Magnetoimpedance in soft magnetic amorphous and nanostructured wires

The surface impedance tensor approach has been used to review the impedance response in a variety of amorphous and nanocrystallized wires. An experimental study on the torsion annealing effect on the magnetoimpedance (MI) behaviour for positive and negative magnetostriction amorphous wires of FeSiB and CoSiB compositions, respectively, has been carried out. Moreover, the influence of the onset nanocrystallization on the MI behaviour in Finemet-type alloys, with particular attention focussed on the case that the wires are annealed under applied torsional stress, is also presented. The analysis of the MI and torsion-impedance (TI) effects allows us to compare the different magnetic characteristics observed in a variety of wires. Special attention is paid to new results of the off-diagonal MI huge responses of these ferromagnetic samples, which are also a very useful source of information on the magnetic properties of the wires.     

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.     

Effect of Mechanical Stresses and Annealing on the Magnetic Structure and the Magnetic Impedance of Amorphous CoFeSiBCr Microwires

The structural and magnetic properties of amorphous ferromagnetic microwires can undergo significant measurements under the action of external mechanical stresses and heat treatment. The study of transformations occurring in this case is important for designing various sensors of mechanical stresses, loading, and temperature and also for inducing in the wires a certain type of magnetic anisotropy that plays a significant role in the realization of various effects in them. In this work, the influence of external stresses and annealing on the processes of the magnetization and the magnetic impedance of Co₇₁Fe₅B₁₁Si₁₀Cr₃ microwires having a low positive magnetostriction (~10^-8) in amorphous state has been studied. The influence of external stresses leads to a sharp change in the character of the magnetization reversal curve, which was due to the change in the sign of the magnetostriction and the type of magnetic anisotropy. The amplitude of higher harmonics and the value of the magnetic impedance, respectively, are sensitive to mechanical stresses. Elastic stresses in the wires with a partial crystallization do not lead to a marked change in the magnetic properties; however, annealing can lead to a substantial increase in the axial magnetic anisotropy of the wires existing in the stressed state. The experimental results are analyzed in the framework of a magnetostriction model of induced magnetic anisotropy.     

Recent advances in studies of magnetically soft amorphous microwires

In this paper, we present the giant magneto-impedance (GMI) effect (real part of longitudinal impedance, Z, and of the off-diagonal impedance) and hysteretic magnetic properties of amorphous glass-coated microwires with different compositions possessing nearly zero, positive and negative magnetostriction constant and metallic nucleus diameter ranging between 6 and 16 μm. Enhanced soft magnetic properties (low coercivity of about 4 A/m) and high-GMI effect have been observed in Co-rich microwires with vanishing magnetostriction constant. The magnetic anisotropy field of these microwires depends on the ratio between metallic diameter, d and total microwires diameter, D. Stress-sensitive magnetic properties have been obtained in Fe-rich microwires after stress annealing: hysteresis loop stress-annealed (SA) microwires drastically changes under applied stress. A variety of hysteresis loops with different hysteresis loops can be obtained in Fe-rich microwires changing the conditions (time and/or temperature) of the stress annealing. The obtained results allow us to tailor the microwire magnetic properties for magnetic sensors applications through selection of their composition and/or geometry and by thermal treatment.     

Micromagnetic study of magnetic domain structure and magnetization reversal in amorphous wires with circular anisotropy

In this work we present a detailed numerical investigation on the magnetic domain formation and magnetization reversal mechanism in sub-millimeter amorphous wires with negative magnetostriction by means of micromagnetic calculations. The formation of circular magnetic domains surrounding a multidomain axially oriented central nucleus was observed for the micromagnetic model representing the amorphous wire. The magnetization reversal explained by micromagnetic computations for the M-H curve is described in terms of a combined nucleation-propagation−rotational mechanism after the saturated state. Results are interpreted in terms of the effective magnetic anisotropy.     

Novel trends in the study of magnetically soft Co-based amorphous glass-coated wires

An overview of the recent progress and state-of-the-art results in the investigation of the amorphous glass-coated wires with nearly zero magnetostriction is presented. These versatile microwires display enhanced soft magnetic properties, which make them suitable as sensing elements in various sensors for biomedical and automotive applications. Current results on their magnetic characteristics refer to a major refinement of their core-shell magnetic structure by taking into account the interdomain wall and to the thorough analysis of the magnetization within the outer shell. Experimental techniques such as giant magneto-impedance, magneto-resistance, and magneto-optical Kerr effect measurements are employed to prove the outcome of the theoretical calculations. The impact of the magnetic structure of the outer shell on the propagation of domain walls in bistable amorphous wires is analyzed. Very recent results on the magnetization process in nearly zero magnetostrictive amorphous glass-coated wires with submicron dimensions are also reviewed.     

The Analysis of Large Barkhausen Effect in the FeSiB Amorphous Wire

Amorphous FeSiB wires with positive magnetostriction are very perspective soft magnetic materials for many applications, e.g. torque, field or current sensors, pulse generators and highly sensitive magnetometers. The appearance of the Large Barkhausen Effect (LBE) during slow magnetization of FeSiB wires is described by means of the core-shell model assuming a residual radial tensile stresses in the as-cast state. In this work, the LBE during magnetization reversal of Fe_77.5Si_7.5B₁₅ amorphous wire in the as-cast state was analysed. We have studied the kinetics of the reverse domain in the core region of the wire by means of Sixtus-Tonks method of two small pick-up coils placed in an asymmetric way with respect to the ends of the wire. We estimated the velocity of the reverse domain wall and the core region volume of the wire. It was found that the residual radial tensile stress distribution of the shell region strongly influences the magnetization reversal in the FeSiB wire.     

Compressive Stress Dependence of Magnetic Properties of Co66Fe4Ni1B14Si15 Alloy

The influence of compressive stresses on magnetic properties of Co₆₆Fe₄Ni₁B₁₄Si₁₅ amorphous alloy rings was investigated. In spite of low magnetostriction the stress sensitivity is significant. New method of applying stresses to a ring core is presented.     

Magnetostriction of transition metal substituted Co-based amorphous alloys

The effects of the substitutions of 3d-, 4d- and 5d-transition metals for silicon on magnetostriction of Co-based amorphous alloys have been investigated by measuring the tension dependence of anisotropy field in magnetic hysteresis loop. Fractional changes of the magnetostriction with transition metal solutes in Co-based amorphous alloys are qualitatively discussed in terms of the magnetic moment and coordination number, and classified into 3 types, i.e., positive, negative and complicated types, from a viewpoint of the contribution to magnetostriction.     

Studies of the relaxed amorphous phase in the Fe80Nb6B14 alloy

The paper presents measurements of magnetic permeability, magnetic after-effects, magnetostriction, DSC and XPS for the Fe₈₀Nb₆B₁₄ amorphous alloys preliminary annealed for 1 h at temperatures ranging from 300 to 770 K. It was shown that annealing out of free volume and internal stresses causes a decrease of magnetostriction coefficient and leads to the formation of the energetically stable relaxed amorphous state. The XPS spectra show local fluctuation of boron density. This effect was attributed to the formation of small iron clusters—the characteristic feature for the relaxed amorphous phase.     

Magnetic Properties of Iron-Based Amorphous Metal Wires

It has been studied how the conditions of machining and the elastic tensile stresses affect the magnetic properties of amorphous metal wires of composition Fe₇₅Si₁₀B₁₅ produced by drawing from a melt. The magnetic characteristics of wires subjected to both thermal treatment and treatment with a continuous electric current of different magnitude have been investigated. The residual induction of wires is their magnetic parameter most sensitive to the conditions of treatment. The dependences of the residual induction on temperature and on the magnitude of the treating electric current are qualitatively similar. The greatest changes in residual induction are observed in the range of treating electric currents from 0.5 to 0.8 A, which can be associated with the processes of structural relaxation and crystallization occurring in the wires. The run of the dependence of the residual induction on the magnitude of tensile stresses is nonmonotonic in character and is determined by the level of internal hardening stresses of the test wires.     

巨磁阻抗效应实验研究

利用物理实验室中常见的设备,搭建了一组巨磁阻抗效应实验装置,测量了在不同轴向外磁场及不同频率下钴基非晶态软磁合金细丝的阻抗.阻抗实部虚部均随外磁场增大迅速减小,而随频率增大缓慢增大.与交流阻抗的电磁理论比较,该实验结果与理论一致.实验观察到在40 MHz的频率、10 mT外磁场中阻抗变化达80%.根据数据和理论公式拟合,发现材料复磁导率随磁场强度增大迅速减小.     

Measurement of the stress sensitivity of magnetostriction in electrical steels under distorted waveform conditions

Electrical steel laminations used in the construction of transformer cores are subject to stresses introduced during their construction and analysis of the effect of this on the magnetostriction of the lamination has been investigated previously. It has been shown that higher harmonics of magnetostriction are of greater importance than the fundamental when considering transformer noise. Whereas previous studies have concentrated on the magnetostriction harmonics generated by sinusoidal magnetization, this investigation seeks to understand the relationship between harmonics present in the magnetization waveform and those in the magnetostriction waveform. A measurement system has been designed based on a similar principle to one previously described. In this case, a single Labview Virtual Instrument (VI) is used for the control of the applied stress, controlled magnetization and measurement of magnetostriction together with other magnetic parameters such as specific total loss, specific apparent power, permeability, coercivity and remanence. An adaptive digital feedback algorithm is utilized for control of arbitrary waveform which may be constructed from discrete harmonics or read from an input waveform. As well as measuring peak magnetostriction the software utilizes an FFT to calculate the harmonics of magnetostriction at each stress point. The effect of harmonics introduced into the magnetization waveform on the magnetostriction harmonics will be shown at various applied stresses. A harmonic, Harm_B in the flux density waveform is shown to have the effect of producing a dominant harmonic in the magnetostriction given by (Harm_B+1)/2.     

非晶和纳米晶软磁丝状样品的环向磁化过程

采用阻抗法测定了不同磁结构的软磁丝状样品(非晶及纳米晶合金)的环向磁导率随环向磁场 强度和频率的变化. 按照Chen等的理论公式计算了样品的环向磁化曲线，结果发现，这一实 验原理公式对具有较大损耗的磁化过程并不适用. 因此，将其发展给出了更一般情况下的理 论公式. 此外，通过分析复数磁导率对环向磁场的依赖关系，确定了两类不同畴结构样品的 不同的环向矫顽力机理. 研究了交流频率对磁化过程的影响. 关键词： 非晶和纳米晶软磁丝 阻抗 环向磁化曲线     

Magnetoelastic interactions in multilayer microwires

Amorphous magnetic microwires exhibit outstanding magnetic characteristics as giant Barkhausen jump or nearly non-hysteretic behaviour that make them very suitable as sensing elements in various devices. In this article, we overview the different families of microwires and summarise most relevant results in connection with the magnetoelastic interlayer interactions in multilayer microwires. Improved magnetic behaviour can be observed in bi-layer microwires consisting of a magnetic nucleus coated by insulating cover. By employing combined sputtering and electroplating techniques, a novel series of multilayered magnetic microwires have been recently introduced. They consist typically of a magnetic nucleus and several shells having insulating and/or metallic nature, the latter magnetic or not. The magnetic character of the multilayer microwire will then depend on the magnetoelastic coupling between layers. External layers induce compressive stresses on the soft amorphous nucleus resulting in induced axial or circular anisotropies depending on magnetostriction sign. In a similar way, reduction of measuring temperature results in tensile stress as a consequence of different thermal expansion coefficients of various layers.     

钴基非晶丝的巨磁阻抗效应实验装置设计

介绍了巨磁阻抗效应的原理,提出了测量方法,分析巨磁阻抗效应随频率、外加磁场的变化规律.研究表明在6~21 MHz范围内,100 mT磁场下巨磁阻抗可达-50%以上,并且在30~40 mT磁场下巨磁阻抗即可达到-40%.     

Relaxation phenomena in amorphous Co75−x Mn x B25 induced by stress annealing

The change of the magnetization direction in amorphous ferromagnets by the application of a magnetic field and a tensile stress is investigated by magnetoresistivity measurements. Attention is focussed on permanently remaining influences on the domain structure induced by stress annealing treatments. To observe any dependence on the sign of the magnetostrictionλ _s, we investigated the amorphous Co_75?x Mn _x B₂₅ system where the sign ofλ _s changes from negative to positive values with increasing Mn content. Surprisingly the stress-annealed samples showed a similar behavior independent of the sign of the magnetostriction.     

Surface magnetic anisotropy in glass-coated amorphous microwires as determined from ferromagnetic resonance measurements

The ferromagnetic resonance frequency of different Co base glass-coated amorphous magnetic microwires about 3.5 μm in diameter with negative, vanishing and positive magnetostriction has been investigated from power absorption measurements in the microwave frequency range. The experimental technique employed here involves the replacement of the dielectric of a coaxial transmission line by the sample to be measured. From the evolution of the resonance frequency with DC applied magnetic field, the surface magnetic anisotropy field of the microwires has been quantitatively obtained and, as expected, found to depend on the sign and strength of the magnetostriction. Similar values for the surface anisotropy are obtained in comparison with bulk anisotropy as determined from quasi-static hysteresis loops measurements.     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.