Dae-Yun Kim

Maximizing domain-wall speed via magnetic anisotropy adjustment in Pt/Co/Pt films

We report an experimental observation that indicates that a direct relation exists between the speed of the magnetic domain-wall (DW) motion and the magnitude of the perpendicular magnetic anisotropy (PMA) in Pt/Co/Pt films. It is found that by changing the thicknesses of the nonmagnetic Pt layers, the PMA magnitude can be varied significantly and the field-driven DW speed can also be modified by a factor of up to 50 under the same magnetic field. Interestingly, the DW speed exhibits a clear scaling behavior with respect to the PMA magnitude. A theory based on the DW creep criticality successfully explains the observed scaling exponent between the DW speed and the PMA magnitude. The presented results offer a method of maximizing the DW speed in DW-mediated nanodevices without altering the thickness of the magnetic Co layer.

Determination of magnetic domain-wall types using Dzyaloshinskii–Moriya-interaction-induced domain patterns

The Neel-type domain-wall (DW) configuration caused by the Dzyaloshinskii–Moriya interaction has attracted significant attention because of its crucial role in current-induced DW motion. Here, we propose an experimental technique to determine the DW types (Neel or Bloch) by analyzing the asymmetry in domain expansion patterns. Such asymmetry is caused by the counterbalance between the Dzyaloshinskii–Moriya and Zeeman interactions, which results in the elongation of the domains either longitudinal (for Neel-type DWs) or transverse (for Bloch-type DWs) to the in-plane magnetic field. Therefore, the DW types can be determined simply by examining the elongation axis. The present technique is applicable even to a single image of the domain expansion pattern and provides a rapid determination of the DW configuration for exploring high-efficiency materials for current-induced DW motion in device applications.

Intrinsic asymmetry in chiral domain walls due to the Dzyaloshinskii–Moriya interaction

We present an analytical description of the energy density of chiral magnetic domain walls (DWs) that considers variations in DW width. Surprisingly, under the application of a longitudinal in-plane magnetic field, the DW width varies abnormally, resulting in an asymmetric variation of the DW energy density. Such asymmetry is attributable to the nonlinear contribution to the effective magnetic field from the Dzyaloshinskii–Moriya interaction. The formation of such asymmetric DWs is confirmed by a micromagnetic simulation. The present prediction proposes a possible origin of the experimental asymmetry related to chiral damping.

Free energy approach to the formation of an icosahedral structure during the freezing of gold nanoclusters

The freezing of metal nanoclusters such as gold, silver, and copper exhibits a structural evolution. The formation of the icosahedral (Ih) structure is dominant despite its energetic metastability. The dynamical aspects of the structural transformations, which are eventually responsible for the kinetics, are studied by calculating free energies of gold nanoclusters. The transition barriers have been determined by using the umbrella sampling technique. Our calculations show that the formation of Ih gold nanoclusters is attributed to the lower free energy barrier from the liquid to the Ih phase compared to the barrier from the liquid to the face-centered-cubic crystal phase.

Spin-orbit torque-induced switching in ferrimagnetic alloys: Experiments and modeling

We investigate spin-orbit torque (SOT)-induced switching in rare-earth-transition metal ferrimagnetic alloys using W/CoTb bilayers. The switching current is found to vary continuously with the alloy concentration, and no reduction in the switching current is observed at the magnetic compensation point despite a very large SOT efficiency. A model based on coupled Landau-Lifschitz-Gilbert (LLG) equations shows that the switching current density scales with the effective perpendicular anisotropy which does not exhibit strong reduction at the magnetic compensation, explaining the behavior of the switching current density. This model also suggests that conventional SOT effective field measurements do not allow one to conclude whether the spins are transferred to one sublattice or just simply to the net magnetization. The effective spin Hall angle measurement shows an enhancement of the spin Hall angle with the Tb concentration which suggests an additional SOT contribution from the rare earth Tb atoms.

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.

Wide-Range Probing of Dzyaloshinskii–Moriya Interaction

The Dzyaloshinskii–Moriya interaction (DMI) in magnetic objects is of enormous interest, because it generates built-in chirality of magnetic domain walls (DWs) and topologically protected skyrmions, leading to efficient motion driven by spin–orbit torques. Because of its importance for both potential applications and fundamental research, many experimental efforts have been devoted to DMI investigation. However, current experimental probing techniques cover only limited ranges of the DMI strength and have specific sample requirements. Thus, there are no versatile methods to quantify DMI over a wide range of values. Here, we present such an experimental scheme, which is based on the angular dependence of asymmetric DW motion. This method can be used to determine values of DMI much larger than the maximum strength of the external magnetic field strength, which demonstrates that various DMI strengths can be quantified with a single measurement setup. This scheme may thus prove essential to DMI-related emerging fields in nanotechnology.

Optimal angle of magnetic field for magnetic bubblecade motion

Unidirectional motion of magnetic structures such as the magnetic domain and domain walls is a key concept underlying next-generation memory and logic devices. As a potential candidate of such unidirectional motion, it has been recently demonstrated that the magnetic bubblecade—the coherent unidirectional motion of magnetic bubbles—can be generated by applying an alternating magnetic field. Here we report the optimal configuration of applied magnetic field for the magnetic bubblecade. 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 examining the magnetic bubblecade in Pt/Co/Pt 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 results provide a useful guideline of optimal design for magnetic bubblecade memory and logic devices.

Huge domain-wall speed variation with respect to ferromagnetic layer thickness in ferromagnetic Pt/Co/TiO2/Pt films

In this study, we investigate the influence of the ferromagnetic layer thickness on the magnetization process. A series of ultrathin Pt/Co/TiO2/Pt films exhibits domain-wall (DW) speed variation of over 100,000 times even under the same magnetic field, depending on the ferromagnetic layer thickness. From the creep-scaling analysis, such significant variation is found to be mainly attributable to the thickness-dependence of the creep-scaling constant in accordance with the creep-scaling theory of the linear proportionality between the creep-scaling constant and the ferromagnetic layer thickness. Therefore, a thinner film shows a faster DW speed. The DW roughness also exhibits sensitive dependence on the ferromagnetic layer thickness: a thinner film shows smoother DW. The present observation provided a guide for an optimal design rule of the ferromagnetic layer thickness for better performance of DW-based devices.

Experimental observation of the correlation between the interfacial Dzyaloshinskii–Moriya interaction and work function in metallic magnetic trilayers

AbstractThe 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. Precise control of the DMI strength is one of the key issues for achieving better stability and functionality of these chiral objects. In this paper, we report that in magnetic trilayer films, the DMI strength exhibits a noticeable correlation with the work functions of the non-magnetic layers interfaced to the magnetic layer. This correlation with the intrinsic material parameters provides a guideline for material selection for engineering the DMI strength.We experimentally observe that D, the strength of the Dzyaloshinskii–Moriya interaction (DMI), is correlated with the difference of work function W of Co and nonmagnetic material X in Pt/Co/X trilayer system. The spin–orbit-scattering-mediated spin–chiral effect may plays leading role in this system, and this phenomena may be affected by work function difference which may be linked to electrostatic potential that may affect the scattering potential at the interface. Such correlation with the intrinsic material parameter provides a guideline for material selection to engineer the DMI and helps control properties for applications of chiral objects such as magnetic skyrmion. Magnetic nanodevices: helping more metals get to workA study that correlates magnetic nanostructure formation with an easy-to-measure property of metals may help engineers design better devices for manipulating and storing data. A team led by Byoung-Chul Min at the Korea Institute of Science and Technology in Seoul and Sug-Bong Choe from Seoul National University investigated thin films that sandwich a ferromagnetic cobalt layer between two non-magnetic metals. In this system, magnetic spins can interact with other atoms where the layers contact, and create asymmetric swirling patterns known as skyrmions that are useful in high-speed spin-based computing. Experiments demonstrated that the strength of this magnetic interaction relates directly to the energy it takes to remove electrons from the non-magnetic layers. These energy values may serve as accessible guidelines for selecting materials for use in magnetic thin-film devices.