Cylindrical magnetization model for glass-coated microwires with circular anisotropy: Statics

The static magnetization profile of glass-coated microwires with effective circular anisotropy is investigated using micromagnetics. In this family of microwires, the ferromagnetic nucleus with an amorphous character presents a magnetic structure composed of an inner region with axial domains and an outer region with circular domains, due to magnetoelastic anisotropy. A one-dimensional micromagnetic model is developed, taking into account both the exchange and magnetoelastic anisotropy energies, and solved quasi analytically. The total energy, magnetization profiles and magnetization curves are investigated as a function of radius and anisotropy constant of the nucleus. This work represents a fundamental study of the magnetization process in these amorphous microwires and provides guidelines for the production of microwires with tailored magnetic properties. En passant, the nucleation problem in an infinite cylinder, introduced by W.F. Brown, is revisited.     

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.     

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.     

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.     

Study of domain structure and magnetization reversal after thermal treatments in Fe40Co38Mo4B18 microwires

We have studied the effect of thermal treatment on the magnetic domain structure and magnetic reversal process of amorphous and nanocrystalline Fe₄₀Co₃₈Mo₄B₁₈ microwires. The domain structure and the magnetization reversal of amorphous FeCoMoB microwires reflect the complex stress distribution introduced by the glass coating. Hence, the thickness of radial domain structure decreases with temperature and the temperature dependence of the switching field presents a discontinuous behavior. After nanocrystallization, the domain structure of FeCoMoB microwire is almost constant within the temperature range 10-400 K and the switching field decreases almost linearly with temperature mostly because of the decrease of saturation magnetization.     

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.     

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.     

Investigation of magnetic properties and micromagnetic structure of multicomponent Fe61.4Ni3.6Cr3.2Si2.4Nb7.8Mn3.6B18 amorphous ribbons

The magnetic properties and micromagnetic structure (equilibrium distribution of magnetization) of multicomponent Fe_61.4Ni_3.6Cr_3.2Si_2.4Nb_7.8Mn_3.6B₁₈ amorphous ribbons are studied using scanning Kerr microscopy and a vibrating sample magnetometer. 5-mm-wide and 35-μm-thick ribbons were obtained by hardening of melt in a rapidly rotating drum. Strong difference in the surface and bulk magnetic parameters of the ribbons is established. Domain walls (DW) parallel to the ribbon length are detected. It is shown that quasistatic magnetization reversal of ribbons mainly occurs due to the DW displacement.     

A quantitative analysis of magnetic vortices in Permalloy nanoparticles characterized by electron holography

The present work investigates experimentally curling magnetic configurations locally observed in almost dispersed Permalloy nanoparticles in the remanent state. Magnetic analysis is performed in a field emission TEM using off-axis electron holography. Particularly, electron holography is used to characterize the magnetic microstructure of Fe₃₀Ni₇₀ nanoparticles, whose average diameter (50 nm) is expected to be close to the critical size for a curling magnetic structure (vortex) formation. The vortex core diameter D_core and the bulk magnetic profile of the vortex are measured and compared with a “rigid vortex” micromagnetic model. The connection between vortex structure and the characteristic micromagnetic length of the system deduced from magnetization curve measurements is discussed.     

Domain wall propagation in Fe-rich amorphous microwires

The domain wall (DW) propagation in magnetically bistable Fe₇₄Si₁₁B₁₃C₂ amorphous microwires with metallic nucleus diameters of 12–16 μm has been investigated in order to explain high DW velocities observed in Sixtus–Tonks like experiments. In micrometric wires, the boundary between two head-to-head domains is very elongated. The DW mobility normal to the wall surface is reduced by the domain aspect ratio and is in the range of a few m/s/Oe in the linear regime. The experimental results in the viscous regime could be quantitatively explained in terms of the domain length and normal mobility limited by the eddy currents and spin relaxation losses.     

Influence of glass coating thickness and metallic core diameter on GMI effect of glass-coated Co68Fe4.5Si13.5B14 amorphous microwires

Influences of the size factors (glass coating thickness and metallic core diameter) of microwires on GMI effects of the glass-coated Co₆₈Fe_4.5Si_13.5B₁₄ amorphous microwires were investigated. The results indicated that the GMI effect of the microwires with the same glass coating thickness or the same metallic core diameter was initially increased to a peak and then decreased with an increase in the diameter or the thickness. The glass coating thickness and the metallic core diameter corresponding to the maximum GMI varied with metallic core diameter and glass thickness, respectively. The GMI effect of the microwires with the same geometric size varied remarkably under different cooling rates. Such effect was ascribed to the microstructural changes of the metallic core wire under different cooling rates. The influence of the glass coating thickness on the GMI effect of the microwire was attributed to the synthetical actions of crystallization enthalpy (degree of disorder) and the internal stress.     

Specific features of magnetic properties of “thick” microwires produced by the Ulitovsky-Taylor method

The magnetic properties of initial and heat-treated Co₆₉Fe₄Cr₄Si₁₂B₁₁ microwires in a glass shell with the diameter D = 125 μm and the diameter of the amorphous metallic core d = 90 μm produced by the Ulitovsky-Taylor method have been studied. It has been found that the magnetic characteristics, in particular, the saturation field H _S and the coercive force H _C of the samples annealed at a temperature T < 300°C do not differ from H _S and H _C of the initial microwire, and those of the samples annealed at T ≥ 400°C increase by almost one order of magnitude. The obtained experimental data have been explained by the structural features of the microwires. The near-surface values of H _S and H _C at T < 300°C are found to be larger than the bulk values by a factor of 5–10. These experimental data have been explained by the existence of structural and chemical ingomogeneities in the near-surface layer, which are inherent in amorphous materials. This difference decreases with a further increase in the annealing temperature, but H _S and H _C increase substantially. This fact has been explained by the beginning of the microwire crystallization.     

Effects of dipolar interactions on magnetic properties of Co nanowire arrays

Magnetic properties and magnetization processes of Co nanowire arrays with various packing densities are investigated by means of object-oriented micromagnetic framework(OOMMF) software package with finite difference micromagnetic simulations. The packing density of nanowires is changed with the diameter, number of nanowires and center-to-center spacing between the wires. The magnetization reversal mechanism and squareness of the hysteresis loops of the nanowire arrays are very sensitive to the packing density of nanowires. Clear steps and plateaux on the demagnetization are visible,which turns out that dipolar interactions among the wires have a significant influence on switching field.     

Magnetization switching of CoFeSiB free-layered magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprising amorphous Co_70.5Fe_4.5Si₁₅B₁₀, possessing low saturation magnetization of 560 emu/cm³, as a free layer have been investigated. The switching behaviours were confirmed for the micrometer-sized elements experimentally, using the scanning magneto-optical Kerr effect (scanning MOKE). A micromagnetic modelling study was also carried out for the submicrometer-sized elements. By using either a CoFeSiB single or a synthetic antiferromagnetic free-layer structure, the magnetization switching field became much lower than conventionally used CoFe free layered MTJs.     

Surface-induced structural modification in ZnO nanoparticles

Cobalt?Ccobalt carbide [Co _x C (x?=?2 or 3)] and cobalt (FCC-Co) microwires have been synthesized using a polyol method in the presence of a high external magnetic field of 4.3?kOe. It was reported before that the synthesis of these particles in the absence of magnetic field leads to the formation of spherical particles. Analysis of the X-ray diffraction (XRD) scans indicates that the synthesized Co _x C wires consist of four phases?? ??-Co, ??-Co, Co₃C, and Co₂C. The percent composition of these phases was 53.3?% Co₃C, 26.8?% Co₂C, 12.5?% ??-Co, and 7.4?% ??-Co. XRD analysis of the as-synthesized cobalt wires shows that it consists of single-phase FCC-Co. Based on Scherrer analysis of the XRD data, the average crystallite sizes of the cobalt carbide and the cobalt particles are 18.5 and 16.3?nm, respectively. Scanning electron microscopy (SEM) studies show that the diameter of Co _x C wires is in the range of 1.6(±0.2)???m with their length varying between 18 and 30???m while the diameter of the cobalt wires is 1.65(±0.1). The SEM results infer that the morphology of the growing particles was controlled by the magnetic field with the applied field directs the growth of the particles into wires. The magnetic measurements indicate a superparamagnetic character of the cobalt wires and a soft ferromagnetic nature of the synthesized Co _x C chains. The degree and field range of the interactions between magnetic domains have been investigated using a Henkel plot.     

Magnetization processes in the narrow domain structures of amorphous ferromagnetic alloys

The magnetization processes within the narrow domain laminae of amorphous ferromagnetic alloys have been investigated by means of the magneto-optical Kerr effect. Changes of the domain width of the closure domains by a magnetic field applied perpendicular to the laminae have been determined for the alloys Fe₈₀B₂₀ and Fe₄₀Ni₄₀P₁₄B₆. These results are compared with theoretical calculations, assuming that wall displacements within the closure domains and rotations of the magnetization in the bulk domains take place simultaneously and a stray field free domain structure is developed. It turned out, that the closure domain structure on the surface of the sample vanishes at the same magnetic field where magnetic saturation is approached.     

Evaluation of helical magnetoelastic anisotropy in Fe-based amorphous wire from the decomposed susceptibility spectra

A novel method is introduced here to evaluate the intrinsic magnetic anisotropy from susceptibility spectra. The combination of two techniques namely, the decomposition of susceptibility spectra together with its dependence on applied torsion is employed to determine quantitatively the intrinsic helical anisotropy in an amorphous wire. The susceptibility spectra of Fe_77.5Si_7.5B₁₅ amorphous wires have been experimentally measured as a function of torsion. The reversible susceptibility is ascribed to wall motion of axial domains within the core, and to magnetization rotation within radial domains in the shell. The relaxation frequencies of these magnetization processes are evaluated to be 0.36 and 1.82 MHz, respectively. The static rotational susceptibility shows an asymmetric behavior with regards to positive and negative torsion angles, and is maximum at the torsion angle of 30°, which counterbalances the internal stress. Present results indicate the existence of an intrinsic helical anisotropy corresponding to an average helical stress of 33 MPa.     

Magnetotransport measurements as a tool to probe the micromagnetic configurations in epitaxial Co wires

The micromagnetic structure in epitaxial Co wires is investigated by using the magnetoresistance effect associated with the domain wall formation. This provides an efficient tool to monitor the magnetization reversal in systems with reduced lateral size. The influence of the lateral size and of the magnetic history on the micromagnetic configuration of the wires is considered.     

铁基非晶丝样低频涡流损耗分析

以非晶丝的芯-壳(core-shell)结构模型为基础,对铁基非晶丝考虑其芯内存在一沿轴向的应力磁各向异性K_c, 而壳内是沿径向的应力磁各向异性K_s(K_c和K_s均为应力能常量λ_sσ₀的函数),导出了样品的交流环向磁导率μ_φ(r).由Maxwell方程和Ohm定律计算出此模型下的低频经典涡流损耗反常因子为η＝2．2.理论值与实验结果的比较表明,铁基非晶丝的 关键词： 非晶丝 芯壳结构 涡流损耗反常因子     

High-frequency magnetoresonance absorption in amorphous magnetic microwires

The Fe₆₉Si₁₆B₁₀C₅, Co₇₅Si₁₀B₁₅, Co₆₈Mn₇Si₁₀B₁₅ amorphous microwires have been studied by the magnetoresonance absorption technique in the X (9.5 GHz), K (20–27 GHz) and Q (30–37 GHz) frequency bands. The specimens under study were metal threads of about 5 μm in diameter coated with dielectric Pyrex layer with thickness 5 μm. The dependences of magnetic resonance spectra on frequency and wire orientation have been measured. The analysis of the resonance signal parameters has revealed that well-known classical equations for FMR in a cylindrical-shaped sample could not be applied for these microwires. It is shown that due to the skin depth effect the model of hollow cylindrical tube has to be applied to explain the experimental results in the frequency range measured. The values of saturation magnetization, g-factor and anisotropy field have been estimated from the frequency dependence of the field for resonance.