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We investigate the interaction between two magnetic layers separated with a normal metal insertion layer (Ti, Pt, and Ru) using x-ray ferromagnetic resonance (XFMR). We measure the amplitude and phase of the ferromagnetic resonance of both layers. Our results indicate that a ferromagnetic exchange coupling between two layers is a dominant coupling mechanism for a thick insertion metal layer. Based on the exchange coupling model, we extract the smallest value of the indirect exchange coefficient of 1.2 μJ/m2, which corresponds to an exchange field of about 0.36 mT. While this value is difficult to measure with other experimental tools, we were able to measure the small value because XFMR detects a resonance phenomenon of a thin layer generated by an oscillating indirect exchange and the Oersted fields with a phase and layer resolved observation.

The various spiral structures that exist in nature inspire humanity because of their morphological beauty, and spiral structures are used in various fields, including architecture, engineering, and art. Spiral structures have their own winding directions, and in most spirals, it is difficult to reverse the predetermined winding direction. Here, we show that a rotating spiral exists in magnetic systems for which the winding direction can be easily reversed. A magnetization vector basically has a spiral motion combining a precessional and a damping motion. The application of these basic mechanics to a system composed of magnetic vectors that are affected by a radial current and the Dzyaloshinskii–Moriya interaction forms the rotating magnetic spiral. The winding direction of the magnetic spiral has its own stability, but the direction can be changed using an external magnetic field. This magnetic spiral has a finite size, and the magnetic domain is destroyed at the edge of the spiral, which can create magnetic skyrmions.

Efficient magnetization control is a central issue in magnetism and spintronics1-8. Particularly, there are increasing demands for versatile manipulation of magnetic states in van der Waals (vdW) magnets for spintronic devices with their unconventional functionalities9-13. The electric control of the vdW magnets has been achieved for the magnetization switching via spin torque, but current-induced phase transition between ferromagnetic-to-antiferromagnetic states without external magnetic field is yet to be demonstrated12,14,15. Here, we report the current-induced magnetic phase transition in a vdW ferromagnet Fe5GeTe2, resulting in a giant magnetoresistance. Based on magneto-transport measurements and relevant theoretical analysis, we demonstrate that the transition sequentially occurs through the layers by voltage difference across the vdW gap, induced by an in-plane current. The current-tunability of magnetic phase in Fe5GeTe2 opens a path for electric control of the magnetic properties, expanding our ability to use vdW magnets for diverse spintronic device applications.

The paradigm shift of information carriers from charge to spin has long been awaited in modern electronics. The invention of the spin-information transistor is expected to be an essential building block for the future development of spintronics. Here, we introduce a proof-of-concept experiment of a magnetic skyrmion transistor working at room temperature, which has never been demonstrated experimentally. With the spatially uniform control of magnetic anisotropy, we can maintain the shape and topology of a skyrmion when passing the controlled area. Our findings will open a new route towards the design and realization of skyrmion-based spintronic devices in the near future. This article is protected by copyright. All rights reserved.

Due to its topological protection, the magnetic skyrmion has been intensively studied for both fundamental aspects and spintronics applications. However, despite recent advancements in skyrmion research, the deterministic creation of isolated skyrmions in a generic perpendicularly magnetized film is still one of the most essential and challenging techniques. Here, we present a method to create magnetic skyrmions in typical perpendicular magnetic anisotropy (PMA) films by applying a magnetic field pulse and a method to determine the magnitude of the required external magnetic fields. Furthermore, to demonstrate the usefulness of this result for future skyrmion research, we also experimentally study the PMA dependence on the minimum size of skyrmions. Although field-driven skyrmion generation is unsuitable for device application, this result can provide an easier approach for obtaining isolated skyrmions, making skyrmion-based research more accessible.

A scaling law elucidates the universality in nature, presiding over many physical phenomena which seem unrelated. Thus, exploring the universality class of scaling law in a particular system enlightens its physical nature in relevance to other systems and sometimes unearths an unprecedented new dynamic phase. Here, the dynamics of weakly driven magnetic skyrmions are investigated, and its scaling law is compared with the motion of a magnetic domain wall (DW) creep. This study finds that the skyrmion does not follow the scaling law of the DW creep in 2D space but instead shows a hopping behavior similar to that of the particle-like DW in 1D confinement. In addition, the hopping law satisfies even when a topological charge of the skyrmion is removed. Therefore, the distinct scaling behavior between the magnetic skyrmion and the DW stems from a general principle beyond the topological charge. This study demonstrates that the hopping behavior of skyrmions originates from the bottleneck process induced by DW segments with diverging collective lengths, which is inevitable in any closed-shape spin structure in 2D. This work reveals that the structural topology of magnetic texture determines the universality class of its weakly driven motion, which is distinguished from the universality class of magnetic DW creep.

A moving magnetic skyrmion exhibits transverse deflection. This so-called skyrmion Hall effect has been explained by the Thiele equation. Here, we provide an alternative interpretation of the skyrmion Hall effect based on the dynamics of domain walls enclosing the skyrmion. We relate the spin-torque-induced local rotation of the domain wall segments to the shift of the skyrmion core, explaining the skyrmion Hall effect at the micromagnetic level. Based on our intuitive interpretation, we also show that the skyrmion Hall effect can be suppressed by combining the spin-transfer and spin–orbit torques, thereby removing the major obstacle to utilizing skyrmions in devices.

The magnetic skyrmion is a topologically protected spin texture that has attracted much attention as a promising information carrier because of its distinct features of suitability for high-density storage, low power consumption, and stability. One of the skyrmion devices proposed so far is the skyrmion racetrack memory, which is the skyrmion version of the domain-wall racetrack memory. For application in devices, skyrmion racetrack memory requires electrical generation, deletion, and displacement of isolated skyrmions. Despite the progress in experimental demonstrations of skyrmion generation, deletion, and displacement, these three operations have yet to be realized in one device. Here, a route for generating and deleting isolated skyrmion-bubbles through vertical current injection with an explanation of its microscopic origin is presented. By combining the proposed skyrmion-bubble generation/deletion method with the spin-orbit-torque-driven skyrmion shift, a proof-of-concept experimental demonstration of the skyrmion racetrack memory operation in a three-terminal device structure is provided.

Magnetic domain wall (DW) motion in perpendicularly magnetized materials is drawing increased attention due to the prospect of new type of information storage devices, such as racetrack memory. To augment the functionalities of DW motion‐based devices, it is essential to improve controllability over the DW motion. Other than electric current, which is known to induce unidirectional shifting of a train of DWs, an application of in‐plane magnetic field also enables the control of DW dynamics by rotating the DW magnetization and consequently modulating the inherited chiral DW structure. Applying an external bias field, however, is not a viable approach for the miniaturization of the devices as the external field acts globally. Here, the programmable exchange‐coupled DW motion in the antiferromagnet (AFM)/ferromagnet (FM) system is demonstrated, where the role of an external in‐plane field is replaced by the exchange bias field from AFM layer, enabling the external field‐free modulations of DW motions. Interestingly, the direction of the exchange bias field can also be reconfigured by simply injecting spin currents through the device, enabling electrical and programmable operations of the device. Furthermore, the result inspires a prototype DW motion‐based device based on the AFM/FM heterostructure, that could be easily integrated in logic devices., The role of magnetic field in the ferromagnetic domain‐wall (DW) motions within a micropatterned magnetic layer can be efficiently overtaken by the effective field from adjacent exchange‐coupled antiferromagnet. Using the spin structure of antiferromagnet controllable with spin current, highly adjusted motion of exchange‐coupled DW is demonstrated showing a prospect of field‐free electrical operations of DW‐based spintronics devices.

Magnetic skyrmions, which are topological swirling spin textures, have drawn much attention in spintronics because of their use as an information carrier with distinct robustness rooted in their topological nature. Real-time generation of skyrmions is therefore imperative for realizing skyrmion-based spintronic devices. However, to date, experimental demonstration has been limited to exquisite works with well-tuned samples. Here, we report a method to generate skyrmions by driving the stripe instability via an in-plane magnetic field. We have demonstrated that the key parameter determining the stripe domain instability is the stripe width, regardless of other material parameters. This skyrmion generation method can be applicable to generic magnetic films with perpendicular magnetic anisotropy. Our work will facilitate the development of skyrmion-based devices by offering a general method for controlling a large skyrmion population. Here, we investigated the details of magnetic skyrmion bubble domain formation by a tilted magnetic field. The in-plane component of the magnetic field serves to reduce the width of the stripe domain. These stripe domains are easily broken and bubble domains are formed. We have demonstrated that the key parameter determining the stripe domain instability is the stripe width, regardless of other material parameters. This bubble generation method can be applicable to generic magnetic films with perpendicular magnetic anisotropy. Our work will facilitate the development of skyrmion-based devices by offering a general method for controlling a large skyrmion population.

One long-standing goal in the emerging neuromorphic field is to create a reliable neural network hardware implementation that has low energy consumption, while providing massively parallel computation. Although diverse oxide-based devices have made significant progress as artificial synaptic and neuronal components, these devices still need further optimization regarding linearity, symmetry, and stability. Here, we present a proof-of-concept experiment for integrated neuromorphic computing networks by utilizing spintronics-based synapse (spin-S) and neuron (spin-N) devices, along with linear and symmetric weight responses for spin-S using a stripe domain and activation functions for spin-N. An integrated neural network of electrically connected spin-S and spin-N successfully proves the integration function for a simple pattern classification task. We simulate a spin-N network using the extracted device characteristics and demonstrate a high classification accuracy (over 93%) for the spin-S and spin-N optimization without the assistance of additional software or circuits required in previous reports. These experimental studies provide a new path toward establishing more compact and efficient neural network systems with optimized multifunctional spintronic devices. An array of low-power devices that mimic the behavior of the brain has been constructed by researchers in South Korea. The brain works by passing chemical and electrical signals across a network of cells called neurons, which are connected via synapses. Scientists want to create an artificial neural network to take advantage of the parallel way in which the brain processes information. YeonJoo Jeong from the Korea Institute of Science and Technology, Jin Pyo Hong from Hanyang University, both in Seoul, and their colleagues have demonstrated a proof-of-principle neuromorphic computing network using so-called spintronic devices. Spintronics are low-power devices that use the property of an electron called spin rather than its charge as in conventional electronics. The team’s neural network of electrically connected spin-synapses and spin-neurons successfully performed a simple pattern classification task. We introduced spintronics-based synapses (spin-S) by utilizing a stripe domain ensuring its highly linear and symmetric weight responses, together with domain-wall motion-based neurons (spin-N) for activation functions. In addition, a crossbar array architecture for the spin-S/N has been proposed and experimentally demonstrated. A simple pattern-classification task was tested using an integrated network of electrically connected spin-S and spin-N to mimic a human brain. Our experimental findings provide a new avenue toward establishing more efficient neural network systems with spintronic devices.

Spin–orbit torque (SOT)-induced magnetization switching via electric currents in large spin–orbit-coupled heavy metal (HM)/ferromagnetic (FM) frames is one of the most reliable approaches to achieve increased performances in various spintronic applications. However, its widespread utilization is still challenging owing to the inherent requirement of a high electric current density for efficient magnetization switching. In this paper, we introduce a sandwiched synthetic antiferromagnetic (S-SyAF) frame, comprising CoFeB (FM)/W (HM)/CoFeB (FM) stacks, whose operation is mediated by the Ruderman–Kittel–Kasuya–Yosida interaction. The middle HM layer serves as a spin current source for the top and bottom FM layers simultaneously. In addition, the S-SyAF frame contributes to a substantial reduction in the net magnetization, which ensures a considerably increased SOT switching efficiency; further, the switching current was almost 10 times smaller than that in a conventional HM/FM frame. Thus, we believe that these experimental findings will provide a new avenue for designing future spintronic devices.

Thickness-dependent magnetic domain structure of ultrathin Co wedge films (0.3 nm–1.0 nm) sandwiched by Pt layers was investigated by scanning transmission x-ray microscopy (STXM) employing X-ray magnetic circular dichroism (XMCD), utilizing elliptically polarized soft x-rays and electromagnetic fields, with a spatial resolution of 50 nm. The magnetic domain images measured at the Co L3 edge showed the evolution of the magnetic domain structures from maze-like form to the bubble-like form as the perpendicular magnetic field was applied. The asymmetric domain expansion of a 500 nm-scale bubble domain was also measured when the in-plane and perpendicular external magnetic field were applied simultaneously.

Current-induced magnetic domain wall (DW) motion is an important operating principle of spintronic devices. Injected current generates spin torques (STs) on the DWs in two ways. One is the spin transfer from magnetic domains to the walls by the current flowing in the magnet. Current flow in attached heavy metals also generates another ST because of the spin-Hall effect. Both phenomena explain the wall motions well; therefore, their respective contribution is an important issue. Here, we show the simultaneous measurement of both torques by using magnetic facet domains that form mountain-shaped domains with straight walls. When the STs and the external magnetic field push the walls in opposite directions, the walls should have equilibrium angles to create balanced states. Such angles can be modulated by an additional in-plane magnetic field. Angle measurements distinguish the STs because each torque has a distinct mechanism related to the DW structure.

We establish a model for a stripe domain pattern width change under hard-axis magnetic fields in a perpendicularly magnetized system with Dzyaloshinskii–Moriya interaction. With the presence of hard-axis magnetic fields, tilted magnetizations in domains modulate an exchange interaction, a perpendicular magnetic anisotropy and a dipole–dipole interaction into specific forms, which are the main competing interactions to determine the specific form of the stripe domain. In addition, we found that the stripe domain pattern change under a hard-axis perturbation can be analyzed by only considering the amount of magnetization tilting from the initial easy-axis. The proposed model was used to draw a single universal curve for stripe width change as a function of the hard-axis magnetic fields.

The exotic spin textures known as skyrmions are important for memory and logic applications, due to their robustness against external perturbations. Skyrmions have been considered to exist only in perpendicular magnetic domains, but this study shows that they may also exist in $i\phantom{\rule{0}{0ex}}n\ensuremath{-}p\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e$ domains, as seen by employing the rotation transformation (a type of continuous transformation). The peculiar feature of in-plane skyrmions is that textures of opposite ``charge'' can coexist within the same domain. This work notes the possibility of double-bit transfer on a single track in a memory device.

The current-induced magnetization dynamics is an important subject in spintronics research. It contains many interesting physical phenomena and has a high potential for spintronic applications. Most magnetic memory devices are based on the magnetization switching operation. Thus, finding the switching mechanism, as well as the exact understanding of it, is an essential implement for spintronic devices. Here, we show that the current can switch the magnetization via chiral buckling. To destabilize the uniform magnetization, a critical current value is needed. The Dzyaloshinskii-Moriya interaction (DMI) produces a shift in this critical current and the sign of this shift also depends on the domain polarity. Thus, a certain current can select a certain domain polarity. During the switching process, a fluctuation of magnetization is exponentially amplified by the current that forms the magnetization buckling. We explain these phenomena by linear stability analysis and show that two distinct switching operations are possible depending on the DMI.

Magnetization, the magnetic moment per unit volume, is the basic quantity for describing a magnetic system. Generally, magnetic imaging techniques show the relative magnitude of magnetization, and its direction. Determining the $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e$ value of magnetization requires separate measurement with a reference sample, though. This study describes the stripe state of magnetization, in which the absolute value can be measured simply by magneto-optical imaging. In particular, this approach is applicable to samples so small that they cannot be measured by conventional magnetometers.

Among two-dimensional (2D) layered van der Waals materials, ferromagnetic 2D materials can be useful for compact low-power spintronic applications. One promising candidate material is Fe

In the emerging field of $m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}s$, the aim is to use magnons (quantized spin waves) as information carriers in computing technology. For magnonic operation, it is important to control the direction of spin-wave propagation at will. Employing the concept of a phased array antenna, this study shows that it is possible to control the direction of spin waves in thin films without any waveguide, magnonic crystal, or other specifically designed geometry. Furthermore, the phased-array concept enables spin-wave focusing, leading to local confinement with enhanced amplitude. These results pave the way for future work in magnon-based spintronic applications.

Spin–orbit torque (SOT)-induced magnetization switching via electric currents in large spin–orbit-coupled heavy metal (HM)/ferromagnetic (FM) frames is one of the most reliable approaches to achieve increased performances in various spintronic applications. However, its widespread utilization is still challenging owing to the inherent requirement of a high electric current density for efficient magnetization switching. In this paper, we introduce a sandwiched synthetic antiferromagnetic (S-SyAF) frame, comprising CoFeB (FM)/W (HM)/CoFeB (FM) stacks, whose operation is mediated by the Ruderman–Kittel–Kasuya–Yosida interaction. The middle HM layer serves as a spin current source for the top and bottom FM layers simultaneously. In addition, the S-SyAF frame contributes to a substantial reduction in the net magnetization, which ensures a considerably increased SOT switching efficiency; further, the switching current was almost 10 times smaller than that in a conventional HM/FM frame. Thus, we believe that these experimental findings will provide a new avenue for designing future spintronic devices.

The spin-transfer-torque-driven (STT-driven) dynamics of a domain wall in an easy-axis rare-earth transition-metal ferrimagnet is investigated theoretically and numerically in the vicinity of the angular momentum compensation point $T_A$, where the net spin density vanishes. The particular focus is given on the unusual interaction of the antiferromagnetic dynamics of a ferrimagnetic domain wall and the adiabatic component of STT, which is absent in antiferromagnets but exists in the ferrimagnets due to the dominant coupling of conduction electrons to transition-metal spins. Specifically, we first show that the STT-induced domain-wall velocity changes its sign across $T_A$ due to the sign change of the net spin density, giving rise to a phenomenon unique to ferrimagnets that can be used to characterize $T_A$ electrically. It is also shown that the frequency of the STT-induced domain-wall precession exhibits its maximum at $T_A$ and it can approach the spin-wave gap at sufficiently high currents. Lastly, we report a numerical observation that, as the current density increases, the domain-wall velocity starts to deviate from the linear-response result, calling for a more comprehensive theory for the domain-wall dynamics in ferrimagnets driven by a strong current., Comment: 8 pages, 5 figures

Magnetic skyrmions are expected to offer novel functionalities in emerging magnetic devices. However, when the skyrmion moves along a nanotrack, its trajectory is generally curved by the skyrmion Hall effect, resulting in an annihilation of skyrmion at the edge. Here we propose a method to avoid this edge-induced skyrmion annihilation. We found that the annihilation of skyrmion is largely suppressed when we replace the conventional edge with a chiral domain wall boundary, due to topological repulsion. We also provide device architectures that enable the guiding, sorting and size-tuning of multiple skyrmions, which could be potentially useful for future skyrmion devices.

How might we measure the Dzyaloshinskii-Moriya interaction (DMI), which can be so important in spintronics? When this magnetic exchange interaction is small, knowing the actual value becomes significant, since operating speed is proportional to it in spintronic devices based on domain-wall movement. The authors develop a method for measuring a weak DMI: When current and a magnetic field are applied to a perpendicularly magnetized thin film, the characteristic morphology of the magnetic domains that form can be observed, and the DMI obtained from the angles present in the unusual domain shapes. This will improve the engineering of spintronic logic and memory applications.

We examine the current-induced dynamics of a skyrmion that is subject to both structural and bulk inversion asymmetry. There arises a hybrid type of Dzyaloshinskii-Moriya interaction (DMI) which is in the form of a mixture of interfacial and bulk DMIs. Examples include crystals with symmetry classes C$_n$ as well as magnetic multilayers composed of a ferromagnet with a noncentrosymmetric crystal and a nonmagnet with strong spin-orbit coupling. As a striking result, we find that, in systems with a hybrid DMI, the spin-orbit-torque-induced skyrmion Hall angle is asymmetric for the two different skyrmion polarities ($\pm 1$ given by out-of-plane core magnetization), even allowing one of them to be tuned to zero. We propose several experimental ways to achieve the necessary straight skyrmion motion (with zero Hall angle) for racetrack memories, even without antiferromagnetic interactions or any interaction with another magnet. Our results can be understood within a simple picture by using a global spin rotation which maps the hybrid DMI model to an effective model containing purely interfacial DMI. The formalism directly reveals the effective spin torque and effective current that result in qualitatively different dynamics. Our work provides a way to utilize symmetry breaking to eliminate detrimental phenomena as hybrid DMI eliminates the skyrmion Hall angle., Comment: 10 pages, 4 figures, 2 tableS

Spin-orbit torque, which can be dampinglike or fieldlike, draws much attention as a possible means of spin switching in devices. Accurate measurement of the fieldlike torque is quite important in identifying good spintronic materials for applications. The authors show that in a ferromagnetic resonance experiment, the magnitude of the current density---and therefore that of the fieldlike torque---can be obtained by measuring the shift in resonance frequency as it is tuned with different direct currents. This method could play a key role in characterizing materials for spin-orbit-torque devices.

Among two-dimensional (2D) layered van der Waals materials, ferromagnetic 2D materials can be useful for compact low-power spintronic applications. One promising candidate material is Fe3GeTe2 (FGT), which has a strong perpendicular magnetic anisotropy and relatively high Curie temperature. In this study, we confirmed that an oxide layer (O-FGT) naturally forms on top of exfoliated FGT and that an antiferromagnetic coupling (AFC) exists between FGT and O-FGT layers. From a first-principles calculation, oxide formation at the interface of each layer induces an AFC between the layers. An AFC causes a tailed hysteresis loop, where two-magnetization reversal curves are included, and a negative remanence magnetization at a certain temperature range.

We report here an analytic prediction of the domain-wall (DW) tilting caused by the Oersted field in the current-driven DW motion along ferromagnetic nanowires that have perpendicular magnetic anisotropy. By adopting the variational principle for energy minimization, the DW tilting angle is determined as a function of the current density with a finite threshold current density, above which the DW becomes elongated along the nanowire with two narrow domains at its edges. These results predict the minimum data bit size as well as the maximum current density needed for realizing stable DWs in DW-mediated nanodevices.

We propose here a method for compensating the Joule-heating effects in the current-induced domain wall motion (CIDWM). In CIDWM experiments, the current induces not only the spin-transfer torque (STT) effects but also the Joule-heating effects, and both effects influence the domain wall (DW) motion. It is thus desired to develop a way to compensate the Joule-heating effects, in order to determine the pure STT effects on the DW motion. Up to now, in studies of DW creeping motions, such Joule-heating effects have been eliminated based on the Arrhenius law by assuming the temperature-independent creep scaling constants. However, here we find that such scaling constants are sensitive to the temperature, from the DW creeping experiment in Pt/Co/Pt wires with temperature control in a cryostat. By accounting the temperature dependence of the scaling constants, we demonstrate that all the DW speeds with various temperatures are exactly collapsed onto a single universal curve, which enables us to examine the pure STT effects on the DW motion.

$M\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ $c\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}s$ are metamaterials with engineered magnetic properties that can be used to manipulate spin waves, for next-generation digital logic and storage. Such an artificial crystal could be fashioned from arrays of magnetic $s\phantom{\rule{0}{0ex}}k\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}s$ (hedgehog-like topological objects), in principle. The authors show that by controlling the disposition of two lines of skyrmions with the asymmetric magnetic field from nearby currents, a spin wave could be refracted. This idea avoids the usual patterning of the magnonic crystal, which is the most time-consuming aspect of manufacturing.

The one-dimensional magnetic skyrmion motion induced by an electric current has attracted much interest because of its application potential in next-generation magnetic memory devices. Recently, the unidirectional motion of large (20 μm in diameter) magnetic bubbles with two-dimensional skyrmion topology, driven by an oscillating magnetic field, has also been demonstrated. For application in high-density memory devices, it is preferable to reduce the size of skyrmion. Here we show by numerical simulation that a skyrmion of a few tens of nanometres can also be driven by high-frequency field oscillations, but with a different direction of motion from the in-plane component of the tilted oscillating field. We found that a high-frequency field for small skyrmions can excite skyrmion resonant modes and that a combination of different modes results in a final skyrmion motion with a helical trajectory. Because this helical motion depends on the frequency of the field, we can control both the speed and the direction of the skyrmion motion, which is a distinguishable characteristic compared with other methods.

Unidirectional motion of magnetic domain walls is the key concept underlying next-generation domain-wall-mediated memory and logic devices. Such motion has been achieved either by injecting large electric currents into nanowires or by employing domain-wall tension induced by sophisticated structural modulation. Herein, we demonstrate a new scheme without any current injection or structural modulation. This scheme utilizes the recently discovered chiral domain walls, which exhibit asymmetry in their speed with respect to magnetic fields. Because of this asymmetry, an alternating magnetic field results in the coherent motion of the domain walls in one direction. Such coherent unidirectional motion is achieved even for an array of magnetic bubble domains, enabling the design of a new device prototype—magnetic bubblecade memory—with two-dimensional data-storage capability.

The coherent unidirectional motion of magnetic domain walls (DWs) is a key technology used in memory and logic device applications, as demonstrated in magnetic strips by electric current flow as well as in films by oscillation of a tilted magnetic field. Here we introduce a coherent unidirectional motion of DWs in the strip, utilizing an oscillating field, which is described within a previous 1D model. The essential criterion for DW motion in this approach is the oscillating-field-induced modulation of the DW width, which has not been previously considered. This DW motion driven by width modulation sheds light on high frequency domain manipulation in spin devices. A comprehensive inspection of field angle dependence reveals that unidirectional DW motion in this model requires chiral DWs, followed by asymmetric deformation of the domain shape.

We report local concentration of the Oersted field at the junction of electric contact geometry. The local Oersted field is numerically calculated with a simplified analytical model. This local field is then verified experimentally by observing magnetization reversal due to the current injection into Permalloy nanowires under magnetic field bias. The threshold current of magnetization reversal is found to be linearly dependent on the strength of the magnetic-field bias with a proportional coefficient (6.9 ± 0.2) × 10-11 Oe·m2/A, which provides evidence of the local concentration of the Oersted field at the junction.

It is experimentally reported herein that the injection field of domain walls (DWs) from the nucleation pad to the nanowire is controlled by the angle of the initializing magnetic field with the use of asymmetric sample structures. The injection field is abruptly varied twice between two distinct values at a certain angle. Micromagnetic simulation is used to model the injection of a DW into the nanowire with respect to the angle of the initializing magnetic field. This is ascribed to the different structure of the DW at the junction between the pad and the nanowire, resulting in the different pinning strength of the DW. These observations provide a way to control the injection field of DWs into nanostructures and give a possibility of the fast, reliable motion of the DW with field strengths less than the so-called Walker field on the nanowire by injecting the DW with a known magnetic structure.

Nonlinear dynamics of the magnetic vortex state in a circular nanodisk was studied under a perpendicular alternating magnetic field that excites the radial modes of the magnetic resonance. Here, we show that as the oscillating frequency is swept down from a frequency higher than the eigenfrequency, the amplitude of the radial mode is almost doubled to the amplitude at the fixed resonance frequency. This amplitude has a hysteresis vs. frequency sweeping direction. Our result showed that this phenomenon was due to a Duffing-type nonlinear resonance. Consequently, the amplitude enhancement reduced the vortex core-switching magnetic field to well below 10 mT. A theoretical model corresponding to the Duffing oscillator was developed from the Landau–Lifshitz–Gilbert equation to explore the physical origin of the simulation result. This work provides a new pathway for the switching of the magnetic vortex core polarity in future magnetic storage devices.