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Sug-Bong Choe

Publications and source records attributed to Sug-Bong Choe.

At least 19 recordsLinked to original sources

Emergence of Giant Magnetic Chirality during Dimensionality Crossover of Magnetic Materials

Chirality, an intrinsic preference for a specific handedness, is a fundamental characteristic observed in nature. In magnetism, magnetic chirality arises from the anti-symmetric Dzyaloshinskii-Moriya interaction in competition with the symmetric Heisenberg exchange interaction. Traditionally, the anti-symmetric interaction has been considered minor relative to the symmetric interaction. In this study, we demonstrate an observation of giant magnetic chirality during the dimensionality crossover of magnetic materials from three-dimensional to two-dimensional. The ratio between the anti-symmetric and symmetric interactions exhibits a reversal in their dominance over this crossover, overturning the traditional consideration. This observation is validated theoretically using a non-local interaction model and tight-binding calculation with distinct pairing schemes for each exchange interaction throughout the crossover. Additional experiments investigating the asphericity of orbital moments corroborate the robustness of our findings. Our findings highlight the critical role of dimensionality in shaping magnetic chirality and offer strategies for engineering chiral magnet states with unprecedented strength, desired for the design of spintronic materials.

cond-mat.mtrl-sci

Magnetic Domain Wall Motion due to AC Bias-Driven Resonances

Most of the existing researches on the dynamics of a domain wall (DW) have focused on the effect of DC biases, where the induced velocity is determined by the bias strength. Here we show that AC biases such as a field or a current are also able to move a DW via synchronization between the DW angle and the phase of the AC bias. The resulting DW velocity is proportional to the driving frequency of the AC bias, but independent of the bias strength, offering potentially low-power operations of DW devices. The AC-bias-driven DW motion is shown to exhibit a phase locking-unlocking transition, a critical phenomenon akin to the Walker breakdown of a DC-bias-driven DW motion. Our work shows that a DW can be driven resonantly by synchronizing its angle to AC biases, shedding a light on hitherto overlooked utility of internal degree of freedom for driving magnetic textures.

cond-mat.mes-hall

Quantitative agreement of Dzyaloshinskii-Moriya interactions for domain-wall motion and spin-wave propagation

The magnetic exchange interaction is the one of the key factors governing the basic characteristics of magnetic systems. Unlike the symmetric nature of the Heisenberg exchange interaction, the interfacial Dzyaloshinskii-Moriya interaction (DMI) generates an antisymmetric exchange interaction which offers challenging opportunities in spintronics with intriguing antisymmetric phenomena. The role of the DMI, however, is still being debated, largely because distinct strengths of DMI have been measured for different magnetic objects, particularly chiral magnetic domain walls (DWs) and non-reciprocal spin waves (SWs). In this paper, we show that, after careful data analysis, both the DWs and SWs experience the same strength of DMI. This was confirmed by spin-torque efficiency measurement for the DWs, and Brillouin light scattering measurement for the SWs. This observation, therefore, indicates the unique role of the DMI on the magnetic DW and SW dynamics and also guarantees the compatibility of several DMI-measurement schemes recently proposed.

cond-mat.mtrl-sci

Vanishing skyrmion Hall effect at the angular momentum compensation temperature of a ferrimagnet

Charged particles exhibit the Hall effect in the presence of magnetic fields. Analogously, ferromagnetic skyrmions with non-zero topological charges and finite fictitious magnetic fields exhibit the skyrmion Hall effect, which is detrimental for applications. The skyrmion Hall effect has been theoretically predicted to vanish for antiferromagnetic skyrmions because the fictitious magnetic field, proportional to net spin density, is zero. We experimentally confirm this prediction by observing current-driven transverse elongation of pinned ferrimagnetic bubbles. Remarkably, the skyrmion Hall effect, estimated with the angle between the current and bubble elongation directions, vanishes at the angular momentum compensation temperature where the net spin density vanishes. This study establishes a direct connection between the fictitious magnetic field and spin density, offering a pathway towards the realization of skyrmionic devices.

cond-mat.mes-hall

Correlation between Compensation Temperatures of Magnetization and Angular Momentum in GdFeCo Ferrimagnets

Determining the angular momentum compensation temperature of ferrimagnets is an important step towards ferrimagnetic spintronics, but is not generally easy to achieve it experimentally. We propose a way to estimate the angular momentum compensation temperature of ferrimagnets. We find a linear relation between the compensation temperatures of the magnetization and angular momentum in GdFeCo ferrimagnetic materials, which is proved by theoretically as well as experimentally. The linearity comes from the power-law criticality and is governed by the Curie temperature and the Landé g factors of the elements composing the ferrimagnets. Therefore, measuring the magnetization compensation temperature and the Curie temperature, which are easily assessable experimentally, enables to estimate the angular momentum compensation temperature of ferrimagnets. Our study provides efficient avenues into an exciting world of ferrimagnetic spintronics.

cond-mat.mtrl-sci

Empirical correlation between the interfacial Dzyaloshinskii Moriya interaction and work function in metallic magnetic trilayers

The Dzyaloshinskii Moriya interaction (DMI) generates intriguing chiral magnetic objects such as magnetic skyrmions and chiral domain walls that can be used as building blocks in emerging magnetic nanodevices. To achieve better stability and functionality of these chiral objects, it is essential to achieve a larger DMI. In this paper, we report an experimental observation that in magnetic trilayer films, the DMI strength is mainly determined by the work functions of the nonmagnetic layers interfaced with the magnetic layer. The clear correlation with the intrinsic material parameters provides a guideline for material selection to engineer the DMI strength.

cond-mat.mtrl-sci

Magnetic Domain-Wall Tilting due to Domain-Wall Speed Asymmetry

Chiral magnetic materials provide a number of challenging issues such as the highly efficient domain wall (DW) and skyrmion motions driven by electric current, as of the operation principles of emerging spintronic devices. The DWs in the chiral materials exhibit asymmetric DW speed variation under application of in plane magnetic field. Here, we show that such DW speed asymmetry causes the DW tilting during the motion along wire structure. It has been known that the DW tilting can be induced by the direct Zeeman interaction of the DW magnetization under application of in plane magnetic field. However, our experimental observations manifests that there exists another dominant process with the DW speed asymmetry caused by either the Dzyaloshinskii Moriya interaction (DMI) or the chirality dependent DW speed variation. A theoretical model based on the DW geometry reveals that the DW tilting is initiated by the DW pinning at wire edges and then, the direction of the DW tilting is determined by the DW speed asymmetry, as confirmed by a numerical simulation. The present observation reveals the decisive role of the DW pinning with the DW speed asymmetry, which determines the DW geometry and consequently, the dynamics.

cond-mat.mtrl-sci

Chirality-induced Antisymmetry in Magnetic Domain-Wall Speed

In chiral magnetic materials, numerous intriguing phenomena such as built in chiral magnetic domain walls (DWs) and skyrmions are generated by the Dzyaloshinskii Moriya interaction (DMI). The DMI also results in asymmetric DW speed under in plane magnetic field, which provides a useful scheme to measure the DMI strengths. However, recent findings of additional asymmetries such as chiral damping have disenabled unambiguous DMI determination and the underlying mechanism of overall asymmetries becomes under debate. By extracting the DMI-induced symmetric contribution, here we experimentally investigated the nature of the additional asymmetry. The results revealed that the additional asymmetry has a truly antisymmetric nature with the typical behavior governed by the DW chirality. In addition, the antisymmetric contribution changes the DW speed more than 100 times, which cannot be solely explained by the chiral damping scenario. By calibrating such antisymmetric contributions, experimental inaccuracies can be largely removed, enabling again the DMI measurement scheme.

cond-mat.mtrl-sci

Drastic emergence of huge negative spin-transfer torque in atomically thin Co layers

Current-induced domain wall (DW) motion has drawn great attention in the last decades as the key operational principle of emerging magnetic memory devices. As the major driving force of the current-induced DW motion, the spin-orbit torque (SOT) on chiral DWs has been proposed and extensively studied nowadays. However, we demonstrate here that there exists another driving force, which is larger than the SOT in ultra-thin Co films. Moreover, the direction of the present force is found to be opposite to the prediction of the spin-transfer torque (STT), resulting in the DW motion along the current direction. The symmetry of the force and its peculiar dependence on the DW structure suggest that the present force is, most likely, attributed to considerable enhancement of the nonadiabatic STT with a negative spin polarization in atomically thin Co layers. These findings open a new pathway to enhance the overall spin torque efficiency-the critical parameter in emerging spintronic devices.

cond-mat.mtrl-sci

Angle-dependence and optimal design for magnetic bubblecade with maximum speed

Unidirectional magnetic domain-wall motion is a key concept underlying next-generation application devices. Such motion has been recently demonstrated by applying an alternating magnetic field, resulting in the coherent unidirectional motion of magnetic bubbles. Here we report the optimal configuration of applied magnetic field for the magnetic bubblecade, the coherent unidirectional motion of magnetic bubbles, driven by a tilted alternating magnetic field. The tilted alternating magnetic field induces asymmetric expansion and shrinkage of the magnetic bubbles under the influence of the Dzyaloshinskii-Moriya interaction, resulting in continuous shift of the bubbles in time. By realizing the magnetic bubblecade in PtCoPt films, we find that the bubblecade speed is sensitive to the tilt angle with a maximum at an angle, which can be explained well by a simple analytical form within the context of the domain-wall creep theory. A simplified analytic formula for the angle for maximum speed is then given as a function of the amplitude of the alternating magnetic field. The present observation provides a practical design rule for memory and logic devices based on the magnetic bubblecade.

cond-mat.mtrl-sci

Wide-Range Probing of Dzyaloshinskii-Moriya Interaction

Dzyaloshinskii-Moriya interaction (DMI) in magnetic objects is of enormous interest, because it generates a built-in chirality of magnetic domain walls (DWs) and topologically-protected skyrmions for efficient motion driven by spin-orbit torques. Because of its importance for perspective applications and academic curiosities, many experimental efforts have been devoted to DMI investigation. However, current experimental probing techniques cover only limited ranges of the DMI with specific sample requirements, and there are no versatile techniques covering a wide range of DMI. Here, we present a unique experimental scheme to quantify DMI over a wide range based on the angular dependence of asymmetric DW motion. It can determine DMI even larger than the maximum magnetic field strength, demonstrating that various strengths of DMI can be quantified using a single measurement setup. This scheme provides a standard technique over a wide range of DMI, which is essential to DMI-related emerging fields in nanotechnology.

cond-mat.mtrl-sci

Universality of Dzyaloshinskii-Moriya interaction effect over domain-wall creep and flow regimes

Chirality causes diverse phenomena in nature such as the formation of biological molecules, antimatters, non-collinear spin structures, and magnetic skyrmions. The chirality in magnetic materials is often caused by the noncollinear exchange interaction, called the Dzyaloshinskii-Moriya interaction (DMI). The DMI produces topological spin alignments such as the magnetic skyrmions and chiral domain walls (DWs). In the chiral DWs, the DMI generates an effective magnetic field $H_{DMI}$, resulting in a peculiar DW speed variation in the DW creep regime. However, the role of $H_{DMI}$ over the different DW-dynamics regimes remains elusive, particularly due to recent observation of distinct behaviors between the creep and flow regimes. We hereby demonstrate experimentally that the role of $H_{DMI}$ is invariant over the creep and flow regimes. In the experiments, the pure DMI effect is quantified by decomposing the symmetric and antisymmetric contributions of the DW motion. The results manifest that the antisymmetric contribution vanishes gradually across the creep and flow regimes, revealing that the symmetric contribution from $H_{DMI}$ is unchanged. Though the DW dynamics is governed by distinct mechanisms, the present observation demonstrates the uniqueness of the DMI effect on the DWs over the creep and flow regimes.

cond-mat.mtrl-sci

Skyrmion Motion Driven by Oscillating Magnetic Field

Magnetic skyrmion motion induced by an electric current has drawn much interest because of its application potential in next-generation magnetic memory devices. Recently, unidirectional skyrmion motion driven by an oscillating magnetic field was also demonstrated on large (20 micrometer) bubble domains with skyrmion topology. At smaller length scale which is more relevant to high-density memory devices, we here show by numerical simulation that a skyrmion of a few tens of nanometers could also be driven by high-frequency field oscillations but with the motion direction different from the tilted oscillating field direction. We found that high-frequency field for small size skyrmions could excite skyrmion resonant modes and that a combination of different modes would result in the 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.

cond-mat.mtrl-sci

Magnetic Bubblecade Memory

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.

cond-mat.mtrl-sci

Asymmetric Magnetic Domain-Wall Motion by the Dzyaloshinskii-Moriya Interaction

We demonstrate here that ultrathin ferromagnetic Pt/Co/Pt films with perpendicular magnetic anisotropy exhibit a sizeable Dzyaloshinskii-Moriya interaction (DMI) effect. Such a DMI effect modifies the domain-wall (DW) energy density and consequently, results in an asymmetric DW expansion driven by an out-of-plane magnetic field under an in-plane magnetic field bias. From an analysis of the asymmetry, the DMI effect is estimated to be strong enough for the DW to remain in the Néel-type configuration in contrast to the general expectations of these materials. Our findings emphasize the critical role of the DMI effect on the DW dynamics as the underlying physics of the asymmetries that are often observed in spin-transfer-related phenomena.

cond-mat.mtrl-sci

Magnetic domain wall motion in a nanowire: depinning and creep

The domain wall motion in a magnetic nanowire is examined theoretically in the regime where the domain wall driving force is weak and its competition against disorders is assisted by thermal agitations. Two types of driving forces are considered; magnetic field and current. While the field induces the domain wall motion through the Zeeman energy, the current induces the domain wall motion by generating the spin transfer torque, of which effects in this regime remain controversial. The spin transfer torque has two mutually orthogonal vector components, the adiabatic spin transfer torque and the nonadiabatic spin transfer torque. We investigate separate effects of the two components on the domain wall depinning rate in one-dimensional systems and on the domain wall creep velocity in two-dimensional systems, both below the Walker breakdown threshold. In addition to the leading order contribution coming from the field and/or the nonadiabatic spin transfer torque, we find that the adiabatic spin transfer torque generates corrections, which can be of relevance for an unambiguous analysis of experimental results. For instance, it is demonstrated that the neglect of the corrections in experimental analysis may lead to incorrect evaluation of the nonadiabaticity parameter. Effects of the Rashba spin-orbit coupling on the domain wall motion are also analyzed.

cond-mat.mes-hall

Electric Control of Multiple Domain Walls in Pt/Co/Pt Nanotrack with Perpendicular Magnetic Anisotropy

Electric control of multiple domain walls (DWs) motion is demonstrated by Pt/Co/Pt nanotracks with perpendicular magnetic anisotropy. Due to the weak microstructural disorders with small DW propagation field, the purely current-driven DW motion is achieved in the creep regime at current densities less than 10^7 A/cm^2 at room temperature. It is confirmed that by use of a scanning magneto-optical Kerr effect microscope, several DWs are simultaneously and identically displaced by the same distance in the same direction. Utilizing such DWs motion, we succeed to realize random bits writing and transferring as a device prototype of four-bit shift registers.

cond-mat.mes-hall

Universality between current- and field-driven domain wall dynamics in ferromagnetic nanowires

Spin-polarized electric current exerts torque on local magnetic spins, resulting in magnetic domain-wall (DW) motion in ferromagnetic nanowires. Such current-driven DW motion opens great opportunities toward next-generation magnetic devices controlled by current instead of magnetic field. However, the nature of the current-driven DW motion--considered qualitatively different from magnetic-field-driven DW motion--remains yet unclear mainly due to the painfully high operation current densities J_OP, which introduce uncontrollable experimental artefacts with serious Joule heating. It is also crucial to reduce J_OP for practical device operation. By use of metallic Pt/Co/Pt nanowires with perpendicular magnetic anisotropy, here we demonstrate DW motion at current densities down to the range of 10^9 A/m^2--two orders smaller than existing reports. Surprisingly the current-driven motion exhibits a scaling behaviour identical to the field-driven motion and thus, belongs to the same universality class despite their qualitative differences. Moreover all DW motions driven by either current or field (or by both) collapse onto a single curve, signalling the unification of the two driving mechanisms. The unified law manifests non-vanishing current efficiency at low current densities down to the practical level, applicable to emerging magnetic nanodevices.

cond-mat.mtrl-sci