Magnetostrictive Amorphous Microwires Research Articles

Helical magnetic structures in magnetostrictive amorphous microwires

Surface magnetic structures have been studied in amorphous glass-covered microwires using the magneto-optical technique. Different types of helical structure have been recognized depending on the external conditions. The simulation of the magnetic structure in magnetic cylinder demonstrates dependence of the type of the structure on the value of the magnetostriction coefficient. The performed comparative analysis of experiment and results of the simulation permitted to establish the direct relation between the value of the magnetostriction coefficient and the type of the helical structure.

Direct Magnetoelectric Effect in a Sandwich Structure of PZT and Magnetostrictive Amorphous Microwires.

The magnetoelectric (ME) response in a trilayer structure consisting of magnetostrictive Fe77.5B15Si17.5 amorphous microwires between two piezoelectric PZT (PbZr0.53Ti0.47O3) layers was investigated. Soft magnetic properties of wires make it possible to operate under weak bias magnetic fields below 400 A/m. Enhanced ME voltage coefficients were found when the microwires were excited by ac magnetic field of a frequency of 50–60 kHz, which corresponded to the frequency of electromechanical resonance. The as-prepared microwires were in a glass coat creating a large thermoelastic stress and forming a uniaxial magnetic anisotropy. The effect of glass-coat removal and wire annealing on ME coupling was investigated. The glass coat not only affects the wire magnetic structure but also prevents the interfacial bonding between the electric and magnetic subsystems. However, after its removal, the ME coefficient increased slightly less than 10%. Refining the micromagnetic structure and increasing the magnetostriction by stress release during wire annealing (before or after glass removal) strongly increases the ME response up to 100 mV/(cm × Oe) and reduces the characteristic DC magnetic field down to 240 A/m. Although the achieved ME coefficient is smaller than reported values for multilayered films with layers of PZT and soft magnetic alloys as Metglass, the proposed system is promising considering a small volume proportion of microwires.

Controlling depinning and propagation of single domain-walls in magnetic microwires

The magnetization reversal in magnetostrictive amorphous microwires takes place by depinning and propagation of a single domain wall. This is a consequence of the particular domain structure determined by the strong uniaxial anisotropy from the reinforcement of magnetoelastic and shape contributions. In the present study, after an overview on the current state-of-the art on the topic, we introduce the general behaviour of single walls in 30 to 40 cm long Fe-base microwires propagating under homogeneous field. Depending on the way the walls are generated, we distinguish among three different walls namely, standard wall, DWst, depinned and propagating from the wire’s end under homogeneous field which motion is the first one to switch on; reverse wall, DWrev, propagating from the opposite end under non-homogeneous field, and defect wall, DWdef, nucleated around local defect. Both, DWrev and DWdef are observed only under large enough applied field. In the subsequent section, we study the propagation of a wall under applied field smaller than the switching field. There, we conclude that a minimum field, Hdep,0, is needed to depin the DWst, as well as that a minimum field, Hprop,0, is required for the wall to propagate long distances. In the last section, we analyse the shape of induced signals in the pickup coils upon the crossing of the walls and its correlation to the domain walls shape. We conclude that length and shape of the wall are significantly distorted by the fact that the wall is typically as long as the measuring coils.

Outer shell structure in nearly zero magnetostrictive amorphous microwires

Nearly zero magnetostrictive microwires have been proven to be very versatile, due to their extremely soft magnetic properties, the changes in their magnetization mechanism with dimensions, and the appearance of sensitive application-related effects, such as the large Barkhausen and the giant magnetoimpedance effects. Their sensitive magnetoimpedance response originates in the specific domain structure, composed of an inner core and an outer shell (OS), since the OS with a circumferential easy axis is the main region involved in the magnetoimpedance effect. The circumferential OS is believed to display a bamboolike structure with consecutive rings circumferentially magnetized in opposite directions. However, earlier theoretical studies predicted a uniformly magnetized OS as being energetically favored. This paper aims to clarify this contradiction. The variation of the magnetostatic energy from the bamboo domain to the uniformly magnetized state has been calculated and it has been found to be null. Therefore, a uniformly magnetized OS has lower energy due to the absence of domain walls. Calculation results are supported by magneto-optical Kerr effect surface domain images and surface hysteresis loops. For large values of the metallic nucleus diameter, as well as for microwires with the glass coating removed, the OS vanishes, being replaced with a surface region with helical magnetization, which expands toward the microwire surface.

Effect of surface domain structure on wall mobility in amorphous microwires

Recently reported results on domain wall propagation within the inner core of bistable Fe-based positive magnetostrictive amorphous microwires opened up the way for future spintronic applications of amorphous microwires. Domain wall propagation has also been investigated in Co-based nearly zero magnetostrictive microwires, which become bistable under certain conditions. Wall velocity and mobility values were found to be superior in the latter type of microwires due to their much smaller magnetoelastic anisotropy. In this paper, the key role played by the surface domain structure of microwires in determining the wall mobility is investigated. Wall velocity measurements have been performed on (Co0.94Fe0.06)72.5Si12.5B15 microwires in as-cast glass-coated state and after glass removal with a hydrofluoric acid solution. Surface magnetization has been studied employing magneto-optical Kerr effect. The results show that both as-cast glass-coated microwires and microwires with the glass coating removed, which are bistable, display a helical magnetization in the surface region. The direction of the magnetization in this region affects the mobility of the propagating wall due to the stray fields associated with the preponderant components of the magnetization.

Domain Wall Propagation in Nearly Zero Magnetostrictive Amorphous Microwires

Results on the investigation of the propagating 180deg domain walls in bistable amorphous glass-coated microwires with nearly zero magnetostriction are reported for the first time. As-cast glass-coated microwires are bistable only if their metallic nucleus diameter is larger than 20 mum. Glass removal induces bistability in microwires with metallic nucleus diameters below 20 mum. Nearly zero magnetostrictive glass-coated microwires display larger domain wall velocities and mobilities as compared to positive magnetostrictive microwires. Samples that become bistable after glass removal display smaller values of the wall mobility as compared to as-cast bistable microwires. Mobility can be increased by annealing. The experimental results have been explained based on the damping mechanisms of the domain wall motion, specifically on the spin relaxation damping, whose coefficient is proportional to the anisotropy constant from the microwire's inner core. Stress relief determined by glass removal and annealing have been considered. The results are important for future applications of nearly zero magnetostrictive microwires in spintronic devices.

The influence of the composition on the GMI effect in low magnetostrictive amorphous microwires

Abstract The influence of the composition on both the magnetostriction and the GMI effect in Co 72.5− x Fe x Si 12.5 B 15 ( x =5.5–6.2) amorphous glass-covered wires is investigated. The basic magnetic properties of these materials as well as internal stresses induced by the glass cover during the preparation process strongly influence the GMI effect.

Dynamic hysteresis in magnetostrictive amorphous microwires

Dynamic hysteresis in bistable amorphous wires is studied at frequencies up to 200 kHz. As frequency is increased, the magnetization loop exhibits progressive widening, maintaining its bistable property. The switching process is analyzed within the Neel-Brown theory of the magnetization reversal due to activation over an energy barrier. The frequency dependence of the switching field is evaluated by considering a "waiting time" for closure domain wall to escape the pinned state, which gives a very good agreement with the experiment. The time of the magnetization reversal decreases with frequency down to few microseconds at 200 kHz, owing to the switching field increase.

Effect of Applied Mechanical Stressses on the Impedance Response in Amorphous Microwires with Vanishing Magnetostriction

The effect of external (torsion and tensile) stresses on the electrical impedance of nearly-zero magnetostrictive amorphous microwires (conventional and glass covered) is reported. Glass coated Co 68.5 Mn 6.5 Si 10 B 15 microwire exhibits a maximum relative change in magnetoimpedance ratio up to around 130% at a frequency of 10 MHz, magnetic dc field of about 180 A/m and under tension of 60 MPa. This giant magnetoimpedance (GMI) effect of the microwire is affected by the magneto-elastic anisotropy induced in the sample by applying tensile stress. In addition, from the linear variations of the magnetic field H m corresponding to the maximum AZ/Z ratio, with the applied tensile stress, that is the H m (σ) curve, the magnetostriction constant of this amorphous microwire (λ s -2 × 10 -7 ) is estimated. The torsion giant impedance (TGI) ratio (ΔZ/Z) ξ has been investigated in as-cast and annealed (Co 0.94 Fe 0.06 ) 72.5 B 15 Si 12.5 conventional amorphous microwire. As-cast conventional microwire shows a slightly asymmetric Torsion Giant Impedance (TGI) and such asymmetry can be eliminated by current annealing or enhanced by current annealing under torsion. The asymmetry of (ΔZ/Z) ξ in the as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical magnetic anisotropy, which can be compensated by the application of certain torsional stress.

Tailoring of Magnetic Properties of Glass coated Microwires

ABSTRACTThe magnetic behaviour of the glass-covered metallic microwires with a wide compositional range of composition with soft and hard magnetic character is presented. The effect of conventional furnace and dc current annealing under dc axial magnetic field or without it as well as of a chemical etching on the magnetic properties of Co and Fe-based glass coated microwires has been studied. Such treatments modify the magnetic parameters. In particular, annealing under applied magnetic field (field annealing) can improve significantly such magnetic parameters as permeability. Such phenomenology can be interpreted considering the noticeable magnetic anisotropy induced by the combined effects of the magnetic field and strong internal stresses arising from the coating. Giant magnetoimpedance (GMI) effect was observed in nearly-zero magnetostrictive amorphous microwires. Upon careful heat treatment, FeCuNbSiB amorphous microwires devitrificate into nanocrystalline structure with large variety of magnetic parameters. Hard magnetic materials with coercivity up to 800 Oe were obtained as a result of decomposition of metastable phases in Co-Ni-Cu microwires as well as after crystallization process of FeCuNbSiB amorphous microwires.