Domain Wall Propagation Research Articles

In situ observation of the magnetization configuration and reversal in cylindrical nanowires

Curvilinear magnetic structures often have unique magnetic behavior compared to their rectilinear counterparts. This is due to the unique curvilinear boundary conditions as well as the curvature induced Dzyaloshinskii–Moriya-like interaction and the curvature induced anisotropy. The effects of a curvilinear geometry are best studied in 3D structures, where the curvature can have a significant spatial extent. Of these 3D structures, the simplest structure to study is the cylindrical nanowire. Here, we have simulated the magnetization reversal in cylindrical NiFe nanowires and present in situ Lorentz TEM images to support the findings of the simulations. We studied the domain formation and reversal of nanowires with two distinct diameters that give rise to a different reversal behavior. We have, thus, found that the zero-field magnetization configuration in these wires can take on a double helix type of configuration. The reversal in these structures then proceeds through the winding and unwinding of these helical configurations rather than through domain wall propagation.

Combining two-photon lithography with laser ablation of sacrificial layers: A route to isolated 3D magnetic nanostructures

Three-dimensional (3D) nanostructured functional materials are important systems allowing new means for intricate control of electromagnetic properties. A key problem is realising a 3D printing methodology on the nanoscale that can yield a range of functional materials. In this article, it is shown that two-photon lithography, when combined with laser ablation of sacrificial layers, can be used to realise such a vision and produce 3D functional nanomaterials of complex geometry. Proof-of-principle is first shown by fabricating planar magnetic nanowires raised above the substrate that exhibit controlled domain wall injection and propagation. Secondly, 3D artificial spin-ice (3DASI) structures are fabricated, whose complex switching can be probed using optical magnetometry. We show that by careful analysis of the magneto-optical Kerr effect signal and by comparison with micromagnetic simulations, depth dependent switching information can be obtained from the 3DASI lattice. The work paves the way for new materials, which exploit additional physics provided by non-trivial 3D geometries.

Strong interfacial Dzyaloshinskii–Moriya induced in Co due to contact with NiO

The magnetic properties of NiO/Co/Pt as a function of Co layer thickness were investigated by polar magneto-optical Kerr effect (PMOKE) (magnetometry and microscopy) and Brillouin Light Scattering (BLS) spectroscopy. PMOKE measurements revealed strong surface anisotropy (1.8 mJ/m2) favoring perpendicular magnetic anisotropy and asymmetric domain wall propagation explained by anticlockwise chirality. BLS measurements show that this chirality is induced by strong interfacial Dzyaloshinskii–Moriya interaction (+ 2.0 pJ/m). This is one of the highest values reported so far for Co layers surrounded by different layers. The observed chirality is opposite to what has been found in Co/oxide interfaces. These results and data published earlier, indicate that the strength of interfacial Dzyaloshinskii–Moriya interaction increases with the amount of stoichiometric NiO. Therefore, this work shows that NiO is the source of the interfacial Dzyaloshinskii–Moriya interaction.

Unveiling the Complex Magnetization Reversal Process in 3D Nickel Nanowire Networks

AbstractUnderstanding the interactions among magnetic nanostructures is one of the key factors to predict and control the advanced functionalities of 3D integrated magnetic nanostructures. In this work, the focus is on different interconnected Ni nanowires forming an intricate, but controlled, and ordered magnetic system: Ni 3D Nanowire Networks (3DNNs). These self‐ordered systems present striking anisotropic magnetic responses, depending on the interconnections’ position between nanowires. To understand their collective magnetic behavior, the magnetization reversal processes are studied within different Ni 3D Nanowire Networks compared to the 1D nanowire 1DNW array counterparts. The systems are characterized at different angles using first magnetization curves, hysteresis loops, and First Order Reversal Curves techniques, which provided information about the key features that enable macroscopic tuning of the magnetic properties of the 3D nanostructures. In addition, micromagnetic simulations endorse the experiments, providing accurate modeling of their magnetic behavior. The results reveal a plethora of magnetic interactions, neither evident nor intuitive, which are the main role players controlling the collective response of the system. The results pave the way for the design and realization of 3D novel metamaterials and devices based on the nucleation and propagation of ferromagnetic domain walls both in 3D self‐ordered systems and future nano‐lithographed devices.

Control of the Walker breakdown by periodical magnetic wire-width modulation

Suppression of the Walker breakdown in confined wires is key to improving the operation and reliability of magnetic domain-wall-based devices, including logic, memory, and sensor applications. Here, via micromagnetic simulations, we demonstrate that periodical wire-width modulation with suitable geometric parameters can fully suppress the Walker breakdown of a field-driven domain wall, conserving its spin structure in the whole operating field range of a device. Key differences in the efficacy of the wire-width modulation are observed for wires with different widths and thicknesses such that different domain wall states are energetically stable. In particular, the approach is found to be effective in expanding the field-operating window of a device in the case of smaller wire widths and thicknesses (below 150 nm wide and 15 nm thick), whereas in larger wires, the advantages from the suppression in the Walker breakdown are counteracted by the increase in domain wall pinning and the reduction in the nucleation field for new domain walls. Simulations on intersecting magnetic wires prove the importance of suppression of the Walker breakdown. Since the domain wall behavior is chirality dependent, introducing periodical wire-width modulation conserves the spin structure, thus reducing stochasticity of the domain wall propagation.

Observation of a phase transition within the domain walls of ferromagnetic Co3Sn2S2

The ferromagnetic phase of Co3Sn2S2 is widely considered to be a topological Weyl semimetal, with evidence for momentum-space monopoles of Berry curvature from transport and spectroscopic probes. As the bandstructure is highly sensitive to the magnetic order, attention has focused on anomalies in magnetization, susceptibility and transport measurements that are seen well below the Curie temperature, leading to speculation that a “hidden” phase coexists with ferromagnetism. Here we report spatially-resolved measurements by Kerr effect microscopy that identify this phase. We find that the anomalies coincide with a deep minimum in domain wall (DW) mobility, indicating a crossover between two regimes of DW propagation. We demonstrate that this crossover is a manifestation of a 2D phase transition that occurs within the DW, in which the magnetization texture changes from continuous rotation to unidirectional variation. We propose that the existence of this 2D transition deep within the ferromagnetic state of the bulk is a consequence of a giant quality factor for magnetocrystalline anisotropy unique to this compound. This work broadens the horizon of the conventional binary classification of DWs into Bloch and Néel walls, and suggests new strategies for manipulation of domain walls and their role in electron and spin transport.

Estimation of changes in the length of a moving conical domain wall in bistable microwire

The single domain wall process of magnetization reversal in a bi-stable microwire is studied. Models for static planar and folded planar domain walls are analyzed. The results obtained show that a folded domain wall can have lower total energy. Based on this finding, the dynamic properties of the more probable conical shape of a moving domain wall were studied. Using a simple model of the folded conical domain wall, the formulas for length, velocity, and mobility of the domain wall as a function of the applied magnetic field were derived. The main result of the proposed model is that it predicts domain wall shortening with an increasing applied magnetic field. Fitting the model function to the experimental data made it possible to estimate the characteristic length of the domain wall and changes in its shape in the magnetic field applied. Comparison of the model and experimental data indicates that there is a difference in eddy current damping for fast and slow domain walls, which may be responsible for the unidirectional effect in domain wall propagation.

Enhancement of magnetic properties and microstructural changes in TbCu7-type Sm-Fe-Co-Nb-B melt-spun ribbons

The effects of annealing conditions on magnetic properties, phase, and microstructural changes in Sm-Fe-Co-Nb-B melt-spun ribbons were investigated, and the dependence of magnetic properties on Sm and B composition was also investigated. SmxFe80.9−(x+y)Co16.4Nb2.7By amorphous ribbons were prepared by melt-spinning and then annealing. After annealing at 625 °C, the Sm6.2Fe66.4Co16.4Nb2.7B8.3 ribbons exhibited crystallization of hard-magnetic TbCu7-type phase, and the coercivity increased as the annealing time was increased to 9 h. The grain size of the TbCu7-type phase as estimated from transmission electron microscopy (TEM) images was 20–50 nm for ribbons annealed for 9 h. Three-dimensional atom probe (3DAP) analysis revealed that Nb and B became enriched at grain boundaries and tended to condense at the grain boundaries with increasing annealing time. Non-magnetic Nb and B enrichment at the grain boundaries in fine TbCu7-type grains thus promotes magnetic decoupling between TbCu7-type phases and suppresses domain wall propagation, resulting in high coercivity. Composition dependence on magnetic properties was investigated. Optimum annealing time tended to increase with increasing Sm content. The Sm6.7Fe65.9Co16.4Nb2.7B8.3 ribbons exhibited a maximum coercivity of 695 kA⋅m−1 by annealing at 625 °C for 21 h and subsequent slow cooling at a cooling rate of 4 °C⋅h−1. As B content decreased, coercivity decreased and remanence increased. The Sm6.2Fe68.7Co16.4Nb2.7B6.0 ribbons exhibited a high σr of 100 A⋅m2⋅kg−1 and moderate HcJ of 540 kA⋅m−1 after optimal annealing. An isotropic bonded magnet was fabricated from this ribbon. The bonded magnet exhibited high magnetic properties of Br = 0.82 T, HcJ = 538 kA⋅m−1, and (BH)max = 98 kJ⋅m−3, and small temperature coefficients of α(Br) = −0.06%⋅°C−1 and β(HcJ) = −0.27%⋅°C−1.

Chirality-Induced Noncollinear Magnetization and Asymmetric Domain-Wall Propagation in Hydrogenated CoPd Thin Films.

Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits a strong vertical and lateral antiferromagnetic coupling in the perpendicular component between the areas with and without plasma pretreatment, and asymmetric domain-wall propagation in the plasma-pretreated areas during magnetization reversal. These phenomena exhibit evident magnetic chirality and can be interpreted with the Ruderman-Kittel-Kasuya-Yosida coupling and the Dzyaloshinskii-Moriya interaction (DMI). The sample processing demonstrated in this study allows easy incorporation of lithography techniques that can define areas with or without DMI to create intricate magnetic patterns on the sample, which provides an avenue toward more sophisticated control of canted spin textures in future spintronic devices.

Domain wall propagation in Fe-rich magnetic microwires with graded magnetic anisotropy

We observed that stress-annealing of Fe75B9Si12C4 magnetic microwire at variable annealing temperature allows to create graded magnetic anisotropy. We have found that single domain wall (DW) propagation in a media with graded magnetic anisotropy is essentially non-uniform: faster DW propagation is observed in the region with moderate stress-annealing induced magnetic anisotropy. Higher DW velocity in the region with induced magnetic anisotropy is explained by the transverse character of stress-annealing induced magnetic anisotropy which affects the travelling DW in a similar way as application of transversal bias magnetic field. Obtained graded magnetic anisotropy allows engineering the DW dynamics of Fe-rich microwires.

Low pressure drive of the domain wall in Pt/Co/Au/Cr2O3/Pt thin films by the magnetoelectric effect

The magnetoelectric (ME) effect is one of the methods for electrically controlling the magnetization direction. In this study, we investigated the ME-driven domain wall creep and depinning using a Pt/Co/Au/ME-Cr2O3/Pt thin film. The domain switching process is governed by domain wall propagation rather than the nucleation of reversed domains, similar to a pure ferromagnet. The domain wall velocity v increases exponentially with the ME pressure, that is, the simultaneous application of magnetic H and electric E fields. The v–E curve under a constant H can be scaled by the ME pressure with the assistance of the exchange bias. We determined the depinning threshold, pinning energy scale, and depinning velocity based on the model for the magnetic domain wall creep. Compared with the depinning velocity in various other systems, it was suggested that the ME-driven mechanism could yield a fast domain wall velocity utilizing the low pressure.

Structural relaxation in metastable magnetic submicronic wires

Changes triggered by low temperature metastable relaxation annealing in the peculiar magnetic behavior of rapidly solidified submicronic amorphous wires are reported. The overall aim was to understand the effects of the associated stress relief process on magnetization switching and magnetic domain wall propagation in these low dimensional amorphous wires with cylindrical symmetry. The results have shown that, even though the employed relatively low annealing temperatures did not initiate any crystallization, i.e., the amorphous state has been preserved, the consequences in terms of magnetic behavior are significant, with important variations in both magnetic switching field and domain wall velocity values. The observed modifications have been interpreted based on the interplay between the magnetoelastic and magnetostatic anisotropies within the submicronic amorphous wires, as well as on the complex changes in the underlying nonlinear stress distribution generated by the rapid solidification process. The outcomes of this investigation, alongside the general framework for their explanation, have unraveled novel ways of accurately tailoring and controlling the magnetic characteristics of the amorphous submicronic wires for developing new applications in sensors and domain wall logic devices, in which rapid switching and fast domain walls can make an important impact.

(Nd,La,Ce)-Fe-B hot-deformed magnets for application of variable-magnetic-force motors

The recent development of variable-magnetic-force (VMF) motors has raised a new demand for permanent magnets with a moderate coercivity and magnetizing field (Hmag), flat first-order reversal curves (FORCs), and high magnetization. In this study, we explored the potential of low-cost (Nd,La,Ce)-Fe-B hot-deformed magnets for the VMF motor application. We showed that the coercivity can be tuned to a desirable value (∼0.15-0.65 T) using a combined effect of intrinsic and extrinsic factors. Unlike commercial Nd-Fe-B sintered magnets, FORCs of the (Nd,La,Ce)-Fe-B hot-deformed magnets are flat. Microstructure observation and magneto-optical Kerr effect (MOKE) microscopy showed the origin of the flat FORCs is the large volume fraction of the rare-earth-rich intergranular phase in the ultra-fine grained microstructure, which acts as pinning sites against magnetic domain wall propagation. Moreover, a relatively high remanent magnetization of 1.26 T for the (Nd,La,Ce)-Fe-B hot-deformed magnet is an additional merit compared to the previously proposed sintered magnets for the VMF motor application. This work demonstrates the (Nd,La,Ce)-Fe-B hot-deformed magnets are excellent candidates for VMF motor applications and thus widens the application spectrum for low-cost (Nd,La,Ce)-Fe-B magnets with a goal of well-balanced use of critical rare earth elements.

Direct Visualization of a Domain Wall Pinning by Time-Resolved Microscopy in Amorphous Glass-Coated Microwires

Amorphous glass-coated microwires with positive magnetostriction are composite material with very fast domain wall (DW) propagation that can reach up to 18 km/s. Previously it was shown that such high DW velocity can be explained by the tilted orientation of a DW in static or quasi-static experimental conditions. Here we perform time-resolved observation of a DW in propagation. The shape of a DW is studied via magneto-optical Kerr effect (MOKE) imaging. Time-resolved MOKE microscope operating in the single-shot regime and a static mode is used to ascertain the role of surface pinning on DW velocity fluctuation. It is shown that a tilted DW undergoes complex DW distortions on a local scale.

Tuning of Magnetoimpedance Effect and Magnetic Properties of Fe-Rich Glass-Coated Microwires by Joule Heating.

The influence of Joule heating on magnetic properties, giant magnetoimpedance (GMI) effect and domain wall (DW) dynamics of Fe75B9Si12C4 glass-coated microwires was studied. A remarkable (up to an order of magnitude) increase in GMI ratio is observed in Joule heated samples in the frequency range from 10 MHz to 1 GHz. In particular, an increase in GMI ratio, from 10% up to 140% at 200 MHz is observed in Joule heated samples. Hysteresis loops of annealed samples maintain a rectangular shape, while a slight decrease in coercivity from 93 A/m to 77 A/m, after treatment, is observed. On the other hand, a modification of MOKE hysteresis loops is observed upon Joule heating. Additionally, an improvement in DW dynamics after Joule heating is documented, achieving DW propagation velocities of up to 700 m/s. GMI ratio improvement along with the change in MOKE loops and DW dynamics improvement have been discussed considering magnetic anisotropy induced by Oersted magnetic fields in the surface layer during Joule heating and internal stress relaxation. A substantial GMI ratio improvement observed in Fe-rich Joule-heated microwires with a rectangular hysteresis loop and fast DW propagation, together with the fact that Fe is a more common and less expensive metal than Co, make them suitable for use in magnetic sensors.

Stochastic Magnetization Switching in Rapidly Solidified (Co0.94Fe0.06)72.5Si12.5B15 Amorphous Submicronic Wires.

Submicrometric magnetic amorphous wires are good candidates for future development of miniaturized sensors and magnetic logic applications. Here we report the results of an in-depth investigation of magnetization switching in rapidly solidified nearly zero magnetostrictive (Co0.94Fe0.06)72.5Si12.5B15 amorphous samples with diameters of the actual magnetic wires between 300 and 450 nm. All samples were found to be magnetically bistable, displaying characteristic rectangular hysteresis loops. This shows that magnetization reversal occurs through the depinning and subsequent propagation of a magnetic domain wall, whose velocity depends on the applied field and on the sample dimensions. The results of this study reveal stochastic nonlinear dependencies of both the magnetic switching field and the domain wall velocity on the sample diameter. The analysis of the potential causes, which include nonlinear residual stresses, fluctuations in wire dimensions (metal and glass), and competing magnetic anisotropies of different origins, show that a combination of all three factors could lead to the observed stochastic behavior. Calculated values of the switching field, which consider only changes in the wire dimensions, indicate that such influence alone cannot account for the strong nonlinearities. The results are important for the applications of such ultrathin cylindrical magnetic amorphous wires.

Magnetic anisotropy and reversal in epitaxial FeGa/IrMn bilayers with different orientations of exchange bias

Epitaxial FeGa/IrMn bilayers with exchange biases along the FeGa[100] and [110] directions are prepared on MgO(001) single crystal substrates by magnetron sputtering through controlling the orientation of the external field <i>in situ</i> applied during growth. The effect of the exchange bias orientation on the magnetic switching process and the magnetic switching field are studied. The X-ray <i>φ</i>-scan indicates that the FeGa layer is epitaxially grown with a 45° in-plane rotation on the MgO(001) substrate along the FeGa(001)[110] direction and the MgO(001)[100] direction. The measurements of the angular dependence of the ferromagnetic resonance field and the corresponding fitting to the Kittel equation show that the samples have a superposition of fourfold symmetric magnetocrystalline anisotropy <inline-formula><tex-math id="M4">\begin{document}$ {K}_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M4.png"/></alternatives></inline-formula>, unidirectional magnetic exchange bias anisotropy <inline-formula><tex-math id="M5">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M5.png"/></alternatives></inline-formula>, and uniaxial magnetic anisotropy <inline-formula><tex-math id="M6">\begin{document}$ {K}_{\mathrm{u}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M6.png"/></alternatives></inline-formula> with configuration of <inline-formula><tex-math id="M7">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[100\right] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M7.png"/></alternatives></inline-formula> or <inline-formula><tex-math id="M8">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[110\right] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M8.png"/></alternatives></inline-formula>. The combined longitudinal and transverse magneto-optical Kerr effect measurements show that sample with <inline-formula><tex-math id="M9">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[100\right] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M9.png"/></alternatives></inline-formula> exhibits square loops, asymmetrically shaped loops, and one-sided two-step loops in different external magnetic field directions. In contrast, the sample with <inline-formula><tex-math id="M10">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[110\right] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M10.png"/></alternatives></inline-formula> exhibits one-sided two-step and two-sided two-step loops as the magnetic field orientation changes. Because the <inline-formula><tex-math id="M11">\begin{document}$ {K}_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M11.png"/></alternatives></inline-formula> is superimposed by <inline-formula><tex-math id="M12">\begin{document}$ {K}_{\mathrm{u}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M12.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M13">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M13.png"/></alternatives></inline-formula>, the in-plane fourfold symmetry of the magnetic anisotropy energy is broken. The local minima are no longer strictly along the in-plane <inline-formula><tex-math id="M14">\begin{document}$ \left\langle{100}\right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M14.png"/></alternatives></inline-formula> directions, but make a deviation angle which depends on the relative orientation and strength of magnetic anisotropy. A model based on the domain wall nucleation and propagation is proposed with considering the different orientations of <inline-formula><tex-math id="M15">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M15.png"/></alternatives></inline-formula>, which can nicely explain the change of the magnetic switching route with the magnetic field orientation and fit the angular dependence of the magnetic switching fields, indicating a significant change of domain wall nucleation energy as the orientation of <inline-formula><tex-math id="M16">\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M16.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M16.png"/></alternatives></inline-formula> changes.

Vol4 num4-16 Magnetic properties and applications of glass-coated ferromagnetic microwires

The impact of post-processing on soft magnetic properties and the giant magnetoimpedance (GMI) effect of Fe- and Co-based glass-coated microwires is evaluated. A remarkable improvement of magnetic softness and GMI effect is observed in Fe-rich glass-coated microwires subjected to stress annealing. Frequency dependence of GMI ratio of stress-annealed Fe-rich microwires has been discussed considering frequency dependence of the skin penetration depth, δ, as well as magnetic anisotropy distribution within the metallic nucleus. Annealed and stress-annealed Co-rich microwires present rectangular hysteresis loop and single and fast domain wall propagation. However, Co-based stress-annealed microwires present high magnetoimpedance ratio. Observed stress-induced anisotropy and related changes of magnetic properties are discussed considering internal stresses relaxation and “back-stresses”.

Magnetic Vortex Domain Wall Observation on Polycrystalline Imperfect Iron−Cobalt Alloy Nanowires Growing on 1050 Aluminum

Herein, the magnetic vortex domain wall structure of polycrystalline imperfect iron−cobalt alloy nanowires (Nws) growing on 1050 aluminum by pulsed electrodeposition is reported. The magnetic properties are analyzed using magnetometry and off‐axis electron holography. The electrodeposited Nw arrays show homogeneous elemental composition and exhibit a structure composed of piled‐up grains of small crystallites. The saturation magnetization, coercive field, and reduced remanence, measured in directions parallel and perpendicular to the Nw's axis, are studied as a function of the temperature. Although the array of Nws on anodized aluminum grows both straight and within an inclination angle, in both cases, a high shape anisotropy is noticed, which is the most predominant contribution to the magnetic behavior. Misalignments and defects of the Nws in the array, as well as the wide distribution of lengths and diameters, do not contribute significantly to coercivity dispersion. A modified model is used to explain the changes in the magnetic behavior of the Nw's arrays, which accounts for structural imperfections and magnetostatic interactions. The reversal magnetization arising from the vortex domain wall propagation via localized curling is verified by off‐axis electron holography.

Single Diameter Modulation Effects on Ni Nanowire Array Magnetization Reversal.

Geometrically modulated magnetic nanowires are a simple yet efficient strategy to modify the magnetic domain wall propagation since a simple diameter modulation can achieve its pinning during the nanowire magnetization reversal. However, in dense systems of parallel nanowires, the stray fields arising at the diameter interface can interfere with the domain wall propagation in the neighboring nanowires. Therefore, the magnetic behavior of diameter-modulated nanowire arrays can be quite complex and depending on both short and long-range interaction fields, as well as the nanowire geometric dimensions. We applied the first-order reversal curve (FORC) method to bi-segmented Ni nanowire arrays varying the wide segment (45–65 nm diameter, 2.5–10.0 μm length). The FORC results indicate a magnetic behavior modification depending on its length/diameter aspect ratio. The distributions either exhibit a strong extension along the coercivity axis or a main distribution finishing by a fork feature, whereas the extension greatly reduces in amplitude. With the help of micromagnetic simulations, we propose that a low aspect ratio stabilizes pinned domain walls at the diameter modulation during the magnetization reversal. In this case, long-range axial interaction fields nucleate a domain wall at the nanowire extremities, while short-range ones could induce a nucleation at the diameter interface. However, regardless of the wide segment aspect ratio, the magnetization reversal is governed by the local radial stray fields of the modulation near null magnetization. Our findings demonstrate the capacity of distinguishing between complex magnetic behaviors involving convoluted interaction fields.