of ordered arrays of Cobalt nanowires by electrodeposition through porous polycarbonate membranes. Vertically and horizontally aligned nanowires were produced in presence of an external bias field during post deposition etching of the polycarbonate membrane. Structural and compositional analyses have been carried out to establish the material and structural purity. The magneto-optical Kerr effect was employed to measure the magnetic hysteresis loops for the nanowires assembled in the substrate plane. A good magneto-optical signal to noise ratio is observed with clean ferromagnetic hysteresis loops. The loops measured with external magnetic field applied parallel and perpendicular to the axis of the nanowires show a clear difference in the shape and the coercive field, indicating the effect of shape anisotropy in these samples. Micromagnetic simulations were performed to understand the experimental results and to obtain insight to the magnetization reversal mechanism in magnetic nanowires.     

Magnetic measurement of transport AC losses in Bi-2223/Ag tapes

Magnetic nature of the losses in superconducting wire carrying AC current implies that it should be possible to determine these losses in a contactless way. Ribbon-like samples are quite favorable for such an experiment, because a notable portion of magnetic flux related to losses ‘escapes' the sample volume and can be detected by an appropriate pick-up coil. In this case, a model describing the AC current penetration into the tape, based, e.g., on the critical state model, allows one to derive the losses from the pick-up coil signal. Because this signal is proportional to the number of coil turns, extension of the accessible range of measured voltages (and losses) can be achieved. We demonstrate the data obtained on a 1 cm long portion of a low-loss multifilamentary tape carrying AC current with frequency 35 Hz. The pick-up coil technique allowed us to reach loss level more than one order below the experimental limit for direct measurements.     

Linear intermode conversion of electromagnetic wave energy in a magnetically soft gyrotropic material

The excitation of axial radio-frequency (rf) magnetic induction by an axial rf current is observed in a conductor with circular magnetic anisotropy when a weak magnetizing field is applied. The conductor is an amorphous cobalt-based wire, which exhibits azimuthal magnetic anisotropy. It serves as the central conductor in a coaxial line. The axial rf magnetic induction produces an emf in an induction coil coaxial to the conductor. The induction coil is part of a matched receiving circuit. The power conversion coefficient is as high as tens of percent. The measurements demonstrate the high sensitivity of the conversion coefficient to an external field. The theory of ferromagnetic resonance faithfully describes the results of the observations. Zh. Tekh. Fiz. 69, 58–63 (March 1999)     

Experimental Investigation on Low Magnetic Field Operation of an Overmoded Slow-Wave High-Power Microwave Generator

The experimental results of an overmoded slow-wave high-power microwave generator operated at low magnetic field are presented. The feasibility of low magnetic field operation is investigated both theoretically and experimentally based on the characteristics of the overmoded slow-wave device. The experiments were carried out at the Spark-2 accelerator. Under the condition of guiding magnetic field strength of 0.55 T, diode voltage of 4 74 k V,and beam current of 5.2kA, a microwave was generated with power of 510 MW, mode of TM01, and frequency of 9.54 GHz. The relative half-width of the frequency spectrum is less than 1%, and the beam-to-microwave efficiency is about 21% in our experiments.     

An Efficacious Target-Field Approach to Design Shim Coils for Halbach Magnet of Mobile NMR Sensors

The main magnetic fields of mobile nuclear magnetic resonance (NMR) magnets differ from those of conventional NMR and magnetic resonance imaging (MRI) magnets. In the Halbach magnet, the main field B ₀ is perpendicular to the longitudinal axis, the symmetry of the current distribution with respect to the symmetry of the magnetic field differs from that in conventional target-field applications, and the current distribution on the coil surface cannot be expressed in terms of periodic basis functions. To obtain the winding pattern of the coil, an efficacious target-field approach. The surface of a coil is divided into small discrete elements, where each element is represented by a magnetic dipole. From the stream function of the elements, the resultant magnetic field is calculated. The optimization strategy follows an objective function defined by the power dissipation or efficiency of the coil. This leads to the optimum stream function on the coil surface, whose contour lines define the winding patterns of the coil. This paper shows winding patterns designed of shim coils for Halbach magnet and illustrates the craft of a shim coil using flexible printed circuit board. The performance of the coils is verified by simulating the fields they produce over the sensitive volume.     

The peculiarities of coupled electromagnetic and magnetoelastic waves in antiferromagnets

A spectrum of coupled electromagnetic, spin and elastic waves in a two-sublattice antiferromagnet with weak ferromagnetism is theoretically investigated. The influence of the g-factor anisotropy and the transverse and longitudinal relaxation in magnetic subsystem on the spectrum of coupled waves is considered. The most changes of dispersion laws occurred in long-wavelength approximation and near the spin reorientation point, then the vectors of ferromagnetism and antiferromagnetism reoriented onto another crystallographic axis. It is shown that the magnetoelastic, the Dzyaloshinsky and the dipole interactions, the anisotropy of g-factor, the external magnetic field and the longitudinal susceptibility determine the activation of quasiferromagnetic waves. The dispersion laws of quasielectromagnetic and quasielastic waves can change from linear dependence to square. At large damping in magnetic subsystem, one from these modes can become the pure relaxation one.     

Quantum oscillations of rectified dc voltage as a function of magnetic field in an “almost” symmetric superconducting ring

Periodic oscillations in the dependence V _dc(B) of rectified dc voltage on the perpendicular magnetic field have been experimentally observed near the critical temperature in a single superconducting aluminum ring with slight geometric inhomogeneities (without specially formed circular asymmetry), biased by an external ac current (without a dc component). With a change in the external current and temperature, the voltage V _dc(B) behaves like the corresponding voltage on a circularly asymmetric ring but has a much smaller amplitude. The Fourier spectrum of the function V _dc(B) contains the fundamental frequency, corresponding to the ring area, and its highest harmonics. “Satellite” frequencies, dependent on the structure geometry and external parameters, were unexpectedly found in the spectrum.