arXiv subjects

H. W. Zhang

Publications and source records attributed to H. W. Zhang.

At least 19 recordsLinked to original sources

arXiv2025

Stabilization and Re-excitation of Sawtooth Oscillations due to Energetic Particles in Tokamaks

Sawtooth oscillations, driven by internal kink modes (IKMs), are fundamental phenomena in tokamak plasmas. They can be classified into different types, including normal sawteeth, small sawteeth, and in some cases, evolving into the steady-island state, each having a different impact on energy confinement in fusion reactors. This study investigates the interaction between sawtooth oscillations and energetic particles (EPs) using the initial-value MHD-kinetic hybrid code CLT-K, which can perform long-term self-consistent nonlinear simulations. We analyze the redistribution of EPs caused by sawtooth crashes and the effect of EPs on sawtooth behavior and type transitions. The results show that co-passing EPs tend to re-excite sawtooth oscillations, extending their period, while counter-passing EPs promote the system evolution toward small sawteeth, potentially leading to the steady-island state. Additionally, we provide a physical picture of how EPs influence sawtooth type through the mechanism of magnetic flux pumping. We demonstrate that the radial residual flow in the core plays a crucial role in determining the reconnection rate and sawtooth type. Moreover, we observe new phenomena about couplings of various instabilities, such as the excitation of global multi-mode toroidal Alfvén eigenmodes (TAEs) due to EP redistribution following a sawtooth crash and the excitation of the resonant tearing mode (r-TM) when injecting counter-passing EPs. The study also explores the impact of EP energy and the safety factor profile on the development of stochastic magnetic fields and EP transport. These findings emphasize the necessity of multi-mode simulations in capturing the complexity of EP-sawtooth interactions and provide insights for optimizing sawtooth control in future reactors such as ITER.

physics.plasm-ph↗

arXiv2025

Effective resistivity for magnetohydrodynamic simulation of collisionless magnetic reconnection

The electron inertia term and the off-diagonal electron pressure terms are well-known for the frozen-in condition breakdown in collisionless magnetic reconnection, which are naturally kinetic and difficult to be employed in magnetohydrodynamic (MHD) simulations. After considering the shortcomings of MHD and Hall MHD in neglecting the important electron dynamics such as the inertia and the nongyrotropic pressure, the kinetic characteristics of electrons and ions in the diffusion region are studied and an effective resistivity model involving dynamics of charged particles is proposed [Z. W. Ma et al. 2018 Sci. Rep. 8 10521]. The amplitude of the effective resistivity is mainly determined by electrons in most realistic situations with large ion-electron mass ratios. In this work, the effective resistivity model for collisionless magnetic reconnection without the guide field is successfully applied in the 2.5D MHD and Hall MHD simulations, which remarkably improves the simulation results compared with traditional MHD models. For the MHD case, the effective resistivity significantly increased the reconnection rate to the reasonable value of ~0.1$B_0v_A$. For the Hall MHD case with effective resistivity, the peak reconnection rate is ~0.25$B_0v_A$, and the major structures of the reconnecting field and the current sheet agree well with the particle-in-cell (PIC) and hybrid simulations.

physics.plasm-ph↗

arXiv2024

Influences of $δ$B Contribution and Parallel Inertial Term of Energetic Particles on MHD-Kinetic Hybrid Simulations: A Case Study of the 1/1 Internal Kink Mode

The magnetohydrodynamic-kinetic (MHD-kinetic) hybrid model [Park et. al., 1992] has been widely applied in studying energetic particles (EPs) problems in fusion plasmas for past decades. The pressure-coupling scheme or the current-coupling scheme is adopted in this model. However, two noteworthy issues arise in the model application: firstly, the coupled term introduced in the pressure-coupling scheme, $\left( \nabla \cdot \mathbf{P}_{\mathrm{h}} \right)_{\bot}$, is often simplified by $\nabla \cdot \mathbf{P}_{\mathrm{h}}$, which is equivalent to neglecting the parallel inertial term of EPs; secondly, besides the $δf $ contribution caused by changing in the EP distribution function, the magnetic field perturbation (the $δ\mathbf{B} $ contribution) generated during development of the instabilities should also be considered, but it is often ignored in existing hybrid simulations. In this paper, we derive the analytical formulations under these two coupling schemes and then numerically study the representative case of the linear stability of the m/n=1/1 internal kink mode (IKM) [Fu et. al., 2006] by using the CLT-K code. It is found that the approximated models can still yield reasonable results when EPs are isotopically distributed. But it fails completely in cases with anisotropic EP distributions. In addition, we further investigate the influence of EP's orbit width on the stability of IKM and verify the equivalence between pressure-coupling scheme and the current-coupling scheme.

physics.plasm-ph↗

arXiv2020

Robust edge states in magnetic domain-wall racetrack

Controllable artificial pinning is indispensable in numerous domain-wall (DW) devices, such as memory, sensor, logic gate, and neuromorphic computing hardware. The high-accuracy determination of the effective spring constant of the pinning potential, however, remains challenging, because the extrinsic pinning is often mixed up with intrinsic ones caused by materials defects and randomness. Here, we study the collective dynamics of interacting DWs in a racetrack with pinning sites of alternate distances. By mapping the governing equations of DW motion to the Su-Schrieffer-Heeger model and evaluating the quantized Zak phase, we predict two topologically distinct phases in the racetrack. Robust edge state emerges at either one or both ends depending on the parity of the DW number and the ratio of alternating intersite lengths. We show that the in-gap DW oscillation frequency has a fixed value which depends only on the geometrical shape of the pinning notch, and is insensitive to device imperfections and inhomogeneities. We propose to accurately quantify the spring coefficient that equals the square of the robust DW frequency multiplied by its constant mass. Our findings suggest as well that the DW racetrack is an ideal platform to study the topological phase transition.

cond-mat.mes-hall↗

arXiv2020

Twisted magnon as a magnetic tweezer

Wave fields with spiral phase dislocations carrying orbital angular momentum (OAM) have been realized in many branches of physics, such as for photons, sound waves, electron beams, and neutrons. However, the OAM states of magnons (spin waves)$-$the building block of modern magnetism$-$and particularly their implications have yet to be addressed. Here, we theoretically investigate the twisted spin-wave generation and propagation in magnetic nanocylinders. The OAM nature of magnons is uncovered by showing that the spin-wave eigenmode is also the eigenstate of the OAM operator in the confined geometry. Inspired by optical tweezers, we predict an exotic "magnetic tweezer" effect by showing skyrmion gyrations under twisted magnons in exchange coupled nanocylinder$|$nanodisk heterostructure, as a practical demonstration of magnonic OAM to manipulate topological spin defects. Our study paves the way for the emerging magnetic manipulations by harnessing the OAM degree of freedom of magnons.

cond-mat.mes-hall↗

arXiv2020

Evolution of safety factor profiles in sawteeth

Two different definitions of the safety factor are applied to investigate the evolution of the safety factor profile during normal sawteeth, the stationary state, and the incomplete reconnection. It is found that the safety factor profiles from the old definition are sometimes inconsistent with the Poincare plots of the magnetic field during sawteeth. The old safety factor always indicates that the safety factor around the magnetic axis is flattened and equal to 1.0 with the development of the kink instability. However, the Poincare plots of the magnetic field lines indicate that the topology of the magnetic field around the magnetic axis has not been changed. To solve the inconsistency, we propose a new definition of the safety factor, in which the poloidal angle relative to the new twisted magnetic axis is used instead of the poloidal angle to the original axis. With the new definition, the safety factor profiles are consistent with Poincare plots of the magnetic field. We also find that the safety factor profiles are significantly different from the two different q definitions. With the new q definition, the safety factor at the magnetic axis q0 remains unchanged in almost the entire period of a sawtooth and jumps up to 1.0 near the end during normal sawteeth; in the non-axisymmetric equilibrium, q0 is still far below 1.0; q0 remains its initial value throughout the incomplete reconnection.

physics.plasm-ph↗

arXiv2019

Acceleration of three-dimensional Tokamak magnetohydrodynamical code with graphics processing unit and OpenACC heterogeneous parallel programming

In this paper, the OpenACC heterogeneous parallel programming model is successfully applied to modification and acceleration of the three-dimensional Tokamak magnetohydrodynamical code (CLTx). Through combination of OpenACC and MPI technologies, CLTx is further parallelized by using multiple-GPUs. Significant speedup ratios are achieved on NVIDIA TITAN Xp and TITAN V GPUs, respectively, with very few modifications of CLTx. Furthermore, the validity of the double precision calculations on the above-mentioned two graphics cards has also been strictly verified with m/n=2/1 resistive tearing mode instability in Tokamak.

physics.comp-ph↗

arXiv2017

Reconfigurable topological spin wave beamsplitters and interferometers

Conventional magnonic devices use three classes of magnetostatic waves that require detailed manipulation of magnetization structure that makes the design and the device/circuitry scalability difficult tasks. Here, we demonstrate that devices based on topological exchange spin waves do not suffer from the problem with additional nice features of nano-scale wavelength and high frequency. Two results are reported. 1) A perpendicular ferromagnet on a honeycomb lattice is generically a topological magnetic material in the sense that topologically protected chiral edge spin waves exist in the band gap as long as spin-orbit induced nearest-neighbor pseudodipolar interaction (and/or next-nearest neighbor Dzyaloshinskii-Moriya interaction) is present. 2) As a proof of concept, spin wave beamsplitters and spin wave interferometers are designed by using domain walls to manipulate the propagation of topologically protected chiral spin waves. Since magnetic domain walls can be controlled by magnetic fields or electric current/fields, one can essentially draw, erase and redraw different spin wave devices and circuitry on the same magnetic plate so that the proposed devices are reconfigurable and tunable. Devices made from magnetic topological materials are robust against both internal and external perturbations such as the spin wave frequency variation and device geometry as well as defects.

cond-mat.mes-hall↗

arXiv2017

Lateral transport properties of thermally excited magnons in yttrium iron garnet films

Spin information carried by magnons is attractive for computing technology and the development of magnon-based computing circuits is of great interest. However, magnon transport in insulators has been challenging, different from the clear physical picture for spin transport in conductors. Here we investigate the lateral transport properties of thermally excited magnons in yttrium iron garnet (YIG), a model magnetic insulator. Polarity reversals of detected spins in non-local geometry devices have been experimentally observed and are strongly dependent on temperature, YIG film thickness, and injector-detector separation distance. A competing two-channel transport model for thermally excited magnons is proposed, which is qualitatively consistent with the spin signal behavior. In addition to the fundamental significance for thermal magnon transport, our work furthers the development of magnonics by creating an easily accessible magnon source with controllable transport

cond-mat.mtrl-sci↗

arXiv2016

Systematic non-LTE study of the $-2.6 \le$ [Fe/H] $\le 0.2$ F and G dwarfs in the solar neighbourhood. II. Abundance patterns from Li to Eu

For the first time, we present an extensive study of stars with individual non-local thermodynamic equilibrium (NLTE) abundances for 17 chemical elements from Li to Eu in a sample of stars uniformly distributed over the $-2.62 \le$ [Fe/H] $\le +0.24$ metallicity range that is suitable for the Galactic chemical evolution research. The star sample has been kinematically selected to trace the Galactic thin and thick disks and halo. We find new and improve earlier results as follows. (i) The element-to-iron ratios for Mg, Si, Ca, and Ti form a MP plateau at a similar height of 0.3~dex, and the knee occurs at common [Fe/H] $\simeq -0.8$. The knee at the same metallicity is observed for [O/Fe], and the MP plateau is formed at [O/Fe] = 0.61. (ii) The upward trend of [C/O] with decreasing metallicity exists at [Fe/H] $< -1.2$, supporting the earlier finding of Akerman et al. (iii) An underabundance of Na relative to Mg in the [Fe/H] $< -1$ stars is nearly constant, with the mean [Na/Mg] $\simeq -0.5$. (iv) The K/Sc, Ca/Sc, and Ti/Sc ratios form well-defined trends, suggesting a common site of the K-Ti production. (v) Sr follows the Fe abundance down to [Fe/H] $\simeq -2.5$, while Zr is enhanced in MP stars. (vi) The comparisons of our results with some widely used Galactic evolution models are given. The use of the NLTE element abundances raises credit to the interpretation of the data in the context of the chemical evolution of the Galaxy.

astro-ph.SR↗

arXiv2016

Magnetostructural martensitic transformations with large volume changes and magneto-strains in all-d-metal Heusler alloys

The all-d-metal Mn2-based Heusler ferromagnetic shape memory alloys Mn50Ni40-xCoxTi10 (x = 8 and 9.5) are realized. With a generic comparison between d-metal Ti and main-group elements in lowering the transformation temperature, the magnetostructural martensitic transformations are established by further introducing Co to produce local ferromagnetic Mn-Co-Mn configurations. A 5-fold modulation and (3, -2) stacking of [00 10] of martensite are determined by XRD and HRTEM analysis. Based on the transformation, a large magneto-strain of 6900 ppm and a large volume change of -2.54% are observed in polycrystalline samples, which makes the all-d-metal magnetic martensitic alloys of interest for magnetic/pressure multi-field driven applications.

cond-mat.mtrl-sci↗

arXiv2015

Dynamic magnetic susceptibility and electrical detection of ferromagnetic resonance

The dynamic magnetic susceptibility of magnetic materials near ferromagnetic resonance (FMR) is very important in interpreting dc-voltage in electrical detection of FMR. Based on the causality principle and the assumption that the usual microwave absorption lineshape around FMR is Lorentzian, general forms of dynamic susceptibility of an arbitrary sample and the corresponding dc-voltage lineshape are obtained. Our main findings are: 1) The dynamic susceptibility is not a Polder tensor for material with arbitrary anisotropy. Two off-diagonal elements are not in general opposite to each other. However, the linear response coefficient of magnetization to total rf field is a Polder tensor. This may explain why two off-diagonal elements are always assumed to be opposite to each other in analyses. 2) The frequency dependence of dynamic susceptibility near FMR is fully characterized by six numbers while its field dependence is fully characterized by seven numbers. 3) A recipe of how to determine these numbers by standard microwave absorption measurements for an arbitrary sample is proposed. Our results allow one to unambiguously separate the contribution of the anisotropic magnetoresistance to dc-voltage from that of the anomalous Hall effect. With these results, one can reliably extract the information of spin pumping and the inverse spin Hall effect, and determine the spin-Hall angle. 4) The field-dependence of susceptibility matrix at a fixed frequency may have several peaks when the effective field is not monotonic of the applied field. In contrast, the frequency-dependence of susceptibility matrix at a fixed field has only one peak. Furthermore, in the case that resonance frequency is not sensitive to the applied field, the field dependence of susceptibility matrix, as well as dc-voltage, may have another non-resonance broad peak. Thus, one should be careful in interpreting observed peaks.

cond-mat.mes-hall↗

arXiv2015

New Galvanomagnetic Effects of Polycrystalline Magnetic Films

Within the linear response of polycrystalline magnetic films to electric currents, a general analysis predicts three new galvanomagnetic effects originated from the two-dimensional nature of the films. These new galvanomagnetic effects, which differ from the conventional extraordinary Hall effect and anisotropic magnetoresistance, are follows. 1) The longitudinal resistivity depends linearly on the magnetization component perpendicular to a film. 2) A current parallel to the magnetization can generate an electric field in the vertical direction of a film. 3) A current perpendicular to a film can generate an electric field along the magnetization direction.

cond-mat.mes-hall↗

arXiv2015

Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases, but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

cond-mat.mtrl-sci↗

arXiv2014

Galvanomagnetic effects and manipulation of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers

In this work, IrMn$_{3}$/insulating-Y$_{3}$Fe$_{5}$O$_{12}$ exchange-biased bilayers are studied. The behavior of the net magnetic moment $Δm_{AFM}$ in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The $Δm_{AFM}$ is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis are induced by the irreversible switching of the $Δm_{AFM}$. In the training effect, the $Δm_{AFM}$ changes continuously. This work highlights the fundamental role of the $Δm_{AFM}$ in the exchange bias and facilitates the manipulation of antiferromagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2014

A coupling of martensitic and metamagnetic transitions with collective magneto-volume and table-like magnetocaloric effects

A coupling of the first-order paramagnetic-to-induced-ferromagnetic martensitic and the second-order antiferromagnetic-to-ferromagnetic metamagnetic transitions was found in MnNi0.8Fe0.2Ge alloy. Based on the coupling, a magneto-volume effect driven by the martensitic transition and a table-like magnetocaloric effect generated by the successive magnetic phase transitions arise collectively. By using the magneto-volume effect, the internal pressure stress in the volume-expansion martensitic transition was determined at 350 MPa. The magnetocaloric effect, with a wide working temperature range of 26 K around room temperature, shows a small hysteresis loss (5 J/kg) and a large net refrigerant capacity (157 J/kg).

cond-mat.mtrl-sci↗

arXiv2013

Giant magnetocaloric effect in isostructural MnNiGe-CoNiGe system by establishing a Curie-temperature window

An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as -40 J kg-1 K-1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1-xCoxNiGe to become a potential magnetic refrigerant.

cond-mat.mtrl-sci↗

arXiv2013

Disorder-induced significant enhancement in magnetization of ball-milled Fe2CrGa alloy

A new disordered atom configuration in Fe2CrGa alloy has been created by ball-milling method. This leads to a significant enhancement of the magnetic moment up to 3.2~3.9 μB and an increase of Curie temperature by about 200 K, compared with the arc-melt samples. Combination of first-principles calculations and experimental results reveals that Fe2CrGa alloy should crystallize in Hg2CuTi based structure with different atomic disorders for the samples prepared by different methods. It is addressed that magnetic interactions play a crucial role for the system to adopt such an atomic configuration which disobeys the empirical rule.

cond-mat.mtrl-sci↗