Current-induced Domain Wall Motion Research Articles

Decoupling the influences of chiral damping and Dzyaloshinskii-Moriya interaction in chiral magnetic domain walls

We revisit the nature and impact of both the chiral damping (CD) and Dzyaloshinskii-Moriya interaction (DMI) in uniaxial chiral ferromagnetic nanowires with broken inversion symmetry. We propose that CD, akin to its chiral energy counterpart (DMI), can be described in terms of the Lifshitz invariants permissible by the underlying symmetry of the system. This representation offers a clearer foundation for integrating CD into the dynamics of various chiral magnetic textures. We theoretically investigate the current-induced motion of chiral domain walls (DWs), driven by both spin-transfer torque and the spin Hall effect in the presence of CD. We demonstrate that it is possible to unambiguously separate the influence of CD from that of DMI by analyzing the current-induced dynamics. In particular, below the Walker breakdown (WB), the DMI does not affect the DW velocity, whereas increases in the strength of CD result in a decrease in the DW velocity. Moreover, for the spin-orbit torque driven motion, while the DMI enhances both the WB current density and the maximum attainable velocity below the WB, the CD only enhances the WB without affecting the maximum attainable velocity below the WB. Our findings open up intriguing opportunities for exploitation in exotic magnetic textures. Published by the American Physical Society 2024

Current-induced domain wall motion in a van der Waals ferromagnet Fe3GeTe2

The manipulation of spin textures by spin currents is of fundamental and technological interest. A particularly interesting system is the 2D van der Waals ferromagnet Fe3GeTe2, in which Néel-type skyrmions have recently been observed. The origin of these chiral spin textures is of considerable interest. Recently, it was proposed that these derive from defects in the structure that lower the symmetry and allow for a bulk vector Dzyaloshinsky-Moriya interaction. Here, we demonstrate current-induced domain wall motion in Fe3GeTe2 flakes, in which the maximum domain wall velocity is an order of magnitude higher than those reported in previous studies. In heterostructures with Pt or W layers on top of the Fe3GeTe2 flakes, domain walls can be moved via a combination of spin transfer and spin-orbit torques. The competition between these torques leads to a change in the direction of domain wall motion with increasing magnitude of the injected current.

Comparison of Current Induced Domain Wall Motion Driven by Spin Transfer Torque and by Spin Orbit Torque in Ferrimagnetic GdFeCo Wires

Current-induced domain wall motion (CIDWM) in magnetic wires can be driven by spin transfer torque (STT) originating from transferring angular momentums of spin-polarized conducting electrons to the magnetic DW and can be driven by spin orbit torque (SOT) originating from the spin Hall effect (SHE) in a heavy metal layer and Dzyaloshinsky Moriya (DMI) generated at an interface between a heavy metal layer and a magnetic layer. In this work, we carried out a comparative study of CIDWM driven by STT and by SOT in ferrimagnetic GdFeCo wires with magnetic perpendicular anisotropy based on structures of SiN (10 nm)/GdFeCo (8 nm)/SiN (10 nm) and Pt (5 nm)/GdFeCo (8 nm)/SiN (10 nm). We found that CIDWM driven by SOT exhibited a much lower critical current density (JC), and much higher DW mobility (µDW). Our work might be useful for the realization and the development of low-power and high-speed memory devices.

Self-assembly of Co/Pt stripes with current-induced domain wall motion towards 3D racetrack devices

Modification of the magnetic properties under the induced strain and curvature is a promising avenue to build three-dimensional magnetic devices, based on the domain wall motion. So far, most of the studies with 3D magnetic structures were performed in the helixes and nanowires, mainly with stationary domain walls. In this study, we demonstrate the impact of 3D geometry, strain and curvature on the current-induced domain wall motion and spin-orbital torque efficiency in the heterostructure, realized via a self-assembly rolling technique on a polymeric platform. We introduce a complete 3D memory unit with write, read and store functionality, all based on the field-free domain wall motion. Additionally, we conducted a comparative analysis between 2D and 3D structures, particularly addressing the influence of heat during the electric current pulse sequences. Finally, we demonstrated a remarkable increase of 30% in spin-torque efficiency in 3D configuration.

Boosting domain wall velocity and stability in ferrimagnetic GdFe nanowires by applying laser-annealing process

This study highlights the significant achievement of enhancing domain wall (DW) velocity through the laser-annealing (LA) process in GdxFe100−x films, with a specific focus on the Gd26Fe74 composition. By investigating the current-induced DW motion, it was observed that the LA process on the edges and middle of the nanowires resulted in a remarkable improvement in DW velocity from around 600 m/s for non-laser-annealed to 1800 m/s for laser-annealed conditions. Furthermore, we conducted additional experiments to explore the stability and dynamics of a system involving DWs in a single wire. The experimental results revealed a stable and uniform movement of the DWs, as evidenced by a velocity of approximately 1800 m/s, even after the application of 20 pulses with a short pulse width of 1 ns. The resulting stability of the DW motion across a wide range of applied currents demonstrates the practical significance of the LA in achieving high-performance racetrack memories. Our findings suggest that the LA process has a similar effect on both the middle and edge treatments of the wire. This indicates that structural defects on the wire's edges are not the main cause of the observed low velocity or curvy shape of DWs. One possible mechanism for this effect is the reduction of the Ku value through the LA process. This study establishes a ground-breaking advancement by showcasing the effectiveness of the LA in enhancing DW motion velocity in GdFe ferrimagnetic thin films, thereby opening avenues for future research and industrial applications in this field.

Atomic Layer Deposition of Perpendicularly Magnetized Co Layers Showing Current-Induced Domain Wall Motion

Successful preparation of perpendicularly magnetized Co thin layers on Pt seed-layers using atomic layer deposition (ALD) technique with Co(PF3)4H as a precursor is reported. The residual phosphorus content in the samples, due to low deposition temperatures to realize ALD, was reduced effectively by tuning ALD parameters. Spin-orbit-torque driven current-induced domain wall motion was observed in the strip containing ALD Co layer prepared with the low phosphorus condition.

Fabrication of highly oriented Mn4N/Pt epitaxial bilayer structure on MgO(001) for spintronics applications

Magnetization control by electric current or spin current is an attractive technology with potential applications in memory and logic devices, and Mn4N has attracted much attention in recent years as a candidate material for current-induced magnetization switching and magnetic domain wall motion, as well as a skyrmion material. For these applications, a bilayer structure of Mn4N and heavy metals must be formed with high crystalline quality. In this study, we have achieved the epitaxial growth of Mn4N thin films on MgO(001) with a Pt underlayer. The reflection high-energy electron diffraction images and x-ray diffraction ω-rocking curves show that the Mn4N films fabricated on the Pt underlayers exhibit higher crystalline quality than the Mn4N films directly formed on the MgO. Furthermore, anomalous Hall effect loops with bump-like structures were observed. This is considered to be due to the topological spin texture. The Mn4N/Pt epitaxial bilayer structure has potential for use in spin–orbit torque applications and is a candidate configuration for topological spin textures.

Magneto-optical spatial light modulator driven by current-induced domain wall motion for holographic display applications.

We have developed a magneto-optical spatial light modulator (MO-SLM) with a 10 k × 5 k pixel layout and with a pixel pitch horizontally of 1 µm and vertically of 4 µm. An MO-SLM device pixel has a magnetic nanowire made of Gd-Fe magneto-optical material whose magnetization was reversed by current-induced magnetic domain wall motion. We successfully demonstrated the reconstruction of holographic images, showing large viewing zone angles as wide as 30 degrees and visualizing different depths of the objects. These characteristics are unique to holographic images, providing physiological depth cues which may play a vital role in three-dimensional (3D) perception.

Nonuniform magnetic domain-wall synapses enabled by population coding

Traditional artificial intelligence implemented in software is usually executed on accurate digital computers. Nevertheless, the nanoscale devices for the implementation of neuromorphic computing may not be ideally identical, and the performance is reduced by nonuniform devices. In biological brains, information is usually encoded by a cluster of neurons such that the variability of nerve cells does not influence the accuracy of human cognition and movement. Here, we introduce the population encoding strategy in neuromorphic computing and demonstrate that this strategy can overcome the problems caused by nonuniform devices. Using magnetic memristor device based on current-induced domain-wall motion as an example, we show that imperfect storage devices can be applied in a hardware network to perform principal component analysis (PCA), and the accuracy of unsupervised classification is comparable to that of conventional PCA using ideally accurate synaptic weights. Our results pave the way for hardware implementation of neuromorphic computing and lower the criteria for the uniformity of nanoscale devices.

Operating characteristics of domain walls in perpendicularly magnetized ferrimagnetic cylindrical nano-wires for three-dimensional magnetic memory

We observed field- and current-induced domain wall motion (DWM) in a ferrimagnetic cylindrical nano-wire using a micromagnetic simulation. Two types of domain walls, namely the Hedgehog Bloch point (HBP) and transverse domain wall (TDW), can be generated by controlling the initial state of these systems. In the case of the field-induced HBP motion, depinning and Walker breakdown were observed, whereas they did not appear in the field-induced TDW motion. According to the observation of current-induced DWM, we found that the domain wall mobility in ferrimagnetic cylindrical nano-wires is higher than that in conventional ferromagnetic cylindrical nano-wires owing to the small saturation magnetization. The current-induced HBP motion exhibits depinning, unlike the TDW motion, which corresponds to field-driven DWM. These differences are attributed to the symmetry of the magnetization in the domain walls. These results provide notable insight into the design of three-dimensional memory based on current-induced DWM.

Ferrimagnetic-ferromagnetic phase transition in Au-doped Mn4N epitaxial films confirmed by x-ray magnetic circular dichroism

The antiperovskite ferrimagnet Mn4N has perpendicular magnetic anisotropy and small spontaneous magnetization, both of which are favorable properties for current induced domain wall motion. Previously we have investigated the magnetic structure of 3d-element-doped Mn4N thin films and demonstrated ultrafast domain wall velocities reaching 3000 m s−1 in the vicinity of the magnetic compensation composition of Ni-doped Mn4N at the current density of j = 1.2 × 1012 A m−2 at room temperature (RT). In this study, we investigate the effect of Au doping on the magnetic structure of Mn4N films, and present a composition ratio-dependent sign reversal of the anomalous Hall effect at RT. X-ray magnetic circular dichroism measurement revealed that the magnetic moment of the face-centered Mn atoms of Mn4−xAuxN reversed between x = 0.1 and 0.2, and became parallel to that of the corner-site Mn atoms for x = 0.2 and 0.3. This result suggests that the ferrimagnetic-ferromagnetic phase transition occurred in Au-doped Mn4N epitaxial films as in the In-doped Mn4N epitaxial films.

Reversal of current-induced domain wall motion in TbFeCo ferrimagnetic thin films across the magnetization compensation point

We report on a systematic investigation of current-induced domain wall motion in TbFeCo ferrimagnetic thin films with a Pt underlayer. The Tb concentration of the alloy is varied to study the effect of the magnetization compensation on the current-induced motion of domain walls. We find that the direction in which domain walls move changes when the Tb concentration crosses the magnetization compensation point. Interestingly, the domain walls move along (against) the current flow for FeCo-rich (deficient) films. When the film composition is slightly Tb-rich than the magnetic compensation point, the domain wall moves along the current at a lower current but reverses its direction when the current is increased. These results suggest that two competing torques act on the domain walls in the Tb-based ferrimagnets (TbFeCo), causing the domain wall velocity to be more than one order of magnitude smaller than that of Gd-based ferrimagnets.

Domain wall propagation and pinning induced by current pulses in cylindrical modulated nanowires.

The future developments in 3D magnetic nanotechnology require the control of domain wall dynamics by means of current pulses. While this has been extensively studied in 2D magnetic strips (planar nanowires), few reports on this exist in cylindrical geometry, where Bloch point domain walls are expected to have intriguing properties. Here, we report an investigation on cylindrical magnetic Ni nanowires with geometrical notches. An experimental work based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates that large current densities induce domain wall nucleation, while smaller currents move domain walls preferably antiparallel to the current direction. In the region where no pinning centers are present, we found a domain wall velocity of about 1 km s-1. Thermal modelling indicates that large current densities temporarily raise the temperature in the nanowire above the Curie temperature, leading to nucleation of domain walls during the system cooling. Micromagnetic modelling with a spin-torque effect shows that for intermediate current densities, Bloch point domain walls with chirality parallel to the Oersted field propagate antiparallel to the current direction. In other cases, domain walls can be bounced from the notches and/or get pinned outside their positions. We thus found that current is not only responsible for domain wall propagation, but also is a source of pinning due to the Oersted field action.

Imaging current control of magnetization in Fe3GeTe2 with a widefield nitrogen-vacancy microscope

Van der Waals (vdW) magnets are appealing candidates for realising spintronic devices that exploit current control of magnetization (e.g. switching or domain wall motion), but so far experimental demonstrations have been sparse, in part because of challenges associated with imaging the magnetization in these systems. Widefield nitrogen-vacancy (NV) microscopy allows rapid, quantitative magnetic imaging across entire vdW flakes, ideal for capturing changes in the micromagnetic structure due to an electric current. Here we use a widefield NV microscope to study the effect of current injection in thin flakes (∼10 nm) of the vdW ferromagnet Fe3GeTe2 (FGT). We first observe current-reduced coercivity on an individual domain level, where current injection in FGT causes substantial reduction in the magnetic field required to locally reverse the magnetisation. We then explore the possibility of current-induced domain-wall motion, and provide preliminary evidence for such a motion under relatively low current densities, suggesting the existence of strong current-induced torques in our devices. Our results illustrate the applicability of widefield NV microscopy to imaging spintronic phenomena in vdW magnets, highlight the possibility of efficient magnetization control by direct current injection without assistance from an adjacent conductor, and motivate further investigations of the effect of currents in FGT and other vdW magnets.

Current-induced zero-field domain wall depinning in cylindrical nanowires

Multi-segmented cylindrical nanowires have properties that make them attractive for high-density, high-speed logic and memory applications. Investigations of the current-induced domain wall motion in cylindrical nanowires have, so far, typically been conducted with a background magnetic field. However, if performed at zero external field, they would be much more viable for their use in prospective electronic devices. Here, we present an all-magneto electrical method to consistently pin domain walls in multi-segmented nanowires and induce their de-pinning using current pulses. The experiments were conducted with compositionally modulated three-segmented nickel/cobalt/nickel and two-segmented cobalt/nickel nanowires of 190 and 150 nm diameter, respectively, where the soft/hard magnetic texture has been fairly studied. We find that for the 3 segmented nanowire, the domain wall can be de-pinned independent of the polarity of the pulse, while for the 2 segmented nanowire the domain wall de-pins only for one polarity. Applying current pulses of 1 × 1012 A/m2, we use a pulse width of 22 ns to estimate a lower boundary for the domain wall speed of 634.54 m/s in cobalt. We study the resistive heating effect from the DC measurement current to find a temperature increase of no more than 2 °C after more than 20 h of tests.

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.

Ultrafast Racetrack Based on Compensated Co/Gd‐Based Synthetic Ferrimagnet with All‐Optical Switching

AbstractSpin‐orbitronics and single pulse all‐optical switching (AOS) of magnetization are two major successes of the rapidly advancing field of nanomagnetism in recent years, with high potential for enabling novel, fast, and energy‐efficient memory and logic platforms. Fast current‐induced domain wall motion (CIDWM) and single shot AOS have been individually demonstrated in different ferrimagnetic alloys. However, the stringent requirement for their composition control makes these alloys challenging materials for wafer scale production. Here, fast CIDWM and energy efficient AOS in a synthetic ferrimagnetic system based on multilayered [Co/Gd]2 are simultaneously demonstrated. First, it is shown that AOS is present in its full composition range. It is found that current‐driven domain wall velocities over 2000 m s‐1 at room temperature, achieved by compensating the total angular momentum through layer thickness tuning. Furthermore, analytical modeling of the CIDWM reveals that Joule heating needs to be treated transiently to properly describe the CIDWM for our sub‐ns current pulses. The studies establish [Co/Gd]‐based synthetic ferrimagnets to be a unique materials platform for domain wall devices with access to ultrafast single pulse AOS.

Anisotropic magnetoresistance in Mn4−xNixN and the change in the crystalline field

We focus on rare-earth-free anti-perovskite Mn4−xNixN epitaxial films, which can be used for ultrafast current-induced domain wall motion (CIDWM) in magnetic strips. The magneto-transport properties of these materials are very important for a deep understanding of CIDWM. In this study, we investigated the magneto-transport properties of Mn4−xNixN epitaxial films grown on SrTiO3(001) and MgO(001) substrates through anisotropic magnetoresistance (AMR) measurements at temperatures between 2 and 300 K. In samples with a small Ni composition such as x = 0.05−0.1, the AMR ratio of Mn4−xNixN drastically decreased with increasing temperature. We also analyzed the twofold and fourfold symmetries in the AMR curves. Fourfold symmetry is caused by tetragonal crystal fields and is unique to anti-perovskite 3d-metal nitrides such as Mn4N and Fe4N. Only slight fourfold symmetry was observed in Mn4−xNixN. We also performed first-principles calculations with the Vienna ab initio simulation package (VASP) to obtain the projected density of states (PDOS) of d orbitals in Mn4−xNixN, which is responsible for the magnetism of these materials. We conclude that these results are due to the Ni atoms, which function as magnetic impurities and lead to a noticeable change in PDOS, as proved by VASP calculation.

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.

Three-dimensional racetrack memory devices designed from freestanding magnetic heterostructures

The fabrication of three-dimensional nanostructures is key to the development of next-generation nanoelectronic devices with a low device footprint. Magnetic racetrack memory encodes data in a series of magnetic domain walls that are moved by current pulses along magnetic nanowires. To date, most studies have focused on two-dimensional racetracks. Here we introduce a lift-off and transfer method to fabricate three-dimensional racetracks from freestanding magnetic heterostructures grown on a water-soluble sacrificial release layer. First, we create two-dimensional racetracks from freestanding films transferred onto sapphire substrates and show that they have nearly identical characteristics compared with the films before transfer. Second, we design three-dimensional racetracks by covering protrusions patterned on a sapphire wafer with freestanding magnetic heterostructures. We demonstrate current-induced domain-wall motion for synthetic antiferromagnetic three-dimensional racetracks with protrusions of up to 900 nm in height. Freestanding magnetic layers, as demonstrated here, may enable future spintronic devices with high packing density and low energy consumption.