Sang-Cheol Yoo

Magnetic bubblecade memory based on chiral domain walls

Unidirectional motion of magnetic domain walls is the key concept underlying next-generation domain-wall-mediated memory and logic devices.

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.

Emergence of Huge Negative Spin-Transfer Torque in Atomically Thin Co layers

Current-induced domain wall motion has drawn great attention in recent decades as the key operational principle of emerging magnetic memory devices. As the major driving force of the motion, the spin-orbit torque on chiral domain walls has been proposed and is currently extensively studied. However, we demonstrate here that there exists another driving force, which is larger than the spin-orbit torque in atomically thin Co films. Moreover, the direction of the present force is found to be the opposite of the prediction of the standard spin-transfer torque, resulting in the domain wall motion along the current direction. The symmetry of the force and its peculiar dependence on the domain wall structure suggest that the present force is, most likely, attributed to considerable enhancement of a negative nonadiabatic spin-transfer torque in ultranarrow domain walls. Careful measurements of the giant magnetoresistance manifest a negative spin polarization in the atomically thin Co films which might be responsible for the negative spin-transfer torque.

A Method for Compensating the Joule-Heating Effects in Current-Induced Domain Wall Motion

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.

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.

Magnetic bubblecade memory

For next-generation memory and logic devices, there have been suggested many concepts of the unidirectional coherent motion of magnetic domain walls (DWs) [1-3]. Such motion occurs either by injecting large electric currents into nanowires [1] or by employing DW tension induced by sophisticated structural modulation [2, 3]. These schemes, however, require either a high threshold [1] or highly sophisticated nanofabrication processes. Here, we demonstrate a new scheme for unidirectional DW motion without any current injection or structural modulation. This scheme utilizes the recently discovered chiral DWs, due to the Dzyaloshinskii-Moriya interaction, which exhibit asymmetry in their speed with respect to magnetic fields [4]. Because of this asymmetry, an alternating magnetic field results in a coherent motion of the domain walls in one direction. For a proof-of-principle experiment, an arbitrary 5×5 array pattern of bubbles (Fig. 1a) is initially created on the film using the thermomagnetic writing method [5]. Under the application of alternating magnetic pulses, all bubbles exhibit coherent unidirectional motion, as shown by the sequential images (Figs. 1b-d and see Ref. [5]) captured during the pulses. Interestingly, the bubble-array pattern is exactly maintained even after traveling more than 1 mm (Fig. 1d). The observed coherent unidirectional motion of the bubbles possibly replaces the mechanical motion of the magnetic media, enabling a new device prototype - magnetic bubblecade memory - with two-dimensional data-storage capability.