Current-induced Domain Wall Research Articles

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.

Compact Model of Domain Wall MTJ Driven by Spin Orbit Torque and Dzyaloshinskii–Moriya Interaction

Current-induced domain wall motion (CIDWM) shows promising prospects with low power, high density, high speed, and so on. Recent studies have demonstrated that magnetic tunnel junction (MTJ) based on CIDWM has great potential in mimicking a non-volatile artificial neuron and synapse. By combining the effect of the interfacial Dzyaloshinskii–Moriya interaction (DMI) to stabilize the Néel-type domain wall (DW) and the high-speed low-power advantages of spin orbit torque (SOT), the novel MTJ device is experimentally demonstrated with low threshold current and high propagation speed, which shows potential applications in the field of the artificial neuron and synapse. In this article, a compact model for CIDWM-MTJ based on SOT and DMI is presented. The model integrates the SOT mechanism for magnetization reversal and DW nucleation, CIDWM behaviors, and tunnel resistance theory of MTJ nanopillar. The micromagnetic simulation and the temporal evolution of the DW position are implemented by solving the Landau–Lifshitz–Gilbert (LLG) equation and the 1-D model. Based on the developed model, a hybrid CIDWM-MTJ/CMOS circuit is simulated for verification. The presented model combines the CIDWM dynamics and the MTJ magnetic dynamics, showing potential application for the simulation of all-spin artificial neural network (ANN).

Pinning Fault Mode Modeling for DWM Shifting

Extreme scaling for purposes of achieving higher density and lower energy continues to increase the probability of memory faults. For domain wall (DW) memories, misalignment faults arise when aligning domains with access points. A previously understudied type of shifting fault, a pinning fault may occur due to non-uniform pinning potential distribution caused by notches with fabrication imperfections. This non-uniformity can pin a wall during current-induced DW motion. This paper provides a model of geometric variations varying width, depth, and curvature variations of a notch, their impacts on the critical shift current, and a study of the resulting impact on fault rates of DW memory systems. An increase in the effective critical shift current due to 5% variation predicts a pinning fault rate on the order of $10^{-8}$ per shift, which results in a mean-time-to-failure of circa 2s for a DW memory system.

Giant Spin Hall Effect and Spin-Orbit Torques in 5d Transition Metal-Aluminum Alloys from Extrinsic Scattering.

The generation of spin currents from charge currents via the spin Hall effect (SHE) is of fundamental and technological interest. Here, some of the largest SHEs yet observed via extrinsic scattering are found in a large class of binary compounds formed from a 5d element and aluminum, with a giant spin Hall angle (SHA) of ≈1 in the compound Os22 Al78 . A critical composition of the 5d element is found at which there is a structural phase boundary between poorly and highly textured crystalline material, where the SHA exhibits its largest value. Furthermore, a systematic increase is found in the spin Hall conductivity (SHC) and SHA at this critical composition as the atomic number of the 5d element is systematically increased. This clearly shows that the SHE and SHC are derived from extrinsic scattering mechanisms related to the potential mismatch between the 5d element and Al. These studies show the importance of extrinsic mechanisms derived from potential mismatch as a route to obtaining large spin Hall angles with high technological impact. Indeed, it is demonstrated that a state-of-the-art racetrack device has a several-fold increased current-induced domain wall efficiency using these materials as compared to prior-art materials.

Enhanced current-induced domain wall motion by tuning the cubic in-plane magnetic anisotropy

The influence of cubic in-plane magnetic anisotropy on current-induced domain wall (DW) motion was investigated by micromagnetic simulation. DW velocity is enhanced when the nanostripes are placed in an optimized cubic in-plane magnetic anisotropy field that almost exactly compensates the demagnetizing field. An increase is also observed in the walker limit current, which is the linearity loss of the DW velocity as a function of spin current density when the latter exceeds a critical value. The microwave-assisted DW motion that is usually proposed to increase DW velocity is further improved by adding the cubic magnetic anisotropy field on the nanostripes. These effects of enhancing current-induced DW motion are useful for device applications.

Molecular beam epitaxial growth and magneto-transport properties of Mn4−xInxN films on SrTiO3(001) and MgO(001) substrates

Ferrimagnetic Mn4N films have a small saturation magnetization with perpendicular magnetic anisotropy (PMA) and are thereby considered suitable materials for ultrafast current-induced domain wall motion (CIDWM). We recently achieved ultrafast CIDWM, reaching 3,000 m/s at room temperature, in compensated ferrimagnet Mn4−xNixN (x = 0.2) films on SrTiO3[STO](001) substrates. In this work, we grew nonmagnetic element In-doped Mn4N epitaxial films (Mn4−xInxN) on two different substrates, STO(001) and MgO(001), by molecular beam epitaxy. The epitaxial growth of these Mn4−xInxN films was demonstrated up to x = 1.40 for those on MgO(001), but limited only to x = 0.12 for those on STO(001). The PMA vanished for greater amounts of In for both substrates. We attribute such a large difference in x between the two substrates to the difference in lattice mismatch between Mn4−xInxN/STO(001) and Mn4−xInxN/MgO(001). We also investigated anomalous Hall effect of the Mn4-xInxN epitaxial films, and discussed their anomalous Hall angles and coercive fields dependence on x.

Domain wall motion driven by a wide range of current in coupled soft/hard ferromagnetic nanowires.

Racetrack memory with the advantages of small size and high reading speed is proposed based on current-induced domain wall (DW) motion in a ferromagnetic (FM) nanowire. Walker breakdown that restricts the enhancement of DW velocity in a single FM nanowire can be depressed by inter-wire magnetostatic coupling in a double FM nanowire system. However, this magnetostatic coupling also limits the working current density in a small range. In the present work, based on micromagnetic calculation, we have found that when there is a moderate difference of magnetic anisotropy constant between two FM nanowires, the critical current density for triggering the DW motion can be reduced while that for breaking the inter-wire coupling can be enhanced significantly to a magnitude of 1013 A m−2, which is far above the working current density in current electronic devices. The manipulation of working current density is relevant to the modification of DW structure and inter-wire magnetostatic coupling due to the difference of the anisotropy constants between the two nanowires and paves a way to develop racetrack memory that can work in a wide range of current.

Field-free spin–orbit torque switching induced by interlayer exchange coupling in Pt/Co/Ru/Ni/Pt multilayer

High-efficient operation of spin–orbit torque (SOT) is central for the exciting field of spin-orbitronics. To realize a deterministic field-free SOT switching, here, we investigate the enhancement mechanism of SOT with the interlayer exchange coupling in a Pt/Co/Ru/Ni/Pt multilayer. We find that the decrease of the domain wall energy in an interlayer exchange coupling system is the pivotal reason for achieving current-induced field-free domain wall switching. Comparing with the direct ferromagnetic couplings, the switching ratio and efficiency present a significant improvement in the interlayer exchange coupling system. Combining micromagnetic simulations and magneto-optical Kerr effect measurements, the domain wall motions are investigated and the switching mechanism is confirmed to be the domain wall depinning regime. Furthermore, the SOT switching direction is proven to be dominated by the field-modulated domain wall motions. This work paves a potential way toward practical spin–orbit torque-based memory, oscillating, and logic devices.

High speed area efficient 5-bit hybrid Flash ADC using spin devices and CMOS

This paper uses current induced domain wall motion in a magnetic stripe and pre-charge sense amplifiers (PCSA) to design a high speed comparator of a magnetic flash ADC. Current-induced domain-wall (DW) motion is a prominent switching mechanism promising, high-density and high-speed circuits. Domain-wall motion in a magnetic stripe based magnetic flash ADC (Analog to Digital Converter) is area efficient and faster compared to other ADCs. Our proposed 5-bit Flash ADC can operate at 500MS/s with power consumption of 10.5mW. The power consumption can be further reduced to 5.06mW by choosing a smaller step size of 10mV. By using MTJ and domain wall motion in a magnetic stripe with PCSA, SPICE compatible Verilog-A model and CMOS 45nm design kit, its performance such as power and delay have been simulated and compared with other ADCs.

Ionitronic manipulation of current-induced domain wall motion in synthetic antiferromagnets

The current induced motion of domain walls forms the basis of several advanced spintronic technologies. The most efficient domain wall motion is found in synthetic antiferromagnetic (SAF) structures that are composed of an upper and a lower ferromagnetic layer coupled antiferromagnetically via a thin ruthenium layer. The antiferromagnetic coupling gives rise to a giant exchange torque with which current moves domain walls at maximum velocities when the magnetic moments of the two layers are matched. Here we show that the velocity of domain walls in SAF nanowires can be reversibly tuned by several hundred m/s in a non-volatile manner by ionic liquid gating. Ionic liquid gating results in reversible changes in oxidation of the upper magnetic layer in the SAF over a wide gate-voltage window. This changes the delicate balance in the magnetic properties of the SAF and, thereby, results in large changes in the exchange coupling torque and the current-induced domain wall velocity. Furthermore, we demonstrate an example of an ionitronic-based spintronic switch as a component of a potential logic technology towards energy-efficient, all electrical, memory-in-logic.

Present Status of Rare-earth Free Ferrimagnet Mn4N and Future Prospects of Mn4N-based Compensated Ferrimagnets

The rare-earth-free ferrimagnet Mn4N has attractive features for spintronics applications, because it possesses a perpendicular magnetization, due to a relatively large magnetic anisotropy constant of ∼105 J m−3 and a small spontaneous magnetization of ∼100 kA m−1, and a large spin polarization (P = 0.8). More crucially, it is possible to reach the magnetic compensation at room temperature, by tuning its constituent elements in compounds such as Mn4−xNixN and Mn4−xCoxN. This is particularly interesting for spin-torque-based spintronics applications, because at the vicinity of the magnetization and/or angular momentum compensation points, the switching currents can be reduced significantly, thereby leading to lower switching energies and higher switching speed. In this review article, we emphasize the importance of epitaxial growth of Mn4N films by comparing the results obtained on two different substrates, MgO(001) and SrTiO3(001). We then present the achievement of ultrafast current-induced domain wall motion (CIDWM) in Mn4N microstrips, and study the magnetic compensations in Mn4−xNixN and Mn4−xCoxN at room temperature, proved by x-ray magnetic circular dichroism measurements. Finally, we offer future prospects for such Mn4N-based compensated

Surface acoustic wave-assisted spin–orbit torque switching of the Pt/Co/Ta heterostructure

Current-induced spin–orbit torque (SOT) allows for highly efficient control of magnetization for the magnetic memory and the logic application, but its high critical current density Jc limits its applications. Here, we report that the surface acoustic wave (SAW) presents a promising approach for reducing Jc in Pt/Co/Ta heterostructures with perpendicular anisotropy. By the second harmonic Hall measurements, we find that the damping-like SOT effective field is almost the same with or without SAW, implying that the reduction of Jc does not originate from the enhancement of the SOT. However, the current-induced domain wall velocity v under SAW is greatly enhanced. By fitting with the creep law under SAW, we find that lnv is linear with SAW power (P), which reveals that the effective pinning barrier is reduced linearly with increasing P. This feature can be well described by the time average of the periodically accumulated nucleation probability under SAW. The results shed light on the application of SAW-assisted SOT devices for low consumption storage.

Current-induced domain wall motion: Comparison of STT and SHE

In this work, the current-induced domain wall (DW) motion driven by spin Hall effect (SHE) is theoretically investigated via an atomistic model. The SHE is taken into account in the atomistic model as a Slonczewski torque term. We first consider a bilayer system consisting of a ferromagnetic layer (FM) adjacent to a heavy metal (HM). To study the effect of spin Hall angle and FM thickness on DW motion in perpendicularly magnetized FM, an in-plane current is injected into HM. The results show that the critical current density, DW velocity and DW displacement strongly depend on the spin Hall angle and thickness of FM. To demonstrate the efficiency of SOT, we also study the DW motion driven by spin-transfer torque (STT) in a FM/NM/FM system by injecting a charge current perpendicularly to the plane of the structure. The DW velocity and DW displacement of two cases are compared. At the same current density, it is clearly observed that the DW in the presence of SOT is more easily moved with higher velocity and DW displacement. In addition, the critical current density of SHE driven case is smaller compared with spin torque case. To move the DW with the velocity of 100 m/s, the injected current density required for the STT case could be 10 times as high as the SHE case. The proposed model can be used to optimize all factors for spintronic device design with low power consumption, fast speed and high endurance such as the DW-based devices and the perpendicularly magnetized SOT-MRAM.

Increased Efficiency of Current-Induced Motion of Chiral Domain Walls by Interface Engineering.

Magnetic racetrack devices are promising candidates for next-generation memories. These spintronic shift-register devices are formed from perpendicularly magnetized ferromagnet/heavy metal thin-film systems. Data are encoded in domain wall magnetic bits that have a chiral Néel structure that is stabilized by an interfacial Dzyaloshinskii-Moriya interaction. The bits are manipulated by spin currents generated from electrical currents that are passed through the heavy metal layers. Increased efficiency of the current-induced domain wall motion is a prerequisite for commercially viable racetrack devices. Here, significantly increased efficiency with substantially lower threshold current densities and enhanced domain wall velocities is demonstrated by the introduction of atomically thin 4d and 5d metal "dusting" layers at the interface between the lower magnetic layer of the racetrack (here cobalt) and platinum. The greatest efficiency is found for dusting layers of palladium and rhodium, just one monolayer thick, for which the domain wall's velocity is increased by up to a factor of 3.5. Remarkably, when the heavy metal layer is formed from the dusting layer material alone, the efficiency is rather reduced by an order of magnitude. The results point to the critical role of interface engineering for the development of efficient racetrack memory devices.

Theoretical study of current-induced domain wall motion in magnetic nanotubes with azimuthal domains

We report a theoretical overview of the magnetic domain wall behavior under an electric current in infinitely-long nanotubes with azimuthal magnetization, combining the $1$D analytic model and micromagnetic simulations. We highlight effects that, besides spin-transfer torques already largely understood in flat strips, arise specifically in the tubular geometry: the \OErsted field and curvature-induced magnetic anisotropy resulting both from exchange and material growth. Depending on both the geometry of the tube and the strength of the azimuthal anisotropy, Bloch or N\'eel walls arise at rest, resulting in two regimes of motion largely dominated by either spin-transfer torques or the \OErsted field. We determine the Walker breakdown current in all cases, and highlight the most suitable parameters to achieve high domain wall speed.

Ultralow Switching-Current Density in All-Amorphous W−Hf/Co−Fe−B/TaOx Films

We study current-induced deterministic magnetization switching and domain wall motion via polar Kerr microscopy in all-amorphous W$_{66}$Hf$_{34}$/CoFeB/TaO$_\text{x}$ with perpendicular magnetic anisotropy and large spin Hall angle. Investigations of magnetization switching as a function of in-plane assist field and current pulse-width yield switching current densities as low as $3\times 10^{9}$ A/m$^2$. We accredit this low switching current density to a low depinning current density, which was obtained from measurements of domain wall displacements upon current injection. This correlation is verified by investigations of a Ta/CoFeB/MgO/Ta reference sample, which showed critical current densities of at least one order of magnitude larger, respectively.

Dynamics of all-optically switched magnetic domains in Co/Gd heterostructures with Dzyaloshinskii-Moriya interaction

Given the development of hybrid spintronic-photonic devices and chiral magnetic structures, a combined interest in all-optical switching (AOS) of magnetization and current-induced domain wall motion in synthetic ferrimagnetic structures with strong Dzyaloshinskii-Moriya Interaction (DMI) is emerging. In this study, we report a study on single-pulse all-optical toggle switching and asymmetric bubble expansion in specially engineered Co/Gd-based multilayer structures. In the absence of any external magnetic fields, we look into the AOS properties and the potential role of the DMI on the AOS process as well as the stability of optically written micro-magnetic domains. Particularly, interesting dynamics are observed in moon-shaped structures written by two successive laser pulses. The stability of domains resulting from an interplay of the dipolar interaction and domain-wall energy are compared to simple analytical models and micromagnetic simulations.

Electric current and field control of vortex structures in cylindrical magnetic nanowires

Magnetization dynamics in a cylindrical Permalloy nanowire under simultaneously applied electric current and field is investigated by means of micromagnetic simulations. The reversal process starts with the creation of open vortex structures with different rotation senses at the nanowire ends. Our results conclude that the current alone enlarges or reduces the size of these vortex structures according to the rotational sense of the associated Oersted field. Large current intensity creates a vortex structure which covers the whole nanowire surface. At the same time the magnetization in the nanowire core remains the same, i.e., no complete magnetization reversal is possible in the absence of external field. The simultaneous action of the current and field allows for the complete control of the vortex structures in terms of setting the polarity and vorticity. The state diagram for the minimum field and current required for the vorticity and axial magnetization switching is presented. This control is essential for future information technologies based on three-dimensional vertical structures, and the presented state diagram will become very useful for future experiments on current-induced domain wall dynamics in cylindrical magnetic nanowires.

Optimizing the Geometry of Chiral Magnetic Logic Devices

We investigated how geometry affects chiral magnetic logic devices, using micro-magnetic simulation. The logical NOT gate in the device was operated by current-induced domain wall (DW) motion in perpendicularly magnetized nanowires with a locally modulated in-plane magnetic anisotropy (IMA) region, where the up-down DW is switched into a down-up DW by current. We modulated the width of the nanowires as well as the length of the IMA region and found that an optimized geometry exists which depends on the Dzyaloshinskii-Moriya Interaction. Integrating the optimized NOT-gate, we then demonstrated NAND or NOR logic operations. Our results provide design guidelines for the magnetic logic device, paving the way to functional magnetic logic-inmemory devices.

Observation of domain wall creation and propagation using the magnetic fixed region with a synthetic antiferromagnetic configuration

Spin-memristors using a magnetic domain wall (DW) have drawn attention as possible candidates for an essential element of the analogue neuromorphic device. In this study, we fabricated a DW type three-terminal magnetic tunneling junction element with a magnetic fixed region having a synthetic antiferromagnetic configuration. The DW creation and subsequent measurement of current-induced domain wall motion were investigated. The magnetic DW was successfully introduced through a simple operation using the magnetic fixed region as a DW injector. This element structure will be one of the key technologies for realizing an array level production of the spin-memristor.