Kyoung-Woong Moon

Tailoring the topology of an artificial magnetic skyrmion

Despite theoretical predictions, it remains an experimental challenge to realize an artificial magnetic skyrmion whose topology can be well controlled and tailored so that its topological effect can be revealed explicitly in a deformation of the spin textures. Here we report epitaxial magnetic thin films in which an artificial skyrmion is created by embedding a magnetic vortex into an out-of-plane aligned spin environment. By changing the relative orientation between the central vortex core polarity and the surrounding out-of-plane spins, we are able to control and tailor the system between two skyrmion topological states. An in-plane magnetic field is used to annihilate the skyrmion core by converting the central vortex state into a single domain state. Our result shows distinct annihilation behaviour of the skyrmion core for the two different skyrmion states, suggesting a topological effect of the magnetic skyrmions in the core annihilation process.

Detection of the static and kinetic pinning of domain walls in ferromagnetic nanowires

Two distinct pinning mechanisms named as kinetic and static pinning of magnetic domain wall (DW) are experimentally resolved. Both the pinning situations are realized at an artificial notch on U-shaped Permalloy nanowires, depending on the initial DW states, moving or pinned. The kinetic depinning field—a critical field for a moving DW to be trapped at a notch—is revealed to be distinguishably smaller than the static depinning field—a critical field to depin a trapped DW at the notch. Based on one-dimensional collective model, the discrepancy is explained by the tilting angle of the moving DW.

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.

Control of domain wall pinning in ferromagnetic nanowires by magnetic stray fields

We have found that the depinning field of domain walls (DWs) in permalloy (Ni(81)Fe(19)) nanowires can be experimentally controlled by interactions between magnetic stray fields and artificial constrictions. A pinning geometry that consists of a notch and a nanobar is considered, where a DW traveling in the nanowire is pinned by the notch with a nanobar vertical to it. We have found that the direction of magnetization of the nanobar affects the shape and local energy minimum of the potential landscape experienced by the DW; therefore, the pinning strength strongly depends on the interaction of the magnetic stray field from the nanobar with the external pinning force of the notch. The mechanism of this pinning behavior is applied for the instant and flexible control of the pinning strength with respect to various DW motions in DW-mediated magnetic memory devices.

Determination of perpendicular magnetic anisotropy in ultrathin ferromagnetic films by extraordinary Hall voltage measurement

A magnetometric technique for detecting the magnetic anisotropy field of ferromagnetic films is described. The technique is based on the extraordinary Hall voltage measurement with rotating the film under an external magnetic field. By analyzing the angle-dependent Hall voltage based on the Stoner-Wohlfarth theory, the magnetic anisotropy field is uniquely determined. The present technique is pertinent especially for ultrathin films with strong intrinsic signal, in contrast to the conventional magnetometric techniques of which the signal is in proportion to the sample volume and geometry.

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.

Long-Range Domain Wall Tension in Pt/Co/Pt Films with Perpendicular Magnetic Anisotropy

We report an experimental detection of a long-range tension effect on magnetic domain walls in Pt/Co/Pt films. Such a tension effect is observed from circular domains expanding (or shrinking) under a constant applied magnetic field. Interestingly, the domain propagation speed varies with respect to the domain radius. Converting the speed variation to a magnetic field via the creep scaling, it is revealed that there exists an effective field inversely proportional to the radius, in accordance with the domain wall tension and the magnetostatic dipolar interaction. The sizeable tension effect subsists over 20 µm, providing a way to directly determine the wall energy density.

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.

Geometric dependence of static and kinetic pinning of domain walls on ferromagnetic nanowires

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.

Control of magnetic domain-wall polarization by means of angled Oersted field writing

We propose a method to control the polarization of the magnetic domain walls (DWs) in ferromagnetic nanowires. Two neighboring DWs with antiparallel polarization alignment rather than parallel alignment are found to exhibit better stability with a helical magnetic structure that can be hardly be detangled. To achieve such an antiparallel alignment, two co-planar current lines with an angle to the nanowire are designed, from which the Oersted field creates a domain in between the current lines while keeping the polarization of the DWs beneath the current lines, as confirmed by a micromagnetic calculation for ferromagnetic nanowires with perpendicular magnetic anisotropy.