arXiv subjects

H. M. Dong

Publications and source records attributed to H. M. Dong.

7 recordsLinked to original sources

arXiv2026

Symmetry-Tunable Skyrmions and Merons in Magnetic Nanodisks via Spatially Engineered Anisotropy

We demonstrate that spatially engineered magnetic anisotropy can stabilize skyrmion and meron spin textures in magnetic nanodisks even in the absence of Dzyaloshinskii-Moriya interaction (DMI). Using a constrained analytical model and micromagnetic simulations, we show that competing perpendicular and in-plane anisotropies can generate non-collinear topological textures in non-chiral magnetic systems. We further show that DMI and dipolar interactions lift the helicity degeneracy and select preferred chiral configurations; micromagnetic simulations were used to identify physically stable states. These results establish anisotropy-patterned nanodisks as a platform for studying DMI-free topological spin textures and their controllable magnetic response. We also show that arrays of anisotropy-engineered skyrmions can control spin-wave transmission by manipulating their vorticity arrangement, pointing to reconfigurable magnonic elements based on non-chiral topological textures.

cond-mat.mes-hall↗

arXiv2026

Reversible and irreversible dynamical topological transitions of magnetic Hopfions

Magnetic Hopfions are three-dimensional (3D) topological solitons characterized by a nonzero Hopf invariant and offer a promising platform for 3D spintronics. While their static stabilization has been widely studied, their nonlinear dynamics under alternating magnetic (AM) fields remain largely unexplored. We show, using 3D micromagnetic simulations and analytical mode analysis, that an AM field drives two qualitatively distinct dynamical regimes of a confined magnetic Hopfion. In the weak-field regime, resonant excitation of intrinsic Hopfion modes induces a nonlinear instability and an irreversible topological reconfiguration from a Hopfion to a toron. In contrast, in the strong-field regime, the system undergoes reversible field-locked topological switching at GHz frequencies, with the magnetization periodically alternating between a topologically trivial ferromagnetic configuration and a Hopfion state. The switching pathway is selected by the driving frequency: a 2 GHz field drives a breathing pathway associated with the low-frequency collective response, whereas a 40 GHz field produces a nonresonant rotational pathway governed by strong Zeeman-torque-driven precession and field locking. These results identify field amplitude and frequency as independent control knobs and reveal that reversible Hopfion switching can arise either from nonlinear continuation of low-frequency collective motion or from nonresonant high-frequency field locking.

cond-mat.other↗

arXiv2026

Geometric symmetry and size-dependent skyrmion phase transitions in magnetic nanostructures

We investigate the interplay of geometric symmetry, size, and external magnetic fields in regulating individual skyrmion states within magnetic nanostructures. By analyzing nanodisks, nanosquares, and nanorectangles, we demonstrate that rotational symmetry in nanodisks enables rich topological phase transitions, from ferromagnetic states to skyrmions, skyrmioniums, and multi-states, as their diameter increases. In contrast, square and rectangular structures exhibit suppressed topological complexity due to corner-induced demagnetization effects and reduced symmetries. Under perpendicular magnetic fields, nanodisks show field-driven transitions between skyrmionium and skyrmion states. By leveraging asymmetry, square and rectangular nanostructures stabilize skyrmions over a broader parameter range than nanodisks. These findings highlight geometric symmetry as a critical design parameter for tailoring skyrmion stability and functionality in spintronic applications such as multi-state memory and reconfigurable logic devices.

cond-mat.mes-hall↗

arXiv2025

Magnetic field-tuned size and dual annihilation pathways of chiral magnetic bobbers

Magnetic chiral bobbers (CBs) are three-dimensional (3D) topological spin textures that consist of a tapered skyrmion tube terminating in a Bloch point, promising applications in high-density spintronics. However, the mechanisms controlling their size and the dynamics of their annihilation are still not fully understood. In this study, we present an analytical model that predicts the radius $R$ of the CB as a function of the external magnetic field, the Dzyaloshinskii-Moriya interaction (DMI), the magnetic anisotropy, and the exchange interaction. The micromagnetic simulations validate this model across a broad range of parameters. We also identify two mechanisms of annihilation of CBs: (i) a droplet-like instability that occurs under rapid changes in the magnetic field, which we describe using a proposed magnetic Weber number $We$ and its critical field step scaling; and (ii) Bloch point depinning mechanism at interfaces, for which we determine the threshold magnetic field $B_{\text{th}}$ for annihilation. Importantly, we uncover a novel fragmentation pathway in which CBs transform into skyrmion tubes, then into half-CBs, and finally into ferromagnetic states. These findings lay the groundwork for understanding and manipulating 3D CBs as next-generation devices.

cond-mat.mtrl-sci↗

arXiv2016

Exciton states in a circular graphene quantum dot: magnetic field induced intravalley to intervalley transition

The magnetic-field dependence of the energy spectrum, wave function, binding energy and oscillator strength of exciton states confined in a circular graphene quantum dot (CGQD) are obtained within the configuration interaction (CI) method. We predict that: (1) excitonic effects are very significant in the CGQD as a consequence of a combination of geometric confinement, magnetic confinement and reduced screening; (2) two types of excitons (intravalley and intervalley excitons) are present in the CGQD because of the valley degree of freedom in graphene; (3) the intravalley and intervalley exciton states display different magnetic-field dependencies due to the different electron-hole symmetries of the single-particle energy spectra; (4) with increasing magnetic field, the exciton ground state in the CGQD undergoes an intravalley to intervalley transition accompanied by a change of angular momentum; (5) the exciton binding energy does not increase monotonically with the magnetic field due to the competition between geometric and magnetic confinements; and (6) the optical transitions of the intervalley and intravalley excitons can be tuned by the magnetic field and valley-dependent excitonic transitions can be realized in CGQD.

cond-mat.mes-hall↗

arXiv2016

Ultrafast terahertz responses in monolayer graphene

We theoretically investigate the ultrafast terahertz(THz) properties of monolayer graphene. The analytical formulations of the photon carrier, electric polarization and optical current are obtained by solving the Bloch-equations in present of the ultrafast THz Gaussian pulse. Graphene shows a large nonlinear and ultrafast optical response at THz frequencies due to the gapless and relativistic Dirac particles with nearly linear energy dispersion. It is found that the photon carrier density, electric polarization and optical current density increase with increasing the frequency of the THz pulse. These theoretical results are in agreement with recent experimental findings. This study confirms further that graphene exhibits important features and is relevant to the applications in the ultrafast THz fields.

cond-mat.mes-hall↗

arXiv2011

Plasmon and coupled plasmon-phonon modes in graphene in the presence of inter-band excitation channels

Plasmon and coupled plasmon-phonon modes in graphene are investigated the-oretically within the diagrammatic self-consistent field theory. It shows that two plasmon modes and four coupled plasmon-phonon modes can be excited via intra-and inter-band transition channels. It is found that with increasing q and carrier density, the plasmon modes couple strongly with the optic-phonon modes in graphene. The coupled plasmon-phonon modes exhibit some interesting features which can be utilized to realize the plasmonic devices. Our results suggest that the carrier-phonon interaction should be considered to understand and explain the properties of elementary electronic excitations in graphene.

cond-mat.mes-hall↗