K. Y. Jing

Hong Kong University of Science and Technology

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Supermirrors and spin wave amplifications

Applied Physics Letters (2024)

X. R. Wang X. Gong K. Y. Jing

Superradiance, a phenomenon of multiple facets, normally occurs in classical/quantum physics under extreme conditions. Here, we present its manifestation in spin waves under a practical condition. We show that an interface between a current-free ferromagnetic region and a current-flow ferromagnetic region can be a spin wave supermirror whose reflection coefficient is larger than 1. The super-reflection is the consequence of current-induced spectrum inversion where phase and group velocities of spin waves are in the opposite directions. An incident spin wave activates a backward propagating refractive wave inside the current-flow region. The refractive spin wave re-enters the current-free region to constructively interfere with the reflective wave. It appears that the current-flow region coherently emits reflective waves, leading to a super-reflection. The process resembles superradiance of a spinning black hole through the Hawking radiation process or Dicke superradiance of cavity photons inside population inverted media.

Superradiance

Spin wave

Reflection

10.1063/5.0195063

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Spin wave amplification through superradiance

arXiv (Cornell University) (2023)

X. R. Wang Xin-Gao Gong K. Y. Jing

Superradiance is a phenomenon of multiple facets that occurs in classical and quantum physics under extreme conditions. Here we present its manifestation in spin waves under an easily realized condition. We show that an interface between a current-free (normal) ferromagnetic (FM) region and a current-flow (pumped) FM region can be a spin wave super-mirror whose reflection coefficient is larger than 1. The super-reflection is the consequence of current-induced spectrum inversion where phase and group velocities of spin waves are in the opposite directions. An incident spin wave activates a backward propagating refractive wave inside pumped FM region. The refractive spin wave re-enters the normal FM region to constructively interfere with the reflective wave. It appears that the pumped FM region coherently emits reflective waves, leading to a super-reflection. The process resembles superradiance of a spinning black hole through the Hawking radiation process, or Dicke superradiance of cavity photons inside population inverted media.

Superradiance

Reflection

Spin wave

10.48550/arxiv.2308.00962

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Size and profile of skyrmions in skyrmion crystals

Communications Physics (2021)

Hai-Tao Wu X. C. Hu K. Y. Jing X. R. Wang

Abstract Size is a fundamental quantity of magnetic skyrmions. A magnetic skyrmion can be a local circular object and in an isolated form. A skyrmion can also coexist with a group of its siblings in a condensed phase. Each skyrmion in a condensed phase takes a stripe shape at low skyrmion density and a circular shape at high skyrmion density. Skyrmions at high density form a skyrmion crystal (SkX). So far, skyrmion size in an SkX has not been seriously studied. Here, by using a generic chiral magnetic film, it is found that skyrmion size in an SkX has a different parameter dependence as those for isolated skyrmions and stripes. A size formula and a good spin profile for skyrmions in SkXs are proposed. These findings have important implications in searching for stable smaller skyrmions at the room temperature.

Skyrmion

10.1038/s42005-021-00716-y

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Citations (39)

Skyrmion pinning by disk-shaped defects

Physical review. B./Physical review. B (2022)

X. Gong K. Y. Jing Jie Lü X. R. Wang

High precision skyrmion pinning by an intentionally created magnetic structure is important in skyrmion manipulation. Here, we consider skyrmion pinning by various types of disks. Among other findings, we clarify that, in terms of pinning position and pinning potential landscape, one needs to distinguish thin-wall skyrmions from thick-wall skyrmions because of fundamental differences. A skyrmion wall prefers areas with a weaker exchange stiffness, a larger DMI constant, or a smaller magnetic anisotropy while a skyrmion core experiences only the magnetic anisotropy, not exchange stiffness and DMI. Depending on disk type, skyrmion type, and the relative size of the disk (to skyrmion), a skyrmion can be pinned at the symmetric (center) or asymmetric (off-center) point of a disk. In case a skyrmion is pinned by a local energy minimum, thermal agitation can depin the skyrmion, and the pinning lifetime follows an Arrhenius law. Interestingly, when the disk size is comparable to the skyrmion size, the skyrmion deforms greatly such that the skyrmion will shrink or expand to fill the whole disk. These findings should be important for skyrmion manipulations.

Skyrmion

10.1103/physrevb.105.094437

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Current-driven domain wall motion in ferrimagnetic nanowires

Physical review. B./Physical review. B (2024)

K. Y. Jing Z. Z. Sun X. R. Wang

Current-driven DW motion in a ferrimagnetic (FiM) nanowire through spin transfer torques (STTs) is explained by the energy-work principle. An adiabatic STT (a-STT) can be incorporated into the Lagrangian as an energy functional and tends to twist DW planes. DWs in homogeneous FiM nanowires can resist an a-STT from moving below a critical value proportional to the maximal transverse anisotropic field. Second, a nonadiabatic STT (na-STT) cannot be included in the Lagrangian and can enter the spin dynamics through the Rayleigh functional. A static DW cannot exist under a na-STT such that a DW in a homogeneous FiM nanowire must propagate under an arbitrarily small na-STT. Third, STTs do positive work on a DW which must compensate by energy dissipated by moving spins inside the DW through damping. Below the Walker breakdown current, the na-STT does positive work and the a-STT does no work. Above the Walker breakdown, a DW starts to precess around the wire axis during its propagation along the axis. Whether the a-STT does a negative or a positive work depends on the direction of the DW precession, while the na-STT still does a positive work. Last, a DW velocity formula is obtained, which agrees with simulations both below and above the Walker breakdown current. In the vicinity of the angular momentum compensation point, the precession frequency of a DW reaches its maximum, and the DW structure is distorted. The DW distortion and spin wave emission modify the DW motion, which deviates from its linear dependence on current density. This theory explains well the observed DW mobility near the angular momentum compensation point of FiM nanowires and resolves the puzzle of the unphysical negative na-STT problem.

Ferrimagnetism

Precession

10.1103/physrevb.110.054414

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Skyrmion size in skyrmion crystals

arXiv (Cornell University) (2021)

H. T. Wu X. C. Hu K. Y. Jing X. R. Wang

A magnetic skyrmion is a topological object that can exist as a solitary embedded in the vast ferromagnetic phase, or coexists with a group of its "siblings" in various stripy phases as well as skyrmion crystals (SkXs). Isolated skyrmions and skyrmions in an SkX are circular while a skyrmion in other phases is a stripe of various forms. Unexpectedly, the sizes of the three different types of skyrmions depend on material parameters differently. For chiral magnetic films with exchange stiffness constant $A$, the Dzyaloshinskii-Moriya interaction (DMI) strength $D$, and perpendicular magnetic anisotropy $K$, $\kappa\equiv\pi^2D^2/(16AK)=1$ separates isolated skyrmions from condensed skyrmion states. In contrast to isolated skyrmions whose size increases with $D/K$ and is insensitive to $\kappa\ll1$ and stripe skyrmions whose width increases with $A/D$ and is insensitive to $\kappa\gg1$, the size of skyrmions in SkXs is inversely proportional to the square root of skyrmion number density and decreases with $A/D$. This finding has important implications in our search for stable smaller skyrmions at the room temperature in applications.

Skyrmion

10.48550/arxiv.2103.13036

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A Theory of Skyrmion Size in Skyrmion Crystals

Research Square (Research Square) (2021)

X. R. Wang Hao Wu X. C. Hu K. Y. Jing

Abstract A magnetic skyrmion is a topological object that can exist as a solitary embedded in the vast ferromagnetic phase, or coexists with a group of its "siblings" in various stripy phases as well as skyrmion crystals (SkXs). Isolated skyrmions and skyrmions in an SkX are circular while a skyrmion in other phases is a stripe of various forms. Unexpectedly, the sizes of the three different class of skyrmions depend on material parameters differently. For chiral magnetic films with exchange stiffness constant A, the Dzyaloshinskii-Moriya interaction (DMI) strength D, and perpendicular magnetic anisotropy K, κ=π 2 D 2 ⁄(16 AK )=1 separates isolated skyrmions from condensed skyrmion states. In contrast to isolated skyrmions whose size increases with D⁄K and is insensitive to κ<<1 and stripe skyrmions whose width increases with A⁄D and is insensitive to κ>>1, the size of skyrmions in SkXs is inversely proportional to the square root of skyrmion number density and decreases with A⁄D. This finding has important implications in searching for stable smaller skyrmions at the room temperature.

Skyrmion

10.21203/rs.3.rs-373836/v1

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Random walk of antiferromagnetic skyrmions in granular films

Physical review. B./Physical review. B (2021)

K. Y. Jing C. Wang X. R. Wang

The impressive absence of the skyrmion Hall effect of antiferromagnetic (AFM) skyrmions in clean systems has raised high expectations of AFM skyrmions as information carriers and the potential applications in data storage technology. Here, we report an undesired behavior of current-driven AFM skyrmion dynamics in granular thin films. Understanding the motion of AFM skyrmions in disordered systems is crucial because disorders are inevitable in all materials. Micromagnetic simulations show that AFM skyrmions deviate from their trajectory in the clean limit. Interestingly, the transverse (to the current direction) motion is diffusive, i.e., the mean square of transverse displacement is proportional to the traveling time. The longitudinal (along with the current direction) motion is always hindered by disorders. An effective theory based on the stochastic Thiele equation can account for the observed phenomena. At a very strong disorder above a critical value that depends on the current density, AFM skyrmions are pinned. Our findings should be important for future spintronic devices based on AFM skyrmions.

Skyrmion

Mean squared displacement

10.1103/physrevb.103.174430

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Field-driven domain wall motion in ferrimagnetic nanowires

Physical review. B./Physical review. B (2022)

K. Y. Jing X. Gong X. R. Wang

Magnetic domain wall (DW) propagation in nanowires is an important problem whose applications require stable high speeds that are very difficult to achieve with ferromagnets due to the existence of the Walker breakdown, which separates a rigid-body motion from an oscillatory motion. One of the recent fascinating discoveries in magnetism is the high-speed DW motion along ferrimagnetic (FiM) nanowires near the angular momentum compensation point (AMCP). A clear understanding of this fascinating phenomenon is still lacking. Here we use the energy conservation principle and generic FiM dynamics to reveal the physics behind the DW motion and the absence of the Walker breakdown and precessional torque at the AMCP. An almost exact DW velocity formula beyond the Walker breakdown field is obtained that agrees with experiments and simulations. This theory provides useful guidance for the design of high-speed spintronic DW devices.

Ferrimagnetism

Domain wall (magnetism)

10.1103/physrevb.106.174429

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A theoretical study on the statics and dynamics of magnetic domain walls and skyrmions

K. Y. Jing

This thesis conducts a comprehensive theoretical study and numerical simulations on the statics and dynamics of magnetic domain walls (DWs) and skyrmions, two prominent examples of topological solitons in magnetic materials. For DWs, the focus is on their motion in ferrimagnetic nanowires, driven by external magnetic fields or spin-polarized currents. The research addresses challenges like the Walker breakdown, which limits DW speed. A significant finding is that high-speed DW motion occurs near the angular momentum compensation point (AMCP). The study uses energy conservation principles to explain DW dynamics, proving that static DWs cannot exist under uniform external fields and deriving a velocity formula consistent with experimental results. In the skyrmion section, the thesis inv...[ Read more ]

Skyrmion

Statics

Dynamics

Domain wall (magnetism)

10.14711/thesis-991013350259303412

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