Myoung-Woo Yoo

Radial-spin-wave-mode-assisted vortex-core magnetization reversals

The dynamic behaviors of vortex-core magnetization reversals in soft magnetic nanodisks driven by oscillating magnetic fields applied perpendicularly to the disk plane were studied by means of micromagnetic numerical simulations. It was found that when the field frequencies are tuned to the eigenfrequencies of radial spin-wave modes, the threshold field amplitudes required for vortex-core switching are an order of magnitude smaller than those of static perpendicular fields. The reversal mechanism and associated underlying physics are completely different from those of vortex-antivortex-pair-mediated core reversals. The results reflect the achievement of an alternative efficient means of ultrafast vortex-core switching.

Out-of-plane current controlled switching of the fourfold degenerate state of a magnetic vortex in soft magnetic nanodots

We report on an observation of transitions of the fourfold degenerate state of a magnetic vortex in soft magnetic nanodots by micromagnetic numerical calculations. The quaternary vortex states in patterned magnetic dots were found to be controllable by changing the density of out-of-plane dc or pulse currents applied to the dots. Each vortex state can be switched to any of the other states by applying different sequence combinations of individual single-step pulse currents. Each step pulse has a characteristic threshold current density and direction. This work offers a promising way for manipulating both the polarization and chirality of magnetic vortices.

Current-driven skyrmion dynamics in disordered films

A theoretical study of the current-driven dynamics of magnetic skyrmions in disordered perpendicularly magnetized ultrathin films is presented. The disorder is simulated as a granular structure, in which the local anisotropy varies randomly from grain to grain. The skyrmion velocity is computed for different disorder parameters and ensembles. Similar behavior is seen for spin-torques due to in-plane currents and the spin Hall effect, where a pinning regime can be identified at low currents with a transition towards the disorder-free case at higher currents, similar to domain wall motion in disordered films. Moreover, a current-dependent skyrmion Hall effect and fluctuations in the core radius are found, which result from the interaction with the pinning potential.

Perpendicular-bias-field-dependent vortex-gyration eigenfrequency

We found that the angular frequency ω0 of vortex-core gyrations is controllable by the application of static perpendicular bias fields Hp as studied by micromagnetic simulations and Thiele’s-approach-based quantitative interpretation. The observed linear dependence of ω0 on Hp could be explained in terms of the dynamic variables of the vortex, the gyrovector constant G, and the potential stiffness constant κ, for cases of negligible damping. Here we calculated the values of G and κ as a function of Hp directly from the simulation numerical data using Thiele’s equivalent force equations, providing a more correct understanding of the remarkable change of ω0 with Hp. This micromagnetic-simulation-based quantitative analysis is a straightforward, accurate, and effective means of understanding vortex dynamics in nanoscale magnetic elements.

Polarization-selective vortex-core switching by tailored orthogonal Gaussian-pulse currents

Polarization-selective vortex-core switching by tailored orthogonal Gaussian-pulse currents Young-Sang Yu, 1 Ki-Suk Lee, 1 Hyunsung Jung, 1 Youn-Seok Choi, 1 Myoung-Woo Yoo, 1 Dong-Soo Han, 1 Mi-Young Im, 2 Peter Fischer, 2 and Sang-Koog Kim 1,2* National Creative Research Initiative Center for Spin Dynamics & Spin-Wave Devices, and Nanospinics Laboratory, Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA We experimentally demonstrate low-power-consumption vortex-core switching in magnetic nanodisks using tailored rotating magnetic fields produced with orthogonal and unipolar Gaussian-pulse currents. The optimal width of the orthogonal pulses and their time delay are found, from analytical and micromagnetic numerical calculations, to be determined only by the angular eigenfrequency  D for a given vortex-state disk of polarization p, such that   1  D and  t   p  D . The estimated optimal pulse parameters are in good agreement with the experimental results. This work lays a foundation for energy-efficient information recording in vortex-core cross-point architecture.

Edge-soliton-mediated vortex-core reversal dynamics.

We report an additional reversal mechanism of magnetic vortex cores in nanodot elements driven by currents flowing perpendicular to the sample plane, occurring via dynamic transformations between two coupled edge solitons and bulk vortex solitons. This mechanism differs completely from the well-known switching process mediated by the creation and annihilation of vortex-antivortex pairs in terms of the associated topological solitons, energies, and spin-wave emissions. Strongly localized out-of-plane gyrotropic fields induced by the fast motion of the coupled edge solitons enable a magnetization dip that plays a crucial role in the formation of the reversed core magnetization. This work provides a deeper physical insight into the dynamic transformations of magnetic topological solitons in nanoelements.

Resonantly excited precession motion of three-dimensional vortex core in magnetic nanospheres [corrected].

We found resonantly excited precession motions of a three-dimensional vortex core in soft magnetic nanospheres and controllable precession frequency with the sphere diameter 2R, as studied by micromagnetic numerical and analytical calculations. The precession angular frequency for an applied static field HDC is given as ωMV = γeffHDC, where γeff = γ〈mΓ〉 is the effective gyromagnetic ratio in collective vortex dynamics, with the gyromagnetic ratio γ and the average magnetization component 〈mΓ〉 of the ground-state vortex in the core direction. Fitting to the micromagnetic simulation data for 〈mΓ〉 yields a simple explicit form of 〈mΓ〉 ≈ (73.6 ± 3.4)(lex/2R)2.20±0.14, where lex is the exchange length of a given material. This dynamic behavior might serve as a foundation for potential bio-applications of size-specific resonant excitation of magnetic vortex-state nanoparticles, for example, magnetic particle resonance imaging.

Resonant amplification of vortex-core oscillations by coherent magnetic-field pulses

Vortex structures in soft magnetic nanodisks are highly attractive due to their scientific beauty and potential technological applications. Here, we experimentally demonstrated the resonant amplification of vortex oscillations by application of simple coherent field pulses tuned to optimal width and time intervals. In order to investigate vortex excitations on the sub-ns time scale, we employed state-of-the-art time-resolved full-field soft X-ray microscopy of 70 ps temporal and 25 nm lateral resolution. We found that, due to the resonant enhancement of the vortex gyration motion, the signal input power can be significantly reduced to ~ 1 Oe in field strength, while increasing signal gains, by increasing the number of the optimal field pulses. We identified the origin of this behavior as the forced resonant amplification of vortex gyration. This work represents an important milestone towards the potential implementation of vortex oscillations in future magnetic vortex devices.

Injectable intraocular lens using hydrogels

Presbyopia is an age-related condition that begins to develop in humans around the age of forty, resulting in anatomical and physiological changes in the choroids, ciliary body, vitreous, lens capsule, and lens fibers. A suitable biomaterial is needed for the implantation of intraocular lenses (IOL) that could be used to treat the symptoms of presbyopia in people everywhere. In this paper, the characteristics of silicone polymer, polyether, poly acrylic derivatives, polyalcohol, poly(vinyl pyrrolidone) (PVP), and poly(acryl amide) hydrogels were explained, and then their biocompatibilities were compared. These types of hydrogel materials have several advantages over the current fixed-focus IOL including restoration of accommodation, minimal corneascleral trauma, improved physiological position of the IOL, and possible use in the surgical correction of cataracts.

Current-driven skyrmion expulsion from magnetic nanostrips

We study the current-driven skyrmion expulsion from magnetic nanostrips using micromagnetic simulations and analytic calculations. We explore the threshold current density for the skyrmion expulsion, and show that this threshold is determined by the critical boundary force as well as the spin-torque parameters. Using a simple model describing the skyrmion and locally-tilted edge magnetization, we reveal the underlying physics of the dependence of the critical boundary force on the magnetic parameters. This work provides a fundamental understanding of the skyrmion expulsion and the interaction between the skymion and boundaries of devices.