Control Of Domain Wall Motion Research Articles

Magnetic domain wall motion driven by spin-orbit torque based on non-collinear spin source

<p indent="0mm">Recently, the current-induced motion of magnetic domain walls has attracted much attention. It enables electrical control of domain wall motion without the application of an external magnetic field and thus has potential applications in high-performance magnetic racetrack memories, among others. The effective control of domain wall motion under zero magnetic field is one of the important topics in spintronics research. Current-driven magnetization switching via the spin-orbit torque effect is an important approach to realizing the field-free domain wall motion. The traditional research structure of spin-orbit torques is non-magnetic material/magnetic material (NM/FM) heterogeneous films, in which non-magnetic materials are used as the spin source layer to generate and provide spin current through charge-to-spin current conversion. Common non-magnetic materials mainly include heavy metals, such as Pt, β-Ta and β-W. However, in the system with heavy metals as spin sources, the conventional spin Hall effect requires the charge current, spin current and spin polarization directions to be perpendicular to each other, so it is necessary to apply an in-plane auxiliary magnetic field to break the symmetry, especially when driving the perpendicular magnetic moment switching, which increases extra power consumption and is not conducive to practical applications. Recently, it has been found that the spin current generated by the charge current in the outer semimetal WTe₂ is spin polarized in the <bold><italic>z</italic></bold>-direction. It has a unique in-plane symmetry breaking property, where the damping-like torque can contribute more to the domain wall motion of a ferromagnetic layer with perpendicular magnetic anisotropy. Moreover, a unique Rashba-like spin polarization has recently emerged in the noncollinear magnetic structure Mn₃GaN. A charge current is applied in the <bold><italic>x</italic></bold>-direction to generate a spin current in the <bold><italic>z</italic></bold>-direction, with spin polarization along the <bold><italic>x</italic></bold>-direction. The spin polarization along the <bold><italic>x</italic></bold>-direction produces damping-like torque (<bold><italic>τ</italic></bold>_{<italic>x</italic>}_,DL) and field-like torque (<bold><italic>τ</italic></bold>_{<italic>x</italic>}_,FL) in the adjacent in-plane and PMA ferromagnetic layers. The magnetization dynamics of the SOT mentioned above may be quite different. To realize the effectively current-induced spin-orbit torque driven domain wall motion, it is important to understand the spin source with unique physical properties of charge-to-spin conversion. In this work, we study the spin-orbit torque driven domain wall motion by micromagnetic simulations. We consider the mixture of non-collinear spin torques of Rashba-like <bold><italic>S</italic></bold><italic>_y</italic>, Dresselhaus-like <bold><italic>S</italic></bold><italic>_x</italic>, and out-of-plane <bold><italic>S</italic></bold><italic>_z</italic>. The effects of three different domain wall types: Bloch domain wall, Néel domain wall and head-to-head domain wall on motion velocity and the inclination angles of magnetic domain walls are demonstrated. The results for the same magnetic domain show that the current-induced spin polarization perpendicular to the membrane surface generates a damping-like torque perpendicular to the membrane surface, which can efficiently drive the motion of the domain wall. However, in the presence of multiple spin torques, the unconventional damping-like torques in the direction perpendicular to the film surface can be realized to drive domain wall motion at zero field. Our results may provide a new degree of freedom for understanding current-induced spin-orbit torque-driven domain wall motion from non-collinear spin sources for potential applications in spintronic devices.

Advances in Magnetic Domain Walls and Their Applications

This article explores the recent developments in spin-based domain wall (DW) memories. The physics behind the DW motion, device materials, current challenges, and applications have been discussed in detail. DWs can propagate through a magnetic nanowire by the application of external magnetic fields or by spin-polarized electric currents. Great progress has been made in these devices since the introduction of electric current-induced DW motion. However, driving DWs necessitates large spin-current densities that are incompatible with low-power devices. Therefore, significant efforts have been made to achieve highly efficient and controlled DW motion by material engineering and different mechanisms such as spin-orbit-torque (SOT), Dzyaloshinskii-Moriya interaction (DMI), and voltage-controlled magnetic anisotropy. The controlled manipulation of DWs in magnetic materials has inspired numerous strategies for high-density memory and energy-efficient logic implementation. Moreover, these devices are the potential candidates for neuromorphic computing applications that can be combined with logic-in-memory. The displacement of the DW can achieve multiple resistance states that have opened up possibilities of building artificial neurons and synapses. Despite the rapid progress, DW devices face several challenges such as low read margin, low speed, and sub-20nm scalability. Future research directions have to focus on material engineering and fabrication techniques to address these issues. Simultaneously, efforts from the circuit and system perspectives are extensively required in exploring the possible uses of these devices.

On Modelling Approaches to the Influence of Mechanical Deformation on the Micromagnetic Activity in a Mild Steel

In this study an attempt is made to develop a theoretical modelling by which the influence of certain mechanical deformation factors on the micromagnetic emission behavior of a low-carbon steel can reasonably be described and estimated. This modelling consists of a simple kinetics – kinematics – aided approach of the pinning state – controlled domain wall motion by which appropriate specific parameters are introduced. In this aspect the basic notion of specific micromagnetic activity (s.m.a.) is introduced by which the energetic strength of the activity is reflected. In this way, the synergetic effect of the quantitative (count rate) and qualitative (voltage) the detected micromagnetic Barkhausen emission (MBE) is taken into consideration. Thus it is possible, theoretically, to give a prediction of the general trend of changes in the s.m.a. under the influence of the tensile elastic as well as plastic deformation. For instance, one can demonstrate that tensile elastic deformation cannot influence the s.m.a. whereas plastic one leads to an increase in this. Furthermore, one can also predict that increasing permanent (residual) plastic deformation, obtained after unloading from prior tensile loading, leads to an obvious decrease in the s.m.a. Similar decrease in the s.m.a. can also be predicted for increasing rolling deformation by means of the same modelling approach used for the permanent tensile plastic deformation. Owing to the good agreement with the experimental results and the simplicity of the proposed theoretical approaches that can be seen as a promising valuable tool for further similar studies.

Understanding curvature effects on the magnetization reversal of patterned permalloy Archimedean spirals

Geometric curvature in magnetic systems can induce several unique magnetic interactions, the most notable of which are the curvature induced magnetic anisotropy and the curvature introduced Dzyaloshinski–Moria (DM) like interaction. Of the two, the DM-like interaction is particularly interesting as it provides a unique way to control domain wall motion, which is particularly relevant to the field of spintronics. In this work, we study the effect of geometric curvature on magnetization reversal in Permalloy strips patterned into Archimedean spirals with varying widths. We simulated the magnetization reversal of Permalloy strips for several widths ranging from microscale to nanoscale to determine a lengthscale at which the curvature effects are strong. Simulations showed that in the microscale spirals, magnetization reversal primarily occurs through the appearance of magnetization ripple in which domains with reversed magnetization nucleate. On the other hand, the nanoscale spirals showed that reversal primarily proceeded through the motion of domain walls into the arms of the spiral. Our experimental in situ Lorentz transmission electron microscopy data of the microscale spirals, however, showed that magnetization reversal occurred with both mechanisms. At the nanoscale, the effect of local variation of curvature leads to hysteresis loops with stepped behavior. This behavior is characteristic of the curvature induced DM-like interaction affecting domain wall motion in the arms of the nanoscale spirals.

Electron Induced Massive Dynamics of Magnetic Domain Walls.

We study the dynamics of domain walls (DWs) in a metallic, ferromagnetic nanowire, focusing on inertial effects on the DW due to interaction with a conduction electron bath. We develop a Keldysh collective coordinate technique to describe the effect of conduction electrons on rigid magnetic structures. The effective Lagrangian and Langevin equations of motion for a DW are derived microscopically, including the full response kernel which is nonlocal in time. The DW dynamics is described by two collective degrees of freedom: position and tilt-angle. The coupled Langevin equations therefore involve two correlated noise sources, leading to a generalized fluctuation-dissipation theorem (FDT). The DW response kernel due to electrons contains two parts: one related to dissipation via FDT, and another reactive part. We prove that the latter term leads to a mass for both degrees of freedom, even though the intrinsic bare mass is zero. The electron-induced mass is present even in a clean system without pinning or specifically engineered potentials. The resulting equations of motion contain rich dynamical solutions and point toward a way to control domain wall motion in metals via the electronic system properties. We discuss two observable consequences of the mass, hysteresis in the DW dynamics and resonant response to ac current.

Controlled spin-torque driven domain wall motion using staggered magnetic wires

Domain wall (DW) memory devices such as racetrack memory offer an alternative to the hard disk drive in achieving high capacity storage. In DW memory, the control of domain wall positions and their motion using spin-transfer torque is an important challenge. In this Letter, we demonstrate controlled domain wall motion using spin-transfer torque in staggered wires. The devices, fabricated using electron-beam and laser lithography, were tested using magneto-optical Kerr microscopy and electrical transport measurements. The depinning current is found to depend on the device dimensions of the staggering wires. Thus, the proposed staggering configuration can be utilized to fine-tune the properties of DW devices for memory applications.

Magnetic properties of perpendicularly magnetized [Au/Co/Pd]n thin films and nanostructures with Dzyaloshinskii-Moriya interaction

Magnetic force microscopy imaging enable to extract the main magnetic parameters (saturation magnetization, anisotropy, domain size, exchange constant). The magnetoresistance and the extraordinary Hall effect of the material have been characterized using nanostructures patterned on the multilayer. Both the magnetization reversal mode and the magnetotransport properties are found to depend strongly on the nanowire width. The relatively low coercive field, the high anisotropy, and the possibility to control domain wall motion in sub-100 nm wires make this system an interesting alternative to Pt-based multilayers for spin-transfer torques experiments.

Multilevel storage device based on domain-wall motion in a magnetic tunnel junction

We report on a multilevel storage device based on a magnetic tunnel junction (MTJ). Six different resistance states have been observed by controlling domain wall motion in the free layer of a MTJ. It is realized by pinning the domain wall at different positions in the free layer with a special geometric structure. The resistance states can be modulated with the application of an external magnetic field or a d.c. The experimental results are well explained by micromagnetic simulation. The results suggest that our design is expected to have applications in magnetic memory and neuromorphic systems.

Progress of electrical control magnetization reversal and domain wall motion

Electrical control of spins in magnetic materials and devices is one of the most important research topics in spintronics. We briefly describe the recent progress of electrical manipulations of magnetization reversal and domain wall motion.This review consists of three parts:basic concepts,magnetization manipulation by electrical current and voltage methods,and the future prospects of the field.The basic concepts,including the generation of the spin current,the interaction between the spin current and localized magnetization,and the magnetic dynamic Landau-Lifshitz-Gilbert-Slonczewski equation are introduced first.In the second part,we reviewed the progress of the magnetization controlled by electrical current and voltage. Firstly we review the electrical current control of the magnetization and domain wall motion.Three widely used structures, single-layer magnets,ferromagnet/heavy metal and ferromagnet/nonmagnetic metal/ferromagnet,are reviewed when current is used to induce magnetization reversal or drive domain wall motion.In a single-layer magnetic material structure,domain wall can be effectively driven by electrical current through spin transfer torque.The factors influencing the domain wall trapping and motion are also discussed.The electrical current control of the skyrmions has big potential applications due to much lower current density.Using the Dresselhaus and Rashba spin orbital coupling,the electrical current can also directly reverse the magnetization of single magnetic or antiferromagnetic layer.Then,we review the electrical current switching the magnetization of the ferromagnetic layer in ferromagnetic/heavy metal structures,where both spin Hall effect and Rashba effect can contribute to the current switching magnetization in such device structures. To identify the relative contributions of these two mechanisms,several quantitative studies are carried,concluding that spin Hall effect plays a major role,which is summarized in this review.Finally,we review the current switching magnetization of free layers in spin valve and magnetic tunnel junctions (MTJs) by spin transfer torque.We also discuss the approaches to the decrease of the critical current density in MTJs,which is desired for future applications.Alternatively,the electric field can also be used to manipulate the magnetization,where three methods are reviewed. Applying an electric field to the ferromagnetic/piezoelectric heterostructures,which changes the crystal structure of magnetic film through piezoelectric effects,realizes the change of the magnetic anisotropy of the ferromagnetic layer.In ferromagnetic/ferroelectric heterostructures,electric field changes the spin distribution and orbital hybridization at the surface of magnetic film through the magnet-electric coupling effects,and then controls the magnetization of the ferromagnetic layer.In ferromagnetic metal (semiconductor)/dielectric/metal structure,electric field controls the electron accumulation or depletion at the surface of the ferromagnetic metal or semiconductor,the change of the electron density in the magnetic layer in turn affects the magnetic exchange interaction and magnetic anisotropy.Finally,we present the prospects for the development of electrical control magnetization reversal and domain wall motion for future applications.

Manipulating antiferromagnets with magnetic fields: Ratchet motion of multiple domain walls induced by asymmetric field pulses

Future applications of antiferromagnets (AFs) in many spintronics devices rely on the precise manipulation of domain walls. The conventional approach using static magnetic fields is inefficient due to the low susceptibility of AFs. Recently proposed electrical manipulation with spin-orbit torques is restricted to metals with a specific crystal structure. Here, we propose an alternative, broadly applicable approach: using asymmetric magnetic field pulses to induce controlled ratchet motion of AF domain walls. The efficiency of this approach is based on three peculiarities of AF dynamics. First, a time-dependent magnetic field couples with an AF order parameter stronger than a static magnetic field, which leads to higher mobility of the domain walls. Second, the rate of change of the magnetic field couples with the spatial variation of the AF order parameter inside the domain, and this enables a synchronous motion of multiple domain walls with the same structure. Third, tailored asymmetric field pulses in combination with static friction can prevent backward motion of domain walls and thus lead to the desired controlled ratchet effect. The proposed use of an external field, rather than internal spin-orbit torques, avoids any restrictions on size, conductivity, and crystal structure of the AF material. We believe that our approach paves a way for the development of AF-based devices based on the controlled motion of AF domain walls.

Modulated Magnetic Nanowires for Controlling Domain Wall Motion: Toward 3D Magnetic Memories.

Cylindrical magnetic nanowires are attractive materials for next generation data storage devices owing to the theoretically achievable high domain wall velocity and their efficient fabrication in highly dense arrays. In order to obtain control over domain wall motion, reliable and well-defined pinning sites are required. Here, we show that modulated nanowires consisting of alternating nickel and cobalt sections facilitate efficient domain wall pinning at the interfaces of those sections. By combining electron holography with micromagnetic simulations, the pinning effect can be explained by the interaction of the stray fields generated at the interface and the domain wall. Utilizing a modified differential phase contrast imaging, we visualized the pinned domain wall with a high resolution, revealing its three-dimensional vortex structure with the previously predicted Bloch point at its center. These findings suggest the potential of modulated nanowires for the development of high-density, three-dimensional data storage devices.

Domain wall depinning from notches using combined in- and out-of-plane magnetic fields

Controlled domain wall motion and pinning in nanowires with perpendicular magnetic anisotropy are of great importance in modern magnetic memory and logic devices. Here, we investigate by experiment the DW pinning and depinning from a notch in a magnetic nanowire, under the influence of combined in- and out-of-plane magnetic fields. In our experiment, the perpendicular magnetization of the Co/Pt nanowires is tilted with the help of sub-μs in-plane field pulses generated by an on-chip coil. Consequently, the energy density of the DW is decreased and the depinning field of the notch is reduced. A theoretical model is applied and compared to the measurement results. The DW depinning mechanism and the DW type are further investigated by micromagnetic simulations.

Dzyaloshinskii-Moriya domain walls in magnetic nanotubes

We present an analytic study of domain-wall statics and dynamics in ferromagnetic nanotubes with spin-orbit-induced Dzyaloshinskii-Moriya interaction (DMI). Even at the level of statics, dramatic effects arise from the interplay of space curvature and DMI: the domains become chirally twisted leading to more compact domain walls. The dynamics of these chiral structures exhibits several interesting features. Under weak applied currents, they propagate without distortion. The dynamical response is further enriched by the application of an external magnetic field: the domain wall velocity becomes chirality-dependent and can be significantly increased by varying the DMI. These characteristics allow for enhanced control of domain wall motion in nanotubes with DMI, increasing their potential as information carriers in future logic and storage devices.

Controlling domain wall motion in ferroelectric thin films.

Domain walls in ferroic materials have attracted significant interest in recent years, in particular because of the unique properties that can be found in their vicinity. However, to fully harness their potential as nanoscale functional entities, it is essential to achieve reliable and precise control of their nucleation, location, number and velocity. Here, using piezoresponse force microscopy, we show the control and manipulation of domain walls in ferroelectric thin films of Pb(Zr,Ti)O₃ with Pt top electrodes. This high-level control presents an excellent opportunity to demonstrate the versatility and flexibility of ferroelectric domain walls. Their position can be controlled by the tuning of voltage pulses, and multiple domain walls can be nucleated and handled in a reproducible fashion. The system is accurately described by analogy to the classical Stefan problem, which has been used previously to describe many diverse systems and is here applied to electric circuits. This study is a step towards the realization of domain wall nanoelectronics utilizing ferroelectric thin films.

In situ atom scale visualization of domain wall dynamics in VO2 insulator-metal phase transition.

A domain wall, as a device, can bring about a revolution in developing manipulation of semiconductor heterostructures devices at the atom scale. However, it is a challenge for these new devices to control domain wall motion through insulator-metal transition of correlated-electron materials. To fully understand and harness this motion, it requires visualization of domain wall dynamics in real space. Here, domain wall dynamics in VO2 insulator-metal phase transition was observed directly by in situ TEM at atom scale. Experimental results depict atom scale evolution of domain morphologies and domain wall exact positions in (202) and (040) planes referring to rutile structure at 50°C. In addition, microscopic mechanism of domain wall dynamics and accurate lattice basis vector relationship of two domains were investigated with the assistance of X-ray diffraction, ab initio calculations and image simulations. This work offers a route to atom scale tunable heterostructure device application.

Adjustment of Demagnetizing Field in Permalloy Nanowires to Control Domain Wall Motion

The domain wall motion in permalloy nanowires has been investigated widely due to its potential for a new type of memory. In this paper. we use the LLG Simulator based on the Landau-Lifshitz-Gilbert (LLG) equation to investigate the field-driven domain-wall motion in a long, straight ferromagnetic strip. An injection field of 60 Oe is applied to inject a domain wall from an extended disk into the nanowire. We found a dependence of nanowire dimensions with the velocity of domain wall. By increasing the width of the nanowire, the velocity of the domain wall motion also increases, while the Walker breakdown field ( HWB) decreases. On the other hand, increasing the thicknesses of the nanowire, the domain wall velocity, HWB, and demagnetizing field all decrease. By applying a vertical field from 0 to 1000 Oe in order to enhance the demagnetizing field, it is found the HWB is increased from 16 to 20 Oe.

Control of domain wall motion at vertically etched nanotrench in ferromagnetic nanowires

We study field-induced domain wall motion in permalloy nanowires with vertically etched nanotrench pinning site. Micromagnetic simulations and electrical measurements are employed to characterize the pinning potential at the nanotrench. It is found that the potential profile for a transverse wall significantly differs from that of a vortex wall, and there is a correlation between the pinning strength and the potential profile. Reliable domain wall pinning and depinning is experimentally observed from a nanotrench in permalloy nanowires. This demonstrates the suitability of the proposed nanotrench pinning sites for domain wall device applications.

Control of domain wall motion by interference of spin wave

We propose a method to control the motions of magnetic domain walls using the interference phenomenon of the spin wave. It is found that when the distances between two neighboring walls are integer multiples of half wavelengths of the spin wave, the transmission coefficients of the spin wave are local maxima. This interference effect provides a new method for controlling the spin-wave-induced domain-wall motion, not only the speed of the domain wall motion but also its moving direction. In addition, tuning the distance of walls, we can find the direct relationship between the domain-wall velocity and the transmission coefficient, a crucial parameter of the spin-wave-induced domain-wall motions.

Magnonic momentum transfer force on domain walls confined in space

Momentum transfer from incoming magnons to a Bloch domain wall is calculated using one-dimensional continuum micromagnetic analysis. Due to the confinement of the wall in space, the dispersion relation of magnons is different from that of a single domain. This mismatch of dispersion relations can result in reflection of magnons upon incidence on the domain wall, whose direct consequence is a transfer of momentum between magnons and the domain wall. The corresponding counteraction force exerted on the wall can be used for the control of domain wall motion through magnonic linear-momentum transfer, in analogy with the spin transfer torque induced by magnonic angular-momentum transfer.

Direct Observation of Massless Domain Wall Dynamics in Nanostripes with Perpendicular Magnetic Anisotropy

Domain wall motion induced by nanosecond current pulses in nanostripes with perpendicular magnetic anisotropy (Pt/Co/AlO(x)) is shown to exhibit negligible inertia. Time-resolved magnetic microscopy during current pulses reveals that the domain walls start moving, with a constant speed, as soon as the current reaches a constant amplitude, and no or little motion takes place after the end of the pulse. The very low "mass" of these domain walls is attributed to the combination of their narrow width and high damping parameter α. Such a small inertia should allow accurate control of domain wall motion by tuning the duration and amplitude of the current pulses.