Domain Wall Propagation Research Articles

High velocity domain wall propagation using voltage controlled magnetic anisotropy

The use of voltage-controlled magnetic anisotropy (VCMA) via the creation of a sloped electric field has been hailed as an energy-efficient approach for domain wall (DW) propagation. However, this method suffers from a limitation of the nanowire length which the DW can propagate on. Here, we propose the use of multiplexed gate electrodes to propagate DWs on magnetic nanowires without having any length constraints. The multi-gate electrode configuration is demonstrated using micromagnetic simulations. This allows controllable voltages to be applied to neighboring gate electrodes, generating large strength of magnetic anisotropy gradients along the nanowire, and the results show that DW velocities higher than 300 m/s can be achieved. Analysis of the DW dynamics during propagation reveals that the tilt of the DW and the direction of slanted gate electrode greatly alters the steady state DW propagation. Our results show that chevron-shaped gate electrodes is an effective optimisation that leads to multi-DW propagation with high velocity. Moreover, a repeating series of high-medium-low magnetic anisotropy regions enables a deterministic VCMA-controlled high velocity DW propagation.

Fixed vortex domain wall propagation in FeNi/Cu multilayered nanowire arrays driven by reversible magnetization evolution

While studying domain wall (DW) propagation in magnetic nanowires (NWs) may pave the way for future research and technological applications in recording heads and novel sensors, no attention has been paid to the investigation of magnetic reversal modes in multilayered NWs using angular first-order reversal curve (AFORC) analysis. Here, the magnetization reversal process of uniform FeNi/Cu NW arrays with a diameter of 45 nm electrodeposited in the anodic aluminum oxide template is systematically studied by AFORC analysis for the field angle θ (0° ≤ θ ≤ 90°) and compared with the average magnetic behavior of reversal modes based on conventional hysteresis loop measurements. The FeNi segment aspect ratio is kept constant at about 5, whereas the Cu segment length (LCu) increases from 2.5 to 25 nm. AFORC coercivity increases continuously with increasing θ, indicating that the NWs reverse their magnetization by nucleation and propagation of vortex DW (VDW). At θ = 0°, the respective hysteresis loop coercivity and magnetostatic coupling between FeNi segments along the NW length are reduced by increasing LCu from 2.5 to 25 nm, resulting in an enhancement in the reversible fraction of NWs from 10% to 48%. However, the VDW reversal mode is not influenced by the increase in NW reversibility as a function of θ for the different LCu, which arises from constant properties of the FeNi segments. The AFORC analysis of the reversal mechanism is also found to be in agreement with recent angle-dependent anisotropic magnetoresistance measurements in single multilayered NWs.

Field and Current Controlled Domain Wall Propagation in Twisted Glass-Coated Magnetic Microwires

The torsion effect on the field and current driven magnetization reversal and the associated domain wall velocity in cylindrical amorphous and nanocrystalline glass-coated microwires is reported. Samples from three representative compositions have been investigated: (1) amorphous Fe77.5Si7.5B15 with positive magnetostriction, λ ≅ 25 × 10−6, (2) amorphous Co68.18Fe4.32Si12.5B15 with nearly zero negative magnetostriction, λ ≅ −1 × 10−7, and (3) nanocrystalline Fe73.5Si13.5B9Cu1Nb3 (FINEMET) with small positive magnetostriction, λ ≅ 2.1 × 10−6, all having the diameter of the metallic nucleus, d, of 20 µm and the glass coating thickness, tg, of 11 µm. The results are explained through a phenomenological interpretation of the effects of applied torque on the anisotropy axes within the microwires with different characteristics. Among all the complex mechanical deformations caused by the application of torque on magnetic microwire samples, the most important are the axial compression – for axial field-driven domain wall motion, and the circumferential tension – for electrical current/circumferential field-driven domain wall motion. The Co68.18Fe4.32Si12.5B15 microwire, annealed at 300 °C for 1 hour and twisted at 168 Rad/m exhibits the optimum characteristics, e.g. the lowest switching current (down to 9 mA~2.9 × 10−3 A/cm2) and the largest domain wall velocity (up to 2300 m/s).

Modeling magnetic-field-induced domain wall propagation in modulated-diameter cylindrical nanowires

Domain wall propagation in modulated-diameter cylindrical nanowires is a key phenomenon to be studied with a view to designing three-dimensional magnetic memory devices. This paper presents a theoretical study of transverse domain wall behavior under the influence of a magnetic field within a cylindrical nanowire with diameter modulations. In particular, domain wall pinning close to the diameter modulation was quantified, both numerically, using finite element micromagnetic simulations, and analytically. Qualitative analytical model for gently sloping modulations resulted in a simple scaling law which may be useful to guide nanowire design when analyzing experiments. It shows that the domain wall depinning field value is proportional to the modulation slope.

Barkhausen Noise from Precessional Domain Wall Motion.

The jerky dynamics of domain walls driven by applied magnetic fields in disordered ferromagnets-the Barkhausen effect-is a paradigmatic example of crackling noise. We study Barkhausen noise in disordered Pt/Co/Pt thin films due to precessional motion of domain walls using full micromagnetic simulations, allowing for a detailed description of the domain wall internal structure. In this regime the domain walls contain topological defects known as Bloch lines which repeatedly nucleate, propagate, and annihilate within the domain wall during the Barkhausen jumps. In addition to bursts of domain wall propagation, the in-plane Bloch line dynamics within the domain wall exhibits crackling noise and constitutes the majority of the overall spin rotation activity.

Dynamics of magnetoelectric reversal of an antiferromagnetic domain

When electric and magnetic fields are applied together on a magnetoelectric antiferromagnet, the domain state is subject to reversal. Although the initial and final conditions are saturated single-domain states, the process of reversal may decompose into local multi-domain switching events. In thin films of Cr2O3, the magnetoelectric coercivity and the switching speed found from experiments are considerably lower than expected from magnetic anisotropy, similar to Brown's paradox in ferromagnetic materials. Multi-domain effects originate because antiferromagnetic domain walls are metastably pinned by lattice defects, not due to reduction of magnetostatic energy, which is negligible. This paper theoretically analyzes domain reversal in thin-film magnetoelectric antiferromagnets in the form of nucleation, domain wall propagation, and coherent rotation. The timescales of reversal mechanisms are modeled as a function of applied magnetoelectric pressure. The theory is assessed with reference to latest experimental works on magnetoelectric switching of thin-film Cr2O3: domain wall propagation is found to be dominant and responsible for switching in the experiments. The results bear implications in the energy-delay performance of ME memory devices utilizing antiferromagnetic insulators, which are prospective for nonvolatile technology.

Magnetic properties of “thick” glass-coated Fe-rich microwires

We report on preparation and magnetic properties of Fe71.7B13.4Si11Nb3Ni0.9 glass-coated microwire with metallic nucleus diameter d = 103 μm and total diameter D = 158 μm prepared by Taylor-Ulitovsky method. Amorphous structure of as-prepared microwires is confirmed by X-ray diffraction. As-prepared glass-coated microwires present relatively high GMI effect (about 50%) and relatively low coercivity (about 25 A/m). Additionally, as-prepared sample present rectangular hysteresis loop and fast single domain wall propagation with domain wall mobility of about 11,9 m2/As. After annealing we observed considerable improvement of the GMI ratio (from 50% up to 100%). Observed GMI effect improvement has been attributed to the stresses relaxation. From aforementioned studies we can conclude that the Taylor-Ulitovsky technique allows us to obtain thick ferromagnetic microwires with good magnetic properties and GMI effect suitable for industrial applications.

Effect of glass-removal on the magnetostriction and magnetic switching properties in amorphous FeSiB microwires

Abstract Results on the influence of glass-removal on the magnetostriction, magnetic switching properties and domain wall propagation in amorphous Fe77.5Si17.5B15 microwires are reported. Glass removal releases high thermoelastic stresses originated by the difference in thermal expansion coefficients of glass and metal, which reduces the coercivity, increases the domain wall mobility and decreases the local nucleation fields and their distributions. Evening the distribution of nucleation fields and eliminating the local surface defects due to glass removal increase the field range of single domain wall propagation regime. It was also found that the saturation magnetostriction coefficient significantly decreases after glass removal, which strengthens the impact of the reduced magnetoelastic anisotropy. The effect of glass removal on magnetostriction was previously observed only in Co-rich microwires with small negative magnetostriction. Understanding the factors that control the magnetic switching and domain wall dynamics is of interest for practical applications in sensing, storage and logic devices.

Realisation of a frustrated 3D magnetic nanowire lattice

Patterning nanomagnets in three-dimensions presents a new paradigm in condensed matter physics and allows access to a plethora of phenomena including robust spin textures, magnetic metamaterials home to defects carrying magnetic charge and ultrahigh density devices that store information in three-dimensions. However, the nanostructuring of functional magnetic materials into complex three-dimensional geometries has thus far proven to be a formidable challenge. Here we show magnetic nanowires can be arranged into 3D frustrated magnetic nanowire lattices by using a combination of 3D polymer nanoprinting and metallic deposition. The fabricated nanowires are single domain and they switch via nucleation and propagation of domain walls. Deep nanoscale magnetic imaging and finite element simulations elucidate the spin texture present on the 3D nanostructured lattice. Our study demonstrates a generic platform for the production of 3D nanostructured magnetic materials allowing the realisation of racetrack memory devices and 3D nanostructured systems that mimic bulk frustrated crystals.

Suppressing the CeFe2 phase formation and improving the coercivity and thermal stability of Ce-Fe-B alloys by Si substitution

Low anisotropy field and low Curie temperature of Ce2Fe14B phase seriously restrict the application of Ce-Fe-B alloys. Most of the previous work suggest that REFe2 phase inevitably forms in the compositions with high Ce content. In this work, Si substitution for Fe is found to not only suppress the formation of CeFe2 phase, but also significantly improve the coercivity Hc and thermal stability of Ce17Fe78-xSixB6 (x = 0–3.0) alloys. All Si-doped alloys exhibit higher Hc and better thermal stability than Ce17Fe78B6 alloy. With increasing Si doping, a relatively high Hc = 525 kA/m is obtained in the Ce17Fe77.75Si0.25B6 alloy. An abnormal increase of Hc as well as almost unchanged Jr and (BH)max is observed in Ce17Fe78-xSixB6 (x > 1.5) alloys. The refined XRD results indicate that the volume fraction of CeFe2 phase can be dramatically reduced via a moderate content of Si doping. Hard magnetic grains are isolated by forming a structure with thin and continuous intermediate layer, consequently preventing the domain wall propagation. The Curie temperature of the main phase monotonically increases from 424 K for x = 0–445 K for x = 3.0, resulting in improved thermal stability. The highest Hc of 530 kA/m, the lowest temperature coefficients (|β| = 0.551%/°C and |α| = 0.368%/°C), and an acceptable Jr = 0.44 T are obtained in Ce17Fe78-xSixB6 (x = 3.0) alloy. The nucleation and pinning are both responsible for the coercivity mechanism of Ce17Fe75Si3B6 alloy, which should be the reason for its high coercivity.

Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction

Leveraging on interfacial Dzyaloshinskii-Moriya interaction (DMI) induced intrinsic magnetization tilting in nanostructures, a parametric window enabling field-free spin-orbit torque (SOT) magnetization switching in a perpendicular ferromagnet is established. The critical current density (Jc) bounds for SOT switching are highly dependent on the DMI, producing a distorted diamond-shaped region bounded by the Jc-DMI curves. The widest Jc interval is found for DMI values between 0.5 mJ/m2 and 0.8 mJ/m2. Geometrical modulation, of the ferromagnetic layer, reveals that the circular structure is optimum for minimizing the switching energy while maximizing the parametric window. For all the structures investigated, the SOT induced reversal process is via domain wall nucleation and propagation, and the switching is practical at room temperature.

Ultra-Dense Ring-Shaped Racetrack Memory Cache Design

Information storage and transfer via current-induced domain wall (DW) motions exhibit significant density-speed-energy advantages, which inspires numerous emerging devices and circuits, such as racetrack memory (RM). However, the bi-directional propagation of DWs in the conventional tape-shaped nanowire will lead to data overflow issue, implicitly deteriorating storage density and operational performances. In this paper, we propose a non-volatile cache design based on spin-orbit torque-driven ring-shaped RM. The systematical investigations, covering from device modeling, to circuits design, to bit-cell layout design, and to system evaluation have been carried out. Thanks to the cells-overlapping design, the proposed RM L2 cache can achieve $48\times $ , $16\times $ , and $8\times $ improvements in term of capacity, compared with iso-area caches based on static random access memory (SRAM), spin transfer torque magnetic RAM (STT-MRAM), and tape-shaped RM, respectively. As proved by 4-core system experiment results, the proposed RM cache can improve 30.7% instructions per cycle (IPC) and save 58.2% energy compared with SRAM cache.

Exchange-spring behavior below the exchange length in hard-soft bilayers in multidomain configurations

Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enable tailoring the magnetization reversal process. Here, exchange-spring behavior is observed in $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$/FeCo bilayers for soft thicknesses as thin as 2 nm, at least four times below the exchange length of the system. This result is in contrast with the accepted theory for spring magnets that states that the exchange length defines the critical thickness below which both magnetic phases should be rigidly coupled. In combination with micromagnetic calculations, this surprising observation is understood as a consequence of the dominance of domain-wall propagation in the soft phase during the reversal process, so far unaccounted for in theoretical descriptions. Our results emphasize the need to expand the existing spring theory from coherent rotation to domain-wall related processes in multidomain configurations in order to accurately design magnetic heterostructures with controllable reversal.

Control of magneto-static and -dynamic properties by stress tuning in Fe-Si-B amorphous microwires with fixed dimensions

Abstract The factors, which influence the internal stresses in glass coated amorphous microwires were discussed in this work in relation to the magnetization behaviour and domain wall propagation. The approach taking into account the technological parameters of manufacturing by Ulitovsky-Taylor method is presented. The magnetization process and domain wall dynamics in Fe77.5Si7.5B15 microwires produced under different conditions but with identical geometric parameters were investigated experimentally and analysed on the basis of internal stress formation in different microwire states: as prepared, after glass removal and after annealing. It is demonstrated that the residual stresses in wires of the same composition are not defined exclusively by their dimensions but depend on the technological regime. The stress modification due to relaxing treatments also proceeds differently in wires produced under different conditions. These conclusions are evidenced by comparing the domain wall mobilities which strongly depend on the internal stress.

Real-time switching dynamics of ferroelectric tunnel junctions under single-shot voltage pulses

In ferroelectric memory devices, information is stored within the polarization direction whose reversal usually occurs by the nucleation and propagation of domains. In ultrathin ferroelectrics, ultrafast dynamics may be achieved by nucleation-limited switching, avoiding the inherently speed-limited propagation of domain walls. Here, we investigate polarization reversal dynamics in ultrathin ferroelectric films by transient current measurements. Thanks to the tunnel electroresistance, the start of polarization reversal induces sharp variations of the transmitted current under voltage pulses. These single-shot measurements show extremely fast switching with durations down to 3 ns that is only limited by the current device geometry. While the OFF-to-ON switching shows finite nucleation times that scale with the pulse amplitude, the ON-to-OFF switching speed cannot be detected under such rectangular pulses. Resorting to triangular pulse excitations allows us to detect the dynamics of this switching direction. Both cases can be interpreted by nucleation switching models following Merz's law.

Bidirectional propagation of tilting domain walls in perpendicularly magnetized T shaped structure with the interfacial Dzyaloshinskii-Moriya interaction

Understanding of domain wall (DW) propagation in a complex structure is an essential first step toward the development of any magnetic-domain based devices including spin-based logic or magnetic memristors. Interfacial Dzyaloshinskii-Moriya interaction (iDMI) in the structure with broken inversion symmetry induces an asymmetrical DW configuration with respect to the direction of in-plane field. Dynamic behaviors of field-driven DW within the film with perpendicular magnetic anisotropy is influenced by DW tilt from the iDMI effect and the corners in the T-shaped structure of the DW path. Images from Kerr microscopy reveal that the iDMI effective field contributes to a tilted structure of DW configuration and evolution along its propagation. With the combination of iDMI and T-shaped structure, we observed two distinguished bidirectional DW propagations in two output branches and distinct arriving times at the destination pads with a uniform external field. Micromagnetic simulation results is compared with the observed dynamics of a DW configuration in the structure providing an additional confirmation of the interpreted results.

Antiferromagnetic domain wall creep driven by magnetoelectric effect

We observed the magnetoelectric induced domain wall propagation in a Pt/Co/Au/Cr2O3/Pt stacked thin film based on magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. The antiferromagnetic (Cr2O3) domain wall velocity was estimated by a quasi-static approach using a pulsed voltage. At a pulse voltage amplitude of −12 V, corresponding to an electric field of −8.0 × 102 kV/cm, the domain wall velocity was very low, at 0.3 m/s. The domain wall velocity increased with increasing voltage amplitude, reaching 22 m/s at −20 V (−1.3 × 103 kV/cm). The change in the domain wall velocity with the applied voltage amplitude indicates the creep motion of the domain wall. Using a phenomenological model, we estimated the domain wall depinning energy, and found that the bulk and interface terms of the magnetic anisotropy affect the effective magnetic field to the same degree, suggesting that the magnetic domain wall motion may be controllable by the antiferromagnetic layer thickness.

Stress‐Induced Domain Wall Motion in FeCo‐Based Magnetic Microwires for Realization of Energy Harvesting

AbstractEnergy harvesting is getting significant interest due to the emergence of Internet of Things (IoT). As IoT devices are required to generate power on their own, several researchers are studying ferroelectric materials for voltage generation. However, ferroelectric materials suffer from high resistance at low frequencies, which reduces the output power. Domain wall propagation in ferromagnetic materials, as induced by stress and a pickup voltage using coils, is investigated as an alternate form of energy harvesting. Such studies, which are reported in multiferroic structures, feature a voltage applied to induce the stress, which defeats the purpose of self‐power generation. Here, power generation from mechanical vibrations in purely ferromagnetic structures is shown. These results are achieved by showing that the domain walls can be moved entirely by stress in a trilayer stack of ferromagnetic microwires. The use of flexible substrates with low Young's modulus and a trilayer magnetic stack enables the achievement of significant magnetization rotation or domain wall motion even from ambient vibrations. Here, the rotation of magnetization or domain wall motion is exploited to induce voltages in the pickup coils. The results shown here provide an alternative way to power IoT devices.

Field operating window of nanodevices employing the domain wall propagation through the stripes intersection: Numerical optimization

In this paper, we present the detailed numerical study of domain wall (DW) dynamics in thin magnetic stripes and in nanostructures containing intersections of such stripes. We focus on the DW propagation through these intersections (crosses), because these are the key elements in field-based multiturn sensors and magnetic logic devices. The DW propagation through such intersections is the main problem by ensuring the functional reliability of the abovementioned devices due to (i) the strong pinning of a DW at these crosses and (ii) the risk that a wrong cross “shoulder” is reversed by the DW propagation. By means of micromagnetic simulations, the operating field range for the desired DW propagation in devices with a constant stripe width is determined, and the drawbacks of this simple design are discussed. To overcome these drawbacks, we suggest a new design employing the stripes with a modulated width. We show that sensors with this design are expected to have a higher reliability and that their field operating window can be controlled in a more flexible way.

Domain wall nucleation and propagation in spin-transfer torque switching with thermal effects

We conduct micro-magnetic simulations to study spin-transfer torque induced magnetization switching in perpendicular magnetic tunneling junctions. The effects of current densities and temperatures on the switching processes are studied in details. We then proposed an approach to compute the deterministic switching time by taking thermal-effect into account. The switching time is less temperature-dependent under higher current density; however, as the current density decreases, the effect of temperature on the switching time becomes more and more significant. The switching process with micro-magnetic simulations is shown to be via domain wall nucleation and propagation. The phenomena are consistent with the recent experimental found-out. We further propose a method to compute the switching time based on domain wall nucleation and propagation theory, and compare the switching time with those from macro-spin approximation. It is found the switching times from the micro-magnetic simulations are much shorter than that from the macro-spin approximations. Macro-spin approximation over-estimates the switching times due to its coherent rotation assumptions.