Giant magnetoimpedance in composite wires with insulator layer between non-magnetic core and soft magnetic shell

A method for calculation of the magnetoimpedance in composite wires having an insulator layer between non-magnetic core and soft magnetic shell is described. It is assumed that the magnetic shell has a helical anisotropy and the driving current flows through the core only. The distribution of eddy currents and expressions for the impedance are found by means of a solution of Maxwell equations taking into account the magnetization dynamics within the shell governed by the Landau–Lifshitz equation. The effect of the insulator layer on the magnetoimpedance is analyzed.     

Giant magneto impedance in electroplated NiFeMo/Cu microwires

Electroplated soft magnetic FeNiMo alloy films on Cu mircrowires show magneto impedance effects larger than 1000%. The magnetic anisotropy generated by the axial deposition of the alloy seems to be an important factor for the achievement of magnetoimpedance. The extent of this effect also increases with the layer thickness. Furthermore, the impedance is markedly improved after the annealing of the coating under an external magnetic field.     

Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles

The effect of surface anisotropy on the magnetic ground state of hollow maghemite nanoparticles is investigated using atomistic Monte Carlo simulation. The computer modeling is carried on hollow nanostructures as a function of size and shell thickness. It is found that the large contribution of the surface anisotropy imposes a “throttled” spin structure where the moments located at the outer surface tend to orient normal to the surface while those located at the inner surface appear to be more aligned. For increasing values of surface anisotropy in the frame of a radial model, the magnetic moments become radially oriented either inward or outward giving rise to a “hedgehog” configuration with nearly zero net magnetization. We also show the effect of the size of hollow nanoparticle on the spin behavior where the spin non-collinearity increases (for fixed value of surface anisotropy) as the diameter of the hollow nanoparticle increases due to the significant increase in surface-to-volume ratio, the thickness being constant. Moreover, the thickness of the hollow nanoparticle shell influences the spin configuration and thus the relation between surface anisotropy and the size or the thickness of the hollow nanoparticle is established.     

Ferromagnetic resonance of cobalt nanoparticles in the polymer shell

The magnetic properties of cobalt spherical nanoparticles (～ 5–9 nm in size) in a polymer shell are investigated using ferromagnetic resonance (FMR) spectroscopy. The metal-polymer complex is prepared through the frontal polymerization of the cobalt acrylamide (CoAAm) complex, followed by the thermolysis at a temperature of 643 K. Analysis of the ferromagnetic resonance spectra demonstrates that the material has a high blocking temperature of ～700 K. The anisotropy constant equal to 0.5 erg/cm³ is somewhat larger than the anisotropy constants characteristic of cobalt macrostructures. This difference is associated with the predominance of the surface anisotropy of nanoparticles. The surface anisotropy constant is calculated to be 0.17 erg/cm², and the anisotropy field is determined to be ～350 Oe. It is revealed that the polymer shell affects the magnetic properties of nanoparticles.     

Effect of torsional stresses on the magnetoimpedance of amorphous wires with negative magnetostriction

A model to describe the effect of torsional stresses on the magnetoimpedance of amorphous wires with negative magnetostriction is suggested. An approximate expression for the impedance with regard to the spatial distribution of magnetoelastic anisotropy induced by torsional stresses is derived. It is shown that the relative variation of the impedance is maximal near a critical stress value at which the surface magnetic structure of the wire changes. The calculated dependences of the impedance on the external magnetic field and torsional stress are in good qualitative agreement with experimental data for amorphous wires with negative magnetostriction.     

Magnetization Configuration in Magnetic Trilayer System with Out-of-Plane Component Defect

The equilibrium magnetization configuration, the inducing field and the coercive field in trilayer magnetic materials having an out-of-plane anisotropy defect interlayer between two in-plane anisotropy layers are discussed by both analytical and numerical calculations based on a micromagnet approach. It is shown that the above physical parameters strongly depend on the defect layer such as its thickness and exchange stiffness etc., as well as on the applied fields. It is found that there is a special thickness of defect layer, in which the inducing effect begin to occur, and the critical behavior of inducing field in the vicinity of the special thickness is linearly characterized. Particularly, the magnetic hysteresis shows typical soft hysteresis shape, even though the host material is composed of hard magnets, and the coercivity increases with increasing the thickness of the interlayer.     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

Off-diagonal magnetoimpedance in stress-annealed amorphous ribbons

The influence of creep-induced magnetic anisotropy on the off-diagonal magnetoimpedance in amorphous Co₆₇Fe₄Cr₇Si₈B₁₄ ribbon is investigated. Hard-ribbon-axis anisotropy is produced by continuous stress annealing. On applying DC bias current, the off-diagonal impedance becomes an antisymmetric function of applied field with a quasi-linear part around the zero field. Theoretical explanation of the phenomenon is based on classical electrodynamics. It is shown that the maximum on the frequency dependence of off-diagonal impedance, observed around 1 MHz, results from competition between the electromagnetic induction and the skin effect. The quasi-linear characteristic can be utilized in low-cost high-sensitive magnetic field sensors.     

Surface effects in nanoparticles: application to maghemite -Fe O

We present a microscopic model for nanoparticles, of the maghemite (-Fe₂O₃) type, and perform classical Monte Carlo simulations of their magnetic properties. On account of M?ssbauer spectroscopy and high-field magnetisation results, we consider a particle as composed of a core and a surface shell of constant thickness. The magnetic state in the particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and dipolar interactions and bulk and surface anisotropy. We consider the case of ellipsoidal (or spherical) particles with free boundaries at the surface. Using a surface shell of constant thickness ( nm) we vary the particle size and study the effect of surface magnetic disorder on the thermal and spatial behaviors of the net magnetisation of the particle. We study the shift in the surface “critical region” for different surface-to-core ratios of the exchange coupling constants. It is also shown that the profile of the local magnetisation exhibits strong temperature dependence, and that surface anisotropy is responsible for the non saturation of the magnetisation at low temperatures. Received 1 September 1999 and Received in final form 3 November 1999     

Mo覆盖层对MgO/CoFeB/Mo结构磁各向异性的影响

研究了Mo覆盖层厚度对MgO/CoFeB结构磁各向异性的影响. 研究发现, 加平行磁场生长出来的MgO/CoFeB/Mo样品表现为面内各向异性, 并且随着CoFeB的厚度减小, 面内各向异性逐渐减弱; 在CoFeB厚度减小到1.1 nm时, 仍可以保持面内各向异性, 垂直方向的外加饱和场逐渐减少; 厚度在0.9 nm及以下的情况下, 面内各向异性消失. 改变Mo覆盖层厚度, 当t_Mo= 1.6 nm时, 垂直方向的饱和场最小. 当生长过程的磁场变为垂直磁场时, 不同厚度的Mo覆盖层对MgO/CoFeB 的磁各向异性影响不同. Mo厚度在1 nm及以下时MgO/CoFeB/Mo样品表现为面内各向异性, Mo覆盖层厚度在1.2和5 nm之间时样品出现了垂直磁各向异性; 并且垂直方向的矫顽力也发生了变化, Mo覆盖层厚度为1.4 nm时样品的磁滞损耗会大一些.     

Nonlinear reconfiguration of vortexlike domain walls in magnetically uniaxial films under the action of an external field normal to the easy magnetic axis

The effect of external magnetic field H normal to the anisotropy axis on the energy and configuration of vortexlike asymmetric magnetic walls in a magnetically uniaxial film with an easy magnetic axis parallel to its surface is studied. The investigation is based on minimizing the energy functional of the film with due regard to exchange energy, magnetic anisotropy energy, magnetostatic energy, and Zeeman energy. The range of H below the anisotropy field is found where the asymmetric Néel wall is stable, unlike the case H = 0, when the asymmetric Bloch wall is stable. It is shown that an asymmetric Bloch wall becomes absolutely unstable and reconfigures into an asymmetric Néel wall at some critical values of H = H _⊥. The dependences of critical field H _⊥ on the film thickness and saturation induction at different values of the anisotropy field are determined: field H _⊥ depends on the thickness nonlinearly and on the saturation induction nonmonotonically.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Anisotropy of spin splitting and spin relaxation in lateral quantum dots

Inelastic spin relaxation and spin splitting epsilon(s) in lateral quantum dots are studied in the regime of strong in-plane magnetic field. Because of both the g-factor energy dependence and spin-orbit coupling, epsilon(s) demonstrates a substantial nonlinear magnetic field dependence similar to that observed by Hanson et al. [Phys. Rev. Lett. 91, 196802 (2003)]. It also varies with the in-plane orientation of the magnetic field due to crystalline anisotropy of the spin-orbit coupling. The spin relaxation rate is also anisotropic, the anisotropy increasing with the field. When the magnetic length is less than the "thickness" of the GaAs dot, the relaxation can be an order of magnitude faster for B ||[100] than for B || [110].     

Oscillations of the impedance of thin tungsten plates in a magnetic field

The surface impedance derivative with respect to the magnetic field of thin (skin depth comparable with thickness) tungsten plates is investigated experimentally and theoretically in circular polarizations of radio-frequency irradiation $\text{(? = 5 MHz)}$. The magnetic field is directed along the normal to sample surface-face (001). It is shown, that the impedance oscillations in both polarizations are due to the holes, lying on the bend of Fermi surface octahedron. The series of oscillations in “?” polarization is completely due to Gantmakher-Kaner (GK) effect [1]. The impedance oscillations in “+” polarization are caused by both GK effect and excitation of doppleron [2].     

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.     

Effect of surface chemisorption on the spin reorientation transition in magnetic ultrathin Fe film on Ag(0 0 1)

We have investigated the effect of surface chemisorption on the spin reorientation transitions in magnetic ultrathin Fe films on Ag(0 0 1) by means of the polar and longitudinal magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) measurements. It is found by the MOKE that adsorption of O₂ and NO induces the shift of the critical thickness for the transitions to a thinner side, together with the suppression of the remanent magnetization and the coercive field of the Fe film. This implies destabilization of the perpendicular magnetic anisotropy. On the other hand, H₂ adsorption is found not to change the magnetic anisotropy, though the enhancement of the coercive field is observed. The XMCD reveals that although both the spin and orbital magnetic moments along the surface normal are noticeably reduced upon O₂ and NO adsorption, the reduction of the orbital magnetic moments are more significant. This indicates that the destabilization of the perpendicular magnetic anisotropy upon chemisorption of O₂ and NO originates from the change of the spin-orbit interaction at the surface.     

Thickness dependence of microwave magnetic properties in electrodeposited Fe-Co soft magnetic films with in-plane anisotropy

In this work, the thickness effect of Fe₅₂Co₄₈ soft magnetic films with in-plane anisotropy on static and microwave magnetic properties was investigated. The hysteresis loop results indicated that the static in-plane uniaxial anisotropy field increased from almost 0-60 Oe with increasing film thickness from 100 to 540 nm and well-defined in-plane uniaxial magnetic anisotropy can be obtained as the thickness reached 540 nm or larger. Based on Landau-Lifshitz-Gilbert (LLG) equation, the microwave complex permeability spectra were analyzed and well fitted. The LLG curve-fitting results indicated that the initial permeability increased from 106 to 142 and the resonant frequency was shifted from 4.95 to 4.29 GHz as the film thickness was varied from 540 to 1500 nm. Moreover, it was found that there was a discrepancy between the static and the dynamically determined anisotropy field, which can be explained by introducing an additional effective isotropic ripple field. The decreased ripple field was suggested to result in a significant decrease of damping coefficient from 0.109 to 0.038.     

Influence of the surface anisotropy on the r.f. collapse effect

The influence of magnetic surface anisotropy on the fast relaxation of the hyperfine field forced by an r.f. field in invar (r.f. collapse effect) has been studied using the Mössbauer technique. The Mössbauer measurements were performed as a function of sample thickness (2.5–12 μm) and intensity (1–9 Oe) of the 50 MHz r.f. field applied. Due to the very high sensitivity of the r.f. collapse effect to the anisotropy field it was possible to detect the influence of “spin pinning” on the r.f. collapse effect. It is shown that a decrease of the sample thickness causes a decrease of the r.f. collapse effect at a given r.f. field frequency and intensity which is connected with the increase of the anisotropy field due to surface anisotropy. The dependence of the r.f. sidebands effect, which accompanies the r.f. collapse effect, on the sample thickness is discussed. The r.f. sidebands effect increases with decreasing sample thickness, which is in good agreement with the magnetostriction model of sidebands formation.     

垂直各向异性Ho3Fe5O12薄膜的外延生长与其异质结构的自旋输运

垂直磁各向异性稀土-铁-石榴石纳米薄膜在自旋电子学中具有重要应用前景.本文使用溅射方法在(111)取向掺杂钇钪的钆镓石榴石(Gd_0.63Y_2.37Sc₂Ga₃O₁₂,GYSGG)单晶衬底上外延生长了2—100 nm厚的钬铁石榴石(Ho₃Fe₅O₁₂,HoIG)薄膜,并进一步在HoIG上沉积了3 nm Pt薄膜.测量了室温下HoIG的磁各向异性和HoIG/Pt异质结构的自旋相关输运性质.结果显示,厚度薄至2 nm的HoIG薄膜(小于2个单胞层)在室温仍具有铁磁性,且由于外延应变,2—60 nm厚HoIG薄膜都具有很强的垂直磁各向异性,有效垂直各向异性场最大达350 mT;异质结构样品表现出非常可观的反常霍尔效应和“自旋霍尔/各向异性”磁电阻效应,前者在HoIG厚度小于4 nm时开始缓慢下降,而后者当HoIG厚度小于7 nm时急剧减小,说明相较于反常霍尔效应,磁电阻效应对HoIG的体磁性相对更加敏感;此外,自旋相关热电压随HoIG厚度减薄在整个厚度范围以指数方式下降,说明遵从热激化磁振子运动规律的自旋塞贝克效应是其主要贡献者.本文结果表明HoIG纳米薄膜具有可调控的垂直磁各向异性,厚度大于4 nm的HoIG/Pt异质结构具有高效的自旋界面交换作用,是自旋电子学应用发展的一个重要候选材料.     

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.