Under a longitudinal applied magnetic field, microwires increase/decrease length depending on the magnetostriction sign, where the magnetostrictive elongation is due to magnetization rotational processes as well known in classical magnetism. In amorphous alloys, magnetostriction is usually evaluated making use of inverse magnetoelastic effects (e.g., SAMR method) that sometimes is not fully reliable (e.g., for positive magnetostriction) [1]. 
This study in magnetostrictive and non-magnetostrictive amorphous microwires introduces two original aspects: i) Direct magnetostrictive elongation measurements as a function of applied field and tensile stress; and ii) Re-interpretation of the magnetization mechanism by comparing magnetostrictive elongation and magnetization curve. 
A home-made setup with suitable induction/pickup coils, a mechanical system to apply stress and a high-accuracy LVDT position sensor has been used to measure magnetization, M, and magnetostrictive elongation, δl/l,[2] in two amorphous microwire families: i) In-water-quenched (~145 μm diameter) highly magnetostrictive FeSiB, and vanishing positive/negative magnetostriction (CoFe)SiB microwires; and ii) Glass-coated FeSiBC microwire (20 μm diameter).
The figures show high-sensitivity (down to δl/l=10-8) representative data of δl/l vs. M/Mmax and H where the applied stress increases/reduces the elongation for negative/positive magnetostriction. Quantitative information on the rotational magnetization for low/high field regions is obtained: note in Fig. 1 the different elongation slopes for low (domain wall) and high (rotational) magnetization processes. It is also noticeable the magnetostrictive effect in Fig. 2 inset during domain wall motion in the tiny wire due to a helical magnetization component. This method to properly quantify the magnetostrictive elongation allows for a new magnetization/magnetostrictive analysis as required in mechanical sensor devices. 
**References:** [1] C. Gómez-Polo and M. Vazquez, J. Magn. Magn. Mater. 118, 86 (1993); K. Chichay et al., J. Appl. Phys. 116, 173904 (2014). 
[2] J. Moya et al., J. Magn. Magn. Mater. 476, 248 (2019). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/1dc47202896b5978b613db88b267b339.png) 
Fig.1 Magnetostrictive elongation (δl/l) vs. Magnetization (M/Mmax) for a range of applied stresses (σ) in water quenched microwires. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/2899920f693e4ffe650e3eedd4a9c203.png) 
Fig. 2 Magnetostrictive elongation (dl/l) vs. applied Field (H) in FeSiB glass-coated microwire. The inset shows the low-field magnetization and elongation loops.

Magnetization process evaluation and tensile stress effect in direct magnetostriction measurement of amorphous microwires

One of the most promising magnetic systems studied in the last years are the 3D nanostructures with curved geometry where novel and interesting magnetization textures and dynamics are involved [1-2]. One of these nanostructures, cylindrical nanowires, present multiple topologically non-trivial magnetization structures such as Bloch-point domain walls, faster and more stable as compared to those in planar nanowires, vortices, and skyrmion tubes [2-3]. 
This talk presents different ways to manipulate the magnetization configuration and the domain wall dynamics in cylindrical nanowires. The control and stabilization of domains and domain walls have been achieved in multi-segmented structures [4] by alternating the magnetic anisotropy or by imprinting notches along the nanostructures length. The domain wall dynamics, under the influence of magnetic field or current pulses, was investigated by XMCD-PEEM and micromagnetic simulations. The results indicate that large current densities induce domain wall nucleation while smaller currents move domain walls preferably against the current direction but also along it. The pinning of domain walls has been observed not only at geometrical constrictions but also induced by the Oersted field which creates chiral domain walls. 
**References:** [1] R. Streubel et al, J. Phys. D: Appl. Phys. 2012, 49, 363001. 
[2] A. Fernández-Pacheco et al., Nat Commun 2017, 8, 15756. 
[3] J.A. Fernandez-Roldan et al., Nanoscale 2018, 10, 5923. 
[4] C. Bran et al., ACS Nano 2020, 14, 12819–12827.

Nucleation and Motion of Domain Walls in Cylindrical Nanowires

Artificial spin ices (ASI) are arrays of nanoscale magnets, usually arranged in frustrated geometries. Among the ASI, the kagome geometry (AKSI) has been extensively studied and considered to explore a wide range of interesting Physics phenomena, including mimicking bulk spin ice materials, and acting as a metamaterial for applications in magnonics [1]. 
The goal of this work is to study the global and local spatial dependence of the coercivity of a Permalloy AKSI (20 nm thick elliptical magnetic islands). The magnetization curves were acquired by magneto-optic Kerr effect measurements with the laser spot adjusted for the required purpose (either global or local).
The angular dependence of coercivity exhibits a periodic behavior with a period of 60°, which reflects the symmetry of the underlying honeycomb pattern. The coercivity values vary from ~120 Oe to ~145 Oe, resulting in a peak-to-peak amplitude of ~25 Oe. The minima (maxima) occur when the applied field is perpendicular (parallel) to the magnet's long axis. This result agrees with the fact that the long axis direction being an easy axis of a single nanomagnet. A 20 nm permalloy continuous thin film with similar lateral dimensions as the AKSI lattice shows a nearly constant coercivity around only a few Oe. This indicates that the higher magnitude of the AKSI coercivity is due to the thin film patterning.
On the other hand, it is well known that small systems with long range order interactions exhibit pronounced size effects, intuitively understood from the fact that long range interactions are limited by the sample size [2]. By controlling the position of the laser spot to 2 um nominal diameter it was possible to scan the whole sample and determine a spatial map of the coercivity, where each point of the map corresponds to a distinct subregion of the lattice. The results clearly show that the magnetisation reversal properties are locally affected depending on how close to the edge the hysteresis loop is measured. A continuous decrease of the coercive fields close to the array edges reveals that surface effects are present in AKSI and, in principle, could be explained in the light of translation symmetry breaking in systems with long range order interactions. 
**References:** [1] S. Skjærvø, H. Christopher and R. Stamps, Nature Reviews, Vol. 2, p. 13 (2020). 
[2] K. Binder and P. C. Hohenberg, Phys. Rev. B, Vol. 6, p. 3461 (1972).

Surface effects and spatial distribution of magnetic properties in a Kagomé artificial spin ice

As three-dimensional nanomagnetism evolves, novel non-trivial magnetic textures emerge as appealing information carriers for recording and spintronics based on curved nanosystems, and particularly on Cylindrical Nanowires (NWs) [1,2]. One of the most fascinating candidates that is likely to reach the high velocities required for fast recording technologies is the Bloch Point (BP) domain wall (DW). Recently, theoretical evidence indicated that BPs in NWs could reach high velocities close to 2 km/s in the magnonic regime [3]. While the observation of the BP DW in cylindrical NWs is no longer recent [2], scarce numerical studies that combine both current and Oersted field have been published in NWs [4], despite first attempts to measure DW velocities are in progress [5]. In this work we evaluate the dynamics of the BP DW under both current directions in a Ni NW with 100 nm in diameter. We investigate two cases: i) pre-nucleated BP Wall, and ii) the BP DW originated from the transformation of a Vortex-Antivortex DW. The effects of both spin-polarized current and Oersted field are considered. We discuss in detail the role of the chirality of the BP in relation to the Oersted field, also reported previously in precursors of BPs [4]. We show that while the pre-nucleated DW with the same chirality as that of the Oersted field propagates always against the current direction, the BP originated either from the transformation of the BP with the opposite chirality or from the vortex-antivortex DW can either stop the propagation or propagate parallel to the current. We attribute this effect to the role played by the inertia of the BP in its dynamics. Finally, we provide values of the velocities achieved by the BP in the NW as a function of applied current in Fig. 1. We conclude that BPs with vanishing momentum propagate opposite to the current with velocities that may be suppressed by the Oersted field. Importantly for spintronic applications, both momentum and inertial mass play a major role in the dynamics of BPs that has not been envisaged up to know.

![](https://assets.underline.io/uploads/markdown_image/1/image/6ff528b21478a289ec02bc86a4e8eae4.jpg)

The average velocity of a head-to-head BPW driven by a spin-polarized current in a Ni nanowire, far from the ends of the nanowire, as a function of the current density.

Oersted-field- and current- induced Bloch Point domain wall dynamics in cylindrical Ni nanowires

Highly-magnetostrictive amorphous microwires show bistable behaviour, i.e. magnetization reverses by depinning/propagation of a single Domain Wall, DW, along the entire wire. This response makes these microwires an ideal system for fundamental studies on DWs dynamics [1,2]. On the other hand, the classical Matteucci effect, ME, consists of the induction of an emf voltage at the ends of the microwire under applied axial magnetic field [3], and requires a net azimuthal component of magnetization, Mφ. That emf is associated with the DW propagation and its amplitude is proportional to the DW speed, vDW, and the time derivative of Mφ, tailored by the torsion magnetoelastic anisotropy. emf has been measured in a bistable FeSiB microwire (12mm-diam, 10cm-long) under torsion (maximum of ξmax=57 πrad/m) and axial driving magnetic field, Hdr (square wave). The setup is designed to generate unidirectional motion of the DW that unpins from the same end irrespective of the Hdr direction [4], and its speed is quantified by the voltage induced in tiny pickup coils along the microwire. The clockwise/counterclockwise, CW/CCW, torsion dependences of emf and vDW are experimentally determined for parallel/antiparallel Hdr. Asymmetric behaviour is observed for both, the sense of x and the Hdr direction (Fig.1). The DW dynamics is analysed in terms of Landau-Lifschitz-Gilbert eq. considering a bell-like DW (Fig.2). The observed asymmetries are numerically interpreted to derive from the magnetochiral effect related to the DW chirality. The switching of the Hdr direction changes the Head-to-Head, H-H, or Tail-to-Tail, T-T, DW character and chirality (i.e., Mφ circulation). As a consequence of this magnetochiral effect, we proof that H-H and T-T DWs propagate at different velocities, vDW, as experimentally observed. Here we provide a first evidence of DWs magnetochiral effects in microwires that offers an opportunity for investigation of topological properties of spin-textures in microwires that has not been envisaged up to now.

![](https://assets.underline.io/uploads/markdown_image/1/image/3aa87066cea42d5386ea12ea8fba69a6.jpg)

CW, ξ+, and CCW, ξ-, torsion dependence of DW speed under Hdr = 232 A/m

![](https://assets.underline.io/uploads/markdown_image/1/image/83f8fc99b2ba0f6840b6466bd0d04e32.jpg)

Scheme of a longitudinal slice with a propagating bell-shaped DW. Magnetization in the domains makes a small angle with the microwires axis

Magnetochiral effect of single domain wall propagation in the Matteucci effect of magnetostrictive microwire

JOA: Amorphous and Nanocrystalline Soft Magnets I

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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MMM 2022

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MOA: Nanocomposite Magnetic Materials

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**Session Chair:** Yu Shiratsuchi, *Osaka University*

LPA: Structured Materials

poster

**Session chair:** Valentine Novosad, Argonne National Laboratory


HOH - Magnetization Dynamics in Soft Materials

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Manuel Vázquez

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Magnetization process evaluation and tensile stress effect in direct magnetostriction measurement of amorphous microwires

Nucleation and Motion of Domain Walls in Cylindrical Nanowires

Surface effects and spatial distribution of magnetic properties in a Kagomé artificial spin ice

Oersted-field- and current- induced Bloch Point domain wall dynamics in cylindrical Ni nanowires

Magnetochiral effect of single domain wall propagation in the Matteucci effect of magnetostrictive microwire

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