Bistable Microwires Research Articles

Interaction of bistable glass-coated microwires in different positional relationship

The amorphous ferromagnetic glass-coated microwires with positive magnetostriction constant of the metallic core possess the bistable magnetization reversal and the fast domain wall propagation along the microwire axis. These properties and also the magnetization processes in the systems of the microwires are of interest in the magnetic sensing technology, encoding systems and smart composite applications. In this work we present the results of the experimental investigation, simulations and theoretical estimations of the hysteresis process in the systems of the magnetically bistable microwires with different length and positional relationship between them. The location of the short microwires near the long microwire affects the switching fields (external coaxial magnetic field applied for starting of the domain wall propagation along the microwire axis) and the hysteresis process. The changes of these properties are not directly proportional to the value of the shorter microwire shift along the longer one. When the short microwire was placed in the middle of the long one and when the one end of the long microwire coincided with the end of the short one, the two-steps hysteresis loops were observed for both sample orientations: before and after sample rotation on 180°. When the short microwire was placed close to the end of the long microwire (but did not coincide with it) we observed at first the two-steps hysteresis loop and single step behavior for one branch of the hysteresis loop after sample rotation. Moreover, changing of the orientation of the samples results in the shift of the switching field of the shorter microwire when its end was located near the end or coincided with the end of the longer one. This uncommon hysteresis behavior was explained and illustrated using results of the simulations. The values of microwires interaction were also estimated.

Evaluation of use of magnetically bistable microwires for magnetic labels

AbstractThe dynamics of the magnetization reversal by means of head‐to‐head domain‐wall propagation is investigated both experimentally and theoretically for magnetic labels made from Fe‐rich bistable glass‐coated microwires. It is shown that the amplitudes of the electromagnetic harmonics generated by bistable Fe‐rich magnetic labels of the length 1–4 cm with metallic nuclear diameter of 10–20 µm are more than 5 times larger than that of similar labels made of Co‐rich microwires having high permeability and low coercive force. The electromagnetic harmonics can be detected at the distances up to 15–20 cm from the magnetic label at the signal‐to‐noise ratio not lower than 2.0.

Annealing effect on local nucleation fields in bistable microwires

AbstractWe have investigated the influence of annealing on the local nucleation fields distribution in amorphous magnetically bistable microwires. The measurements have quite surprisingly shown a considerable increase of the local nucleation fields after annealing of the wire, although it is generally expected that thermal annealing without a magnetic field relieves the internal stresses, which results in a reduction of the nucleation fields. The increase of the local nucleation fields is explained by the disappearance of local inhomogeneities after annealing.

On different tag reader architectures for bistable microwires

A bistable microwire reader is an inductive type sensor that measures the fast magnetic flux reversals in ferromagnetic microwires having predominant longitudinal magnetisation. The signal processing in such readers is a challenging problem due to short induced pulses, pile up effect, pickup noise, and ringing distortion in the pickup coils. In our present work, analog and digital reading schemes for multi-bit tags made of several bistable microwires have been proposed. The performance of an analog reader can be sufficient for well separated pulses. However, the digital algorithms deliver a clearer output signal with better pulse resolution in the case of the pulse pile up effect. In turn, this enables reliable code recognition. Here, two digital algorithms have been tested. The digital low-pass Gauss filter effectively suppresses the pickup noise and ringing oscillations. The deconvolution algorithm constitutes the inverse task with respect to the convolution operator defining the pickup voltage output. This algorithm recovers the initial pulse profiles and provides the highest possible resolution of a pulse series. The obtained results will be useful for the creation of a robust reader for bistable microwire tags that can be implemented in recycling projects, marking of items, and fraud protection.

Role of Defects on Domain Wall Propagation in Magnetically Bistable Glass-Covered Microwires

We are trying to reveal the contribution of local defects on DW propagation in amorphous microwires. Con- sequently, we present a comparative study of single domain wall dynamics and local nucleation fields in Fe- and Co-rich amorphous glass-coated microwires. For this we modified experimental set-up based on the classical Sixtus-Tonks ap- proach introducing additional pick-up coil in order evaluate defects contribution in domain wall propagation. Below some critical magnetic field, HN , determined by the microwires inhomogeneities, an almost linear DW ve- locity, v, dependence on magnetic field, H , is found. Quite fast DW propagation (v till 3000 m/s at H about 65 A/m) has been observed. When the applied magnetic field ex- ceeds HN , new reverse domains can be nucleated and con- sequently tandem remagnetization mechanism can be real- ized. The role of defects existing in magnetically bistable microwires is related with nucleation of new reversed do- mains.

Bistable FeCoMoB microwires with nanocrystalline microstructure and increased Curie temperature

Novel nanocrystalline glass-coated Fe40Co38Mo4B18 microwires are introduced. They combine the advantages of nanocrystalline alloys exhibiting simultaneously increased Curie temperature and magnetic bistability, which is required for modern sensoric and spintronic devices. Positive magnetostriction of the crystalline FeCo grains results in a magnetic bistability, whereas good soft magnetic properties remain stabilized. As a result of the mechanical stress induced by the glass coating, the optimum temperature range for thermal treatment is enhanced up to 600 °C.

Mechanisms of the ultrafast magnetization switching in bistable amorphous microwires

Two magnetization reversal regimes were found in magnetically bistable Fe-rich microwires. The first one, exhibiting an almost linear dependence of the domain wall velocity v on magnetic field H reaching 1.7 km/s, is related to single DW propagation. The second essentially nonlinear regime is observed when H exceeds some critical magnetic field, HN, determined by the microwire inhomogeneities. At H>HN, new reverse domains can be nucleated, and consequently a tandem remagnetization mechanism can be realized. Ultrafast magnetization switching through additional nucleation centers created artificially can be applied in spintronic devices for enhancing their performance.

Domain Wall Propagation in Nanocrystalline Glass-Coated Microwires

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The characteristics of domain wall propagation in Fe<formula formulatype="inline"> <tex Notation="TeX">$_{73.5}$</tex></formula>Cu<formula formulatype="inline"> <tex Notation="TeX">$_{1}$</tex></formula>Nb<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula>Si<formula formulatype="inline"> <tex Notation="TeX">$_{13.5}$</tex></formula>B<formula formulatype="inline"> <tex Notation="TeX">$_{9}$</tex></formula> amorphous and nanocrystalline glass-coated microwires are investigated in order to determine the changes induced by structural transformation in wall velocity and mobility. An improved method for sensing the wall presence and direction, and for measuring its velocity, is also presented. Amorphous samples are bistable, and their wall velocity displays typical values for microwires with positive magnetostriction. Nanocrystallized samples are not bistable in general, but they become bistable either if the glass coating thickness is large enough to induce a strong axial anisotropy, or if their metallic nucleus diameter is large enough to increase the contribution of the magnetostatic term. Bistable nanocrystalline microwires display smaller switching field values, together with larger wall velocity and mobility values as compared to bistable amorphous samples with the same composition. The results have been explained considering the changes induced in magnetostriction by the nanocrystalline phase formation and the magnetization reversal mechanism by means of wall propagation. </para>

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.

Single-wall dynamics and power law in bistable magnetic microwires

The dynamics of a single domain wall in finite glass-coated microwires is reportedin the low-field regime below the switching field. The power law of the singledomain wall propagating over a large distance is confirmed. Three regionsare determined for the propagating domain wall. Below some critical field,H0, the domain wall is pinned at the wire’s end. Just above that critical field, the wall moves in theadiabatic regime, interacting with the defects during its propagation, with an average velocity ofv = S′(H−H0)β. At high field, the domain wall propagates in the viscous regimeand its average velocity is proportional to the applied magnetic field,H. An analysis of the temperature dependence of the scaling factorβ is further reported. It is also shown that the domain wall mobility parameterS′ is field-independent and is proportional to the domain wall mobilityS in the viscous regime.

Thermal activation over a complex energy barrier in bistable microwires

The thermoactivated-mechanism model is extended to study the thermal activation during the magnetization process in bistable amorphous Fe-base microwires. The complex potential of the single domain wall is taken into account. The thermoactivated-mechanism model gives us a possibility to decompose the domain wall potential into two contributions, magnetoelastic and relaxation. Their role under the different applied measuring condition (stress, time) is studied separately.

Switching-field distribution in amorphous magnetic bistable microwires

We present the temperature dependence of the switching field distribution of amorphous magnetic bistable microwires, which has unusual behavior in comparison with the experimental data presented up to now. The distribution is solved in terms of the thermoactivated model and its unusual temperature dependence is explained by the two contributions to the domain wall pinning in the amorphous microwires. The distribution width is found to be proportional to the switching field.

Magnetization reversal process in bistable microwires and its temperature dependence

The temperature dependence of the magnetic behaviour of Fe-based glass-coated amorphous microwires with positive magnetostriction has been studied in the temperature range from 5 to 300 K. For long enough microwires, hysteresis loops are squared shaped in the whole temperature range denoting the existence of a large domain with axial easy axis. For short microwires, magnetic bistability is partially lost near room temperature but may be recovered at low temperature. This is interpreted considering the influence of the shape anisotropy: in short wires the demagnetizing field may become large enough to destroy the single domain structure. The influence of coating is also considered by studying microwires with different coating thickness. Transverse magnetoelastic anisotropy up to around 400 MPa can be induced at low temperature as a consequence of the difference in thermal expansion coefficients between metallic nucleus and insulating coat. Additional changes in the low-temperature loops have been identified to be a consequence of the way how the microwires are fixed to the sample's holder. Under particular circumstances transverse anisotropy induced at low temperature may destroy the bistable behavior.

Multilayer Microwires: Tailoring Magnetic Behavior by Sputtering and Electroplating

AbstractA novel technique for preparing multilayer microwires with controlled magnetic behavior has been developed. This technique involves combining sputtering and electroplating procedures to deposit (magnetic or non‐magnetic) metallic nano‐ and microlayers onto glass‐coated amorphous magnetic microwires. A suitable choice of magnetostrictive amorphous metallic nucleus, together with the specific stresses induced by the deposited layers, allows the tailoring of specific magnetic behavior. In this way, the preparation of multilayer microwires characterized either by square‐shaped hysteretic loops (typical of magnetically bistable microwires with longitudinal easy axes), or by nearly non‐hysteretic loops (for those microwires with a circumferential magnetization easy axes), can be achieved.

Temperature dependence of remagnetization process in bistable magnetic microwires

Magnetostrictive microwires exhibit bistable magnetic behavior characterized by square hysteresis loops, a consequence of the magnetization reversal by depinning of a single domain wall from the closure structure at one end of the wire. This bistable behavior spontaneously observed in magnetostrictive microwires has been previously thoroughly studied at room temperature. In the present work, the temperature dependence of the remagnetization process is investigated in magnetic microwires, a few micrometres in diameter, covered by glassy coating plus additional metallic coatings. The influence first of geometrical parameters as length and diameter is considered. Later on, an emphasis is placed on the influence of the Pyrex and additional sputtered and electroplated coatings on the temperature dependence of switching field. Bistability can be lost, depending on the character of the coating, when reducing the measuring temperature due to the thermally induced anisotropy arising from the differential thermal expansion coefficient of the composite wire. On the other hand, the temperature dependence of the switching field and its distribution function have been measured. It is shown that the switching field increases as temperature decreases, which is analyzed considering the thermally activated pinning mechanism and the magnetostrictive contribution.

Temperature dependence of the switching field and its distribution function in Fe-based bistable microwires

The switching field distribution for magnetization reversal in a single Barkhausen jump of a bistable Fe-based amorphous microwire as well as its temperature dependence have been investigated in the temperature range from 77 to 450 K. Two processes have been identified to be responsible for the temperature dependence of the switching field: magnetostrictive volume domain wall pinning on stresses and relaxation effects due to local structural rearrangements. While at low temperatures, pinning on the atomic level defects plays the dominant role, magnetostrictive pinning becomes more important at intermediate temperatures. A simple model is proposed considering both energy contributions that fits reasonably well with experimental data and allows us to interpret additionally the observed temperature dependence of the switching field fluctuations.

Spatial and temporal distributions of magnetisation in arrays of interacting bistable microwires

The unbalanced distribution of magnetic poles at the ends of the bistable Fe-based magnetostrictive glass-coated amorphous microwires generates a magnetic stray field that interacts with magnetisation in neighbouring microwires. A theoretical model is developed to analyse the magnetisation process of a two-dimensional array of quasi-identical microwires, taking into account the symmetry of the distribution (hexagonal, cubic, random), the number of microwires, and the distance between them. The conditions are analysed for arrays which behave as a complex system, specifically a system with self-organised criticality where both distributions of microwires with parallel and antiparallel magnetisation follow a fractal patron, and also conditions when individual magnetisation reversals in the array extend to the neighbours, as well as the possibility of application of this effect in magnetic recording devices.

An analysis of interacting bistable magnetic microwires: from ordered to chaotic behaviours

Fe-based magnetostrictive amorphous wires are bistable exhibiting square-shaped longitudinal hysteresis loops. This is a consequence of their particular domain structure that allows them to be approached as magnetic dipoles. Arrays of such bistables wires are accordingly shown to be coupled by magnetostatic interactions. Furthermore, an analysis of these interactions allows us to conclude that depending on the relative strength of the magnetostatic and Zeeman interactions, ordered, complex or chaotic behaviours are found in the temporal variation of the array magnetisation.

Propagation of domain walls in bistable amorphous wires and microwires

The magnetization process of bistable amorphous wires and microwires occurs by a single Barkhausen jump that is ascribed to the propagation of a domain wall along the wire. Recent results on wall propagation are here introduced for both kind of wires. Two experiments are performed based on the classical Sixtus–Tonks method, exciting by alternate homogeneous field and locally under a static field. Velocity and damping characteristics of the propagation wall are obtained.