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Efficient current‐induced switching of perpendicular magnetization is an essential task in spintronics for realizing high‐performance information processing and for storage device application. However, the spin‐orbit torque (SOT) by injection of in‐plane polarized spins cannot deterministically switch the magnetization of ferromagnetic thin films with perpendicular magnetic anisotropy (PMA) without an additionally applied in‐plane external magnetic field to break the symmetry of the PMA. Considering the difficulties of applying the magnetic field to the localized area only within a device structure, it is essential to contrive a facile field‐free SOT switching mechanism. Here, deterministic field‐free SOT switching of perpendicular magnetization is achieved in amorphous and ferrimagnetic Gd/Co multilayers accompanied by a tilted magnetic anisotropy axis. This tilted anisotropy originates from the combined contributions of many internal anisotropies in different orientations from the multilayers and is shown to be controllable. It is expected that the introduction of controlled tilted anisotropy into Gd/Co multilayers over the entire film surface in the present study can be extended to the development of wafer‐scale technologies for the spintronics memory and logic devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Keywords: Dzyaloshinskii-Moriya interaction; ferrimagnets; field-free switching; spin-orbit torque; tilted magnetic anisotropy EN Dzyaloshinskii-Moriya interaction ferrimagnets field-free switching spin-orbit torque tilted magnetic anisotropy 1 1 1 06/28/22 20220624 NES 220624 B Field-Free Switching b In article number 2112561, Jung-Il Hong, Chanyong Hwang, and co-workers demonstrate field-free spin-orbit torque switching of perpendicular magnetization in amorphous and ferrimagnetic Gd/Co multilayers accompanied by a tilted magnetic anisotropy axis. Field-Free Switching of Magnetization by Tilting the Perpendicular Magnetic Anisotropy of Gd/Co Multilayers (Adv. Funct. Dzyaloshinskii-Moriya interaction, ferrimagnets, field-free switching, spin-orbit torque, tilted magnetic anisotropy. [Extracted from the article]

Abstract: 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. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

• The stripe domain pattern behavior under a hard-axis perturbation. • A single universal curve as a function of the hard-axis magnetic fields. • Based on universal curve, the DMI energy and the domain wall type can be known. 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. [ABSTRACT FROM AUTHOR]

We demonstrate here that the current-driven domain wall (DW) in two dimensions forms a "facet" roughness, distinctive to the conventional self-affine roughness induced by a magnetic field. Despite the different universality classes of these roughnesses, both the current- and field-driven DW speed follow the same creep law only with opposite angular dependences. Such angular dependences result in a stable facet angle, from which a single DW image can unambiguously quantify the spin-transfer torque efficiency, an essential parameter in DW-mediated nanodevices. [ABSTRACT FROM AUTHOR]

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\pm 0.2) \times 10^-11 Oe \cdotm^2/A, which provides evidence of the local concentration of the Oersted field at the junction. [ABSTRACT FROM AUTHOR]

Abstract: We present an experimental technique to determine the magnetic anisotropy of ferromagnetic nanowires. In the technique, the magnetization state is monitored by measuring the anisotropic magnetoresistance with rotating the external magnetic field. The measured magnetoresistance curves exhibit basically the same curves typically appeared in the torque magnetometric measurements, which are then readily analyzed based on the Stoner–Wohlfarth theory with a single fitting parameter – the magnetic anisotropy. By applying the present technique to Permalloy nanowires, it is shown that the shape anisotropy in real nanowires is significantly influenced by the edge roughness. [Copyright &y& Elsevier]

We investigate two distinct pinning mechanisms-denoted as static and kinetic pinning of magnetic domain wall (DW) in Permalloy nanowires with different widths. Both pinning situations are realized at an artificial notch on U-shaped Permalloy nanowires, depending on the initial DW states, moving or pinned. We find experimentally that the kinetic and static depinning fields simultaneously increase as the width of the nanowire decreases, whereas a difference between static and kinetic depinning fields monotomically decreases. This is ascribed to the shape anisotropy field of the DWs depending on the geometry of nanowires based on one-dimensional collective model. [ABSTRACT FROM AUTHOR]

• Wedge-shaped perpendicularly magnetized thin-film is fabricated using a physical barrier during sputtering growth. • The axis of perpendicular magnetic anisotropy is observed to be tilted in a wedge-shaped film. • The quantitative analysis of the tilted angle of perpendicular magnetic anisotropy is performed. • Field-free spin–orbit torque switching is achieved utilizing perpendicular magnetic anisotropy tilting. Since the discovery of spin–orbit torque, it has garnered significant attention as one of potential spintronics building blocks, owing to its distinct advantages, such as fast switching capabilities. For the practical application of spin–orbit torque, several technical challenges remain; a primary one being the elimination of the necessity for an external magnetic field during switching process. To address this issue, here, we thoroughly explore the nature of perpendicular magnetic anisotropy tilting via wedge sputtering deposition incorporating a physical barrier. We precisely quantify the tilted angle of magnetic anisotropy, and introduce a way to determine the direction of the magnetic anisotropy tilting. In addition, the realization of field-free magnetization switching using the tilted perpendicular magnetic anisotropy is experimentally demonstrated, ensuring the controllability of polarity of the switching. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Controlling the direction of magnetization with an electric current, rather than a magnetic field, is a powerful technique in spintronics. Spin–orbit torque, which generates an effective magnetic field from the injected current, is a promising method for this purpose. Here we show an approach for quantifying the magnitude of spin–orbit torque from a single magnetic image. To achieve this, we deposited two concentric electrodes on top of the magnetic sample to flow a radial current. By counterbalancing the current effect with an external magnetic field, we can create a stable circular magnetization state. We measure the magnitude of spin–orbit torque from the stable radius, providing a new tool for characterizing spin–orbit torque. [ABSTRACT FROM AUTHOR]

Abstract 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 L 3 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. Highlights • The nanometer-scale magnetic domain imaging is realized by using XMCD-STXM. • The magnetic domain structures of Pt/Co micro-wedge/Pt systems are investigated. • The asymmetric domain expansion of a bubble domain was studied by using XMCD-STXM. [ABSTRACT FROM AUTHOR]

Néel-type magnetic skyrmions in perpendicularly magnetized systems have attracted considerable interest due to their potential in fundamental research on topological objects and spintronics applications. Various systems have been explored to study Néel-type magnetic skyrmions, including repeated magnetic multilayers, two-dimensional materials, and single magnetic thin-films. Among these, single magnetic thin-films, especially a CoFeB single layer, offers multiple benefits, such as reduced defect energy, high mobility, and easy integration with existing magnetoresistive random access memory technology. However, optimizing CoFeB-based skyrmion hosting materials remains challenging and requires further systematic and comprehensive investigation. In this study, we examine the effect of atomic-scale interface engineering by inserting a Ta layer between the CoFeB/MgO interface on perpendicular magnetic anisotropy, saturation magnetization, and Dzyaloshinskii–Moriya interaction. Moreover, we provide a guideline for engineering material parameters and demonstrate the validity of atomic-scale interface engineering. Our findings contribute to the development of optimized CoFeB-based skyrmion hosting materials. [ABSTRACT FROM AUTHOR]

We examine magnetic domain wall motion in metallic nanowires Pt-Co-Pt. Regardless of whether the motion is driven by either magnetic fields or current, all experimental data fall onto a single universal curve in the creep regime, implying that both the motions belong to the same universality class. This result is in contrast to the report on magnetic semiconductor (Ga,Mn)As exhibiting two different universality classes. Our finding signals the possible existence of yet other universality classes which go beyond the present understanding of the statistical mechanics of driven interfaces. [ABSTRACT FROM AUTHOR]

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. [Display omitted] [ABSTRACT FROM AUTHOR]

• The spin-transfer-torque-induced ferrimagnetic domain-wall velocity changes its sign across the angular momentum compensation point. • The frequency of the ferrimagnetic domain-wall precession driven by spin-transfer torque exhibits its maximum at the angular momentum compensation point. • As the current density increases, the ferrimagnetic domain-wall velocity starts to deviate from the linear-response result. 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. [ABSTRACT FROM AUTHOR]

Giant magnetoresistance (GMR) signals and the magneto-optic Kerr effect (MOKE) are combined to investigate the asymmetric domain wall (DW) motion in a GMR spin-valve stripe consisting of a wire and a circular ring. In the propagation of a tail-to-tail DW, the left-hand side, top-half ring, bottom-half ring, and the right-hand side are reversed in sequence. However, in the propagation of a head-to-head DW, the left-hand side, bottom half-ring, right-hand side, and top-half ring are switched in sequence. In addition, the critical current density for DW depinning shows asymmetric behavior. For tail-to-tail DW depinning, the critical current density of negative current pulses are lower than that of current pulses in the positive direction, and vice versa for head-to-head DW depinning. [ABSTRACT FROM AUTHOR]