Electroplated Bimagnetic Microwires: From Processing to Magnetic Properties and Sensor Devices

Multilayer microwires with biphase magnetic behavior are revisited in this work. They are fabricated by the combination of ultrarapid solidification and electroplating techniques, and they are composed by ferromagnetic nucleus, intermediate glass layer, and ferromagnetic outer shell. Different magnetic configurations have been explored: soft/hard (CoFeSiB/CoNi and FeSiB/CoNi), soft/soft (CoFeSiB/FeNi), and hard/soft (FePtSi/FeNi). Their magnetic properties are mainly determined by the magnetic interactions between both magnetic phases: (I) a magnetoelastic coupling that arises from the mechanical stresses induced during the growth of the external magnetic shell and (II) a magnetostatic bias field that arises from uncompensated magnetic charges of the hard layer. Most outstanding static (i.e., low-field hysteresis loops) and dynamic (i.e., magnetoimpedance and ferromagnetic resonance) properties are reviewed in this article. The possibility to tailor the magnetization reversal of the soft phase through the tuning of those magnetic couplings places multilayer biphase microwires in a very competitive position as functional sensing elements suitable for a number of technological applications. In particular, we focus on their use in multifunctional sensor devices and fluxgate applications.     

Effect of composite origin on magnetic properties of glass-coated microwires

We observed that magnetic properties (Giant magneto.-impedance effect and domain wall dynamic) of glass-coated microwires are closely related with the peculiarities of the fabrication technique involving rapid solidification of metallic alloy surrounded by glass coating from the melt.We present studies of the interfacial layer between the metallic nucleus and glass coating and studies of the inhomogeneities related with fabrication process of thin ferromagnetic microwires.We observed gas bubbles within the glass coating with volume content of about 8–12%. The sizes of the bubbles were between 1 and 15 μm. The existence of such bubbles might be the origin of the inhomogeneities in the internal stresses distribution.Using scanning electron microscope JEOL JSM-6610 we obtained the image of the interfacial layer and the elements distribution within the glass coating and metallic nucleus. This allowed us to estimate the thickness of the interfacial layer.Understanding of the origins of the interfacial layer and defects may help for improvement of the existing technology for thin composite wires fabrication and enhance their magnetic properties.     

Korrosionsverhalten wetterfester Baustähle unter Berücksichtigung konstruktionsbedingter Spalte

Corrosion behaviour of weathering steels in consideration of constructional crevices Lattice constructions made from the steels WT St 37‐2, COR‐TEN A, KT 62/ ST 37‐2, and WT Neu had undergone a weathering in Zinnwald/Erzgebirge over a period of 8 years. Stationary protection layers, consisting of an inner amorphous barrier layer and a semicrystalline outer layer, had formed on the free surfaces, whereas this layer formation was not or only partially found in crevices of different geometry. In the present work the thickness‐loss of the steel sheets and profiles is evaluated in dependence of the design of the crevice. Furthermore, the structure and the chemical composition of the rust layers are described with regard to a crevice design which is favourable or unfavourable for the corrosion behaviour.     

Untersuchung des Korrosionsverhaltens dünner magnetischer Metallfilme aus (Co-Ni)- und (Co-Ni-Cr)-Legierungen mit Hilfe der zyklischen Voltammetrie

Investigation into the corrosion behaviour of thin magnetic metal films of (Co-Ni) and (Co-Ni-Cr) alloys by cyclic voltammetry In order to establish the high dense magnetic recording of information it is important to produce very thin metallic magnetic layers. Since these layers show a certain sensitivity to external chemical influences the investigation of their corrosion behaviour is of considerable importance. The investigated specimens are thin layers of (Co-Ni) and (Co-Ni-Cr) alloys on a glass substrate. In the presented paper the layers were investigated by cyclic voltammetry (CV). In doing so, it is possible to determine the extent of corrosion of each single specimen by the determination of the electrical charge below the anodic oxidation peaks. Partly there are considerable differences noticeable between the first and the second scan of polarisation. The height of the oxidation peaks are often much lower in the second scan which indicates the passivation of the layers in the first cycle. This matter is connected especially with the presence of Cr in the layer.     

泰勒法制备的Ni-Mn-In-Co合金纤维的磁热效应

采用泰勒法制备了直径范围在30~100μm之间的玻璃包裹Ni-Mn-In-Co合金纤维。利用综合物性测量系统（PPMS）研究了磁场对制备态和退火态纤维马氏体相变温度的影响,并且从M-B曲线中分别计算得出了制备态和退火态纤维的磁热效应。研究结果表明：制备态纤维在室温下为7M马氏体结构。在制备态和退火态纤维中,奥氏体相变开始温度随外加磁场变化速率（ΔAs/ΔB）分别为-1.6和-4 K/T。退火态纤维在As点附近发生磁场诱发马氏体向奥氏体逆相变。退火态纤维最大磁熵变为3.0 J/（kg·K）,远大于制备态纤维的0.5 J/（kg·K）。Ni-Mn-In-Co合金纤维的大磁熵变和低成本使其成为最具潜力的磁制冷材料之一。     

Designing damage-resistant brittle-coating structures: II. Trilayers

An extension of the FEA-based damage analysis for bilayers in Part I is presented for trilayers consisting of a functional outer layer on an underlying substrate with an intermediate inner core layer. The inner core layer may be used to enhance bonding or load support, but is itself vulnerable to subsurface radial cracking (brittle interlayer) or yield (soft interlayer). A stress analysis is conducted by reducing the trilayer system to an effective bilayer in which the core layer is regarded as either part of the coating or substrate, depending on the damage interface (i.e. outer/inner or inner/substrate). The stress solutions are used to determine generic relations for the critical loads to induce radial cracking or plasticity (or quasiplasticity) in the outer or inner layers, and even in the substrate. A quadratic relationship between critical load and effective coating thickness is preserved; and strength and hardness of the constituent layers remain principal, linearly dependent, material parameters. However, dependence on relative outer/inner layer thicknesses and elastic moduli are not generally amenable to exact solution, limiting useful explicit relations to radial cracking in the inner layer. Use of the analysis in constructing design diagrams is again considered.     

Co/Pt multilayer-based pseudo spin valves with perpendicular magnetic anisotropy

Pseudo spin valves(SVs) exhibiting perpendicular magnetic anisotropy were prepared by magnetron sputtering. Magnetization measurements of the Co/Pt multilayers were performed to select the reference and free layers. The selection criteria are square magnetic hysteresis loops, weaker current shunting effect, and proper coercivity. The optimal reference layer and free layer are Pt(5.0 nm)/[Co(0.4 nm)/Pt(0.6 nm)]3/Co(0.4 nm)/Cu(3.0 nm)and Cu(3.0 nm)/[Co(0.4 nm)/Pt(1.5 nm)]4, respectively.The resulting pseudo SV exhibits two well-separated hysteresis loops when the field is applied perpendicular to the film plane. The minor hysteresis loop corresponding to the free layer shifts toward negative direction of the magnetic field axis, indicating ferromagnetic interlayer exchange coupling between the two magnetic layers. The coupling also enhances the coercivity(HC) of both layers. The perpendicular giant magnetoresistance(GMR) of 2.7 % is achieved with current in plane measurement. The GMR first increases when Pt seed layer is thickened, reaches a maximum of 3.0 % at 4 nm and then decreases with the further increase of thickness. But thicker Cu spacer layer always lowers the GMR of the SV.     

The effect of mechanical stress on Ni63.8Mn11.1Ga25.1 microwire crystalline structure and properties

We studied structure and magnetic properties of Heusler alloy microwires. The dependence of the phase composition of the annealed microwires on the presence of glass coating has been found. The crystalline structure of the glass-coated samples is composed of two cubic crystalline phases of different density. The samples annealed without coating are in the single-phase state. The two-phase state is supposed to be caused by intrinsic stresses resulted from the action of opposite sign stresses in different microwire sections. The magnetic properties of the fabricated Heusler-type microwires differ from their bulk and thin film counterparts. Annealing of microwires considerably affect magnetic properties at room temperature. Unusual magnetic behavior is connected with two-phase structure of microwires.     

High temperature behaviour of NiCrAlY coatings made by laser cladding

The oxidation behaviour of NiCrAlY coatings made by laser cladding on Hastelloy X is presented in this study. Laser cladding is an alternative method to thermal spraying for the production of bond coats. Comparable dense layers with approximately zero porosity should improve the oxidation behaviour. The oxidation behaviour of the coated specimens was assessed by air furnace oxidation tests at 1100 °C for up to 450 h. The coatings were analysed by means of light and electron microscopy techniques, microprobe analysis and X-ray diffraction analysis. The analysis was performed before and after the oxidation tests. The as-clad coating had a columnar dendritic structure and it did not show the presence of the relevant defects. After the oxidation tests an oxide scale was present which consisted of two distinct layers. The layers consisted of an outer layer of mixed spinel-type oxides and an inner continuous layer, in which alumina was present. The obtained results suggested that up to 450 h the system was able to form a continuous alumina layer that could protect the substrate from oxygen diffusion.     

Magnetic and microwave properties of glass-coated amorphous ferromagnetic microwires

Glass-coated amorphous FeCuNbSiB microwires were prepared by Taylor-Ulitovsky technique. X-ray diffractometry and scanning electron microscopy were used to investigate the microstructure and morphology of the glass-coated microwires respectively. The vibrating sample magnetometer and vector network analyzer were used to study the magnetostatic and microwave properties of glass-coated microwires. The experimental results show that the effective anisotropy of an array of 150 microwires of 10 mm in length is large than that of one microwire of 10 mm in diameter and an array of 150 microwires of 1 mm in diameter. The natural ferromagnetic resonance takes place as the microwave magnetic component is perpendicular to the microwires axis, and the electric dipole resonance takes place as the microwire is long or the short microwire concentration is moderate. The natural ferromagnetic resonance shifts to higher frequency with the larger microwire concentration. The electric dipole resonance is governed by the microwires length and concentration. The glass-coated FeCuNbSiB microwires can be used to design EMI filters and microwave absorbing materials.     

Magnetic properties of microwires and filiform nanostructures with elongated magnetic inclusions

We developed technological procedures for preparation of filiform nanostructures (FNSs) on the basis of stretching microwires with magnetic material cores. It is shown that disruptions of the microwire core occur for these materials during technological process of stretching due to relatively high melting temperature of magnetic alloys. As a result, FNSs with elongated magnetic inclusions are produced. An installation for the production of FNSs as well as an experimental complex for measuring magnetic properties of the produced magnetic FNSs is described in this paper. The morphology of the produced FNSs from microwires with magnetic bistability is investigated by means of scanning electron microscopy, while magnetic properties, such as magnetic response with remagnetizing pulses, hysteresis loop, and coercive force, are investigated on the developed experimental complex. The magnetic interaction during remagnetizing processes of several microwires with different coercive forces assembled in a bunch, as well as the reaction of a bunch assembled from a big number of bistable mirowires of two types with different coercive forces (soft magnetic and hard magnetic cores) upon increasing external magnetic field are also investigated. Some potential applications of the produced magnetic structures are suggested.     

Co—Nb／Cr磁性多层膜的振荡耦合

Ｃｏ－Ｎｂ－Ｃｒ磁性多层膜由高频溅射方法制得,由振动样品磁强计测量样品的磁性能。在２．６７Ｐａ溅射氩气压下,发现样品随Ｃｆ层厚度增加,铁磁层间出现铁磁性和反铁磁性的耦合振荡,饱和磁化强度Ｍｓ基本保持不变,而在０．６７Ｐａ溅射氩气压下,样品的饱和磁化强度Ｍｓ发生了振荡,这可能与春铁磁层中顺磁分量振荡变化有关。     

Magnetic behavior of atomically engineered NiO–Co–Cu-based giant magnetoresistance spin valves using Pb as a surface modifier

The present study illustrates the efficacy of using surface modifiers, or simply surfactants, to alter, modify, or control the nature of interfaces and film morphology in order to control physical properties in magnetic multilayers. In particular, the magnetic properties of giant magnetoresistive NiO–Co–Cu-based symmetric spin valves are discussed in relation to the modification of interfaces and nanostructure using surface modifier Pb. Results show that a ML of Pb deposited on the first Co/Cu bilayer of the symmetric spin valve leads to reduced in-plane magnetic anisotropy and coupling. In the presence of Pb, the coherent growth mode of Co–Cu layers is disrupted, leading to a fine grain size (≈1–5 nm) in the metal layers. Although this grain size corresponds to the critical wavelength for maximum coupling due to Néel’s so-called “orange-peel” effect, the absence of topographical correlation between various interfaces prevent the occurrence of this form of coupling. At the same time, averaging of exchange interactions over such a fine grain size, which is much lower than the characteristic exchange length for Co (≈25–45 nm), precludes the display of local magnetocrystalline easy axes, thereby leading to low switching fields. The nature of magnetization reversal in the “free” Co layer of the Pb-free spin valve is highly local in nature and occurs by nucleation, growth and coalescence together of irregular micron or sub-micron sized domains.     

Crystal structure and magnetic properties of Fe/Cr/Gd superlattices

Results of investigations of structural and magnetic properties of Fe/Cr/Gd superlattices that differ in the thicknesses of the Cr interlayer have been reported. The insertion of the Cr interlayer between Gd and Fe layers has been found to lead to structural changes in Gd layers and the appearance of an additional fcc phase in them along with the main hcp phase. The new fcc phase is uniformly distributed across the thickness of the layer and is not localized near layer boundaries or in the center of Gd layers. Polarized-neutron reflectometry was used to show that the aforementioned structural changes are accompanied by a substantial (two-fold to threefold) decrease in the average magnetization of gadolinium over a wide temperature range. Near interfaces of the Gd layer, a layer appears that is two-to-three monatomic layers thick and characterized by increased magnetic moment.     

Magnetic anisotropy of Co/Cu/Co nanocrystalline films upon magnetic annealing

The magnetic anisotropy of Co/Cu/Co films with the thickness of the copper spacer corresponding to the antiferromagnetic and ferromagnetic indirect exchange coupling between Co layers has been studied. The films deposited on naturally oxidized (111) Si single crystals were produced by magnetron sputtering. The films were annealed at 240°C. Such an annealing virtually neither changes the grain size nor leads to the mixing of layers; i.e., no disturbance of the coupling type between the cobalt layers takes place. Changes in the surface and induced magnetic anisotropy and in the effective energy of indirect exchange coupling have been studied upon annealing in the presence and absence of a magnetic field. It has been found that the shape of surface inhomogeneities in the films changes upon annealing in the magnetic field applied along the film plane, which substantially affects, the surface anisotropy. In the films characterized by antiferromagnetic coupling, the easy axis of magnetization is induced only in the magnetic fields exceeding the saturation field. The induced-anisotropy constant estimated theoretically agrees well with those determined experimentally.     

Bond stress development at a surface coating-substrate interface due to the action of a nucleus of thermo-elastic strain

The class of problems dealing with surface reinforced elastic media is encountered in many areas of materials engineering, notably in connection with surface layers that are used to provide protection to an otherwise softer substrate. These problems are of particular importance to the assessment of the mechanical behaviour of thin films and other forms of industrial coatings. This paper examines the problem related to the flexure of a plate-like surface layer that is bonded to an elastic halfspace region, and where the flexure of the coating is induced by a nucleus of thermo-elastic strain acting within the halfspace region.     

Magnetization Mechanism of Electrolessly Deposited NiXP/CoReNiMnP Double Layer Films

This study examines the effect of the particular choice of underlayer materials upon the magnetic properties of electrolessly deposited double layer films that possess perpendicular anisotropy. Double layer films comprising a CoReNiMnP magnetic layer and NiXP (X = Zn, W and Fe) underlayer exhibit the characteristics of both domain wall motion and coherent rotation. Thinner magnetic layers mainly consist of elements recognisable for a wall motion model, whilst an incoherent rotational model is dominant in thicker magnetic layers. A rotational layer emerges at thickness between 80 – 150 nm for each of the underlayers. The transition region between wall motion and rotation is determined by the underlayer materials. The regions for NiWP and NiFeP underlayers are narrower than the corresponding NiP and NiZnP underlayers.     

Magnetic models of cast amorphous microwires

A theory of a magnetic domain model for glass-coated amorphous microwires is studied. The obtained theoretical results are confirmed by experiments. Cast glass-coated amorphous microwires with positive magnetostriction have a rectangle hysteresis loop, which is characterized by a coercive-force stable magnitude. The coercive-force magnitude and the fluctuations of this magnitude are of theoretical and practical interest. The theory of the relaxation mechanism of magnetic reversal is constructed. For the more precise comparison of the theory and the experiment, a series of experimental measurements is needed, which are also discussed in the paper.     

Effect of nanocrystallization on giant magnetoimpedance effect of Fe-based microwires

We studied giant magnetoimpedance (GMI) effect and magnetic properties of Fe_70.8Cu₁Nb_3.1Si_14.5B_10.6 and Fe_71.8Cu₁Nb_3.1Si₁₅B_9.1 Finemet microwires. We observed that GMI effect and magnetic softness of glass-coated microwires produced by the Taylor–Ulitovski technique can be tailored either controlling magnetoelastic anisotropy of as-cast FeCuNbSiB microwires, and/or controlling their structure by heat treatment or by changing the fabrication conditions. High GMI effect has been observed in as-prepared Fe-rich and heat treated microwires with nanocrystalline structure.     

Mechanism of corrosion protection of hot-dip aluminium–silicon coatings on steel studied by electrochemical depth profiling

The electrochemical behaviour of hot dip aluminium–silicon coatings on steel is studied in relation to the in-depth structure and composition, determined by SEM/EDX and EBSD. Progressive GDOES sputtering was used for electrochemical analysis using a micro-capillary three-electrode microcell in craters positioned in the various sublayers. Additionally, cross-sections were analyzed with SKPFM. Results show that the outer free aluminium layer of the coating is able to cathodically protect the underlying layers and substrate. Conversely, the interdiffusion layers are nobler than the steel. The galvanic series implies that cathodic protection will depend on the nature of the defect and the corrosion environment.