External Magnetic Field Research Articles

The role of the external toroidal magnetic field on the large-angle rotation of magnetic flux ropes

It is accepted that magnetic flux ropes might exhibit large-angle rotation in magnetic configurations, including strong external toroidal magnetic fields. The specific mechanisms leading to rotation still remain elusive, however. We examine the mechanisms by which the external toroidal magnetic field facilitates the flux-rope rotation, and we explore potential alternative rotation mechanisms beyond the effects of sheared fields and kink instability. We performed three-dimensional magnetohydrodynamic simulations to model the eruption of magnetic flux ropes in a magnetic configuration with and without external toroidal magnetic fields. We compared the morphology, the dynamics, the magnetic topology, and the Lorentz force acting on the flux rope. The behavior of flux ropes in two simulations is significantly different. The flux rope that evolved without external toroidal fields successfully erupted was accompanied by minor rotation, and the flux rope that evolved with an external toroidal field was confined by large-angle rotation. The evolution of the magnetic topology, the Lorentz force, and the torque showed distinctive patterns in each scenario. Furthermore, the flux ropes in both simulations displayed observable signs of a drifting footpoint. We reveal that external toroidal magnetic fields facilitate the flux-rope rotation, which in turn releases initial twists and amplifies the lateral Lorentz force exerted on the flux rope. The slipping magnetic reconnection between the flux-rope field lines and sheared-arcade field lines can contribute to the rotation, as does the lateral Lorentz force and the kink instability, which is determined by the initial twist number. Moreover, our result suggests that the initial twist of the flux rope cannot determine the rotation angle. The rotation angle of the flux rope does not strictly increase with the initial twist. The rapid release of the twist number necessitates external toroidal magnetic fields.

Tunable Group Delay of Reflected Beam in Multilayered Structures with Antisymmetric Graphene via Magnetic Control

In this paper, we demonstrate the magnetic field-adjustable group delay of the reflected beam in the terahertz frequency range, using a multilayer architecture incorporating three layers of antisymmetric graphene. The observed enhancement in group delay results from localized field amplification, which occurs due to the excitation of surface plasmon polaritons on the graphene at the interface between two dielectric layers. By considering the quantum mechanical response of graphene, the polarity of the group delay can be reversed by exploiting the antisymmetric conductivity characteristics of graphene. Furthermore, the group delay can be dynamically modulated either by varying the external magnetic field or by adjusting the structural parameters. The achieved enhancement and tunability of the group delay offer significant potential for the development of graphene-based terahertz modulation devices and other optical delay applications.

Study and optimization on hyperthermia performance of magnetic fluids modeled by coupled Brownian–Néel rotations

Superparamagnetic nanoparticles (SMNPs) with nonlinear magnetic behavior under alternating magnetic fields hold great potential in biomedical applications, including magnetic hyperthermia, biosensing, magnetic separation, magnetic particle imaging, etc. Magnetic hyperthermia therapy, based on the relaxation movement of SMNPs subjected to an alternating magnetic field, is a promising modality for tumor treatment. Herein, we applied the stochastic Langevin equation to study the magnetic hyperthermia performance of SMNPs by taking the coupled Brownian–Néel rotations into account. The specific absorption rate (SAR) of magnetic fluids is used as a parameter for optimizing the amplitude and frequency of alternating magnetic fields and guiding the design of SMNPs. Specifically, by accounting for the dipole–dipole interactions between SMNPs, it is revealed that these interactions significantly suppress the relaxation behavior of particles, thereby reducing SAR. Furthermore, this study systematically examines the effects of key factors such as particle concentration, particle size, and magnetic anisotropy constant on SAR. The accurate prediction of the rotational and magnetic dynamics of SMNPs under an oscillating magnetic field provides valuable theoretical insights and technical support for the optimized design of external magnetic field systems and the precise fabrication of SMNPs.

Zeeman effect in nuclei

The antiferromagnetic properties of nuclei are studied within the Wigner function moments method. The solution of the time dependent Hartree–Fock–Bogoliubov equations predicts four low-lying [Formula: see text] states. Three of them are known as various scissors modes. Fourth state is disposed below all scissors modes and represents one of three branches of [Formula: see text] multiplet which can exist in spherical nuclei and which is split in deformed nuclei. It is discovered, that the antiferromagnetic properties of nuclei lead to the splitting of [Formula: see text] states already at the zero deformation. It is shown that the splitting caused by nuclear antiferromagnetism is comparable to the Zeeman splitting in an external uniform magnetic field.

Electromagnetic Nanocoils Based on InGaN Nanorings

Energy issues, including energy generation, conversion, transmission and detection, are fundamental factors in all systems. In micro- and nanosystems, dealing with these energy issues requires novel nanostructures and precise technology. However, both concept and setup are not well established yet in the microsystems, especially for those at the nanometer scale. Here, we demonstrate electromagnetic nanocoils with 100 nm diameters based on uniform and periodic InGaN nanoring arrays grown on patterned GaN surfaces using nanoscale selective area epitaxy (NSAE). We observed stronger photoluminescence from the periodic InGaN nanoring arrays compared to the non-uniform InGaN nanorings, which indicates good crystal quality of the InGaN nanostructure with the NSAE. Based on this kind of nanostructure, electromagnetic induction from the nanorings is detected through the rebound movement of high-energy electron diffraction patterns that are influenced by a modulated external magnetic field. Our results clearly show the generation of an inductive current and internal magnetic field in the nanorings. We anticipate this kind of nanostructure to be a potential key element for energy conversion, transfer and detection in nanosystems. For example, it could be used to fabricate microtransformers and micro- and nanosensors for electromagnetic signals.

Quadratic optical response to a magnetic field in the layered magnet CrSBr

The optical properties of layered materials are dominated by intralayer excitons; especially for layered antiferromagnets the layer-to-layer charge hopping, and therefore interlayer excitons, are spin forbidden. An external magnetic field, however, can continuously drive the magnetic order towards layer-to-layer ferromagnetic, which opens spin-allowed charge-transfer channels between the layers. Here, we elaborate how their admixture changes the composition and nature of the excitons, leading to an extension over many layers, and causing a quadratic redshift with respect to the external magnetic field in CrSBr. We present a minimal four-band model to elucidate the interplay between the various interaction and coupling mechanisms which is able to reproduce the findings of our GW–Bethe-Salpeter equation calculations as a function of magnetic field. Our model is generally valid for any coupled layers with different spin directions and the insights help to systematically address excitons and to predict their optical signatures in such systems. Published by the American Physical Society 2025

Single-particle spectral function of fractional quantum anomalous Hall states

Fractional quantum Hall states are the most prominent example of states with topological order, hosting excitations with fractionalized charge. Recent experiments in twisted MoTe2 and graphene-based heterostructures provided evidence of fractional quantum anomalous Hall (FQAH) states, which spontaneously break time-reversal symmetry and persist even without an external magnetic field. Understanding the unique properties of these states requires the characterization of their low-energy excitations. To that end, we construct a parton theory for the energy- and momentum-resolved single-particle spectral function of FQAH states. We explicitly consider several experimentally observed filling fractions as well as a composite Fermi liquid in the half-filled Chern band. Charge fractionalization manifests itself in nearly momentum-independent spectra with a characteristic series of peaks determined from the filling fraction. The parton description qualitatively captures our numerical exact diagonalization results. Additionally, we discuss how the finite bandwidth of the Chern band and the nonideal quantum geometry affect the fractionalized excitations. Our work demonstrates that the energy- and momentum-resolved electronic single-particle spectral function provides a valuable tool to characterize fractionalized excitations of FQAH states in moiré lattices. Published by the American Physical Society 2025

Roll‐to‐Roll (R2R) Processing of Large‐Area Flexible Piezoresistive Films with Magnetically Z‐Aligned Nickel Microcolumnar Morphology

AbstractFlexible pressure sensors are highly sought after for applications like rehabilitation healthcare, advanced wearable electronics, automotive safety, etc. This work demonstrates a scalable method for fabricating functional piezoresistive films using external magnetic fields to engineer the columnar microstructures. The continuous roll‐to‐roll processing involves directed self‐assembly of ferromagnetic Nickel particles within a flexible elastomer to achieve quasi 1–3 composites with low particle concentrations. In this process, polymer precursor‐microparticle system is subjected to a spatiotemporal evolution of temperature and magnetic fields simultaneously. Offline techniques like in‐plane alignment, through‐thickness light transmission, and viscosity evolution determine the influence of transient process conditions on final column morphology. Real‐time visualization in the X‐Y plane quantified by image analysis allowed tracking of morphological evolution where columns are formed separated by particle depletion zones leading to optical transparency. This characteristic is used to determine the kinetics of column formation via out‐of‐plane light transmission during Z‐alignment. Isothermal and non‐isothermal rheokinetics identified the gel point required to “freeze” the column structures. The scalability of the process is demonstrated by producing a 5.18 m long and 15.24 cm wide piezoresistive film with columns in thickness direction. The piezoresistive performance of produced anisotropic films is demonstrated under continuous uniaxial loading‐unloading cycles.

Multimodal Splitting and Reciprocating Transport of Droplets on a Reprogrammable Functional Surface.

Droplet manipulations have important applications in many fields, especially droplet splitting and transport in aseptic operations or biochemical reagent analysis. However, droplet splitting or transport on existing functional surfaces is limited to predesigned microstructures or fixed patterns. It remains a challenge to realize reprogrammable surface microstructures for freely controllable droplet splitting and transport. In this study, a flexible technique for both the multimodal splitting and reciprocating transport of droplets on one surface is proposed. Such a surface is prepared with a facile fabrication method by premixing magnetic particles and softener into the polymer solvent matrix and immersing the solidified matrix in a lubricant. The movable wettability gradient is generated on the surface by an external magnetic field, which can act as an invisible "air knife" to split the droplet in multiple modes. The mechanism and critical conditions of droplet splitting are analyzed and revealed theoretically. Furthermore, the microstructural configurations and surface wettability can be reprogrammed by modulating the magnetic field strength and gradient. Accordingly, the splitting behavior of the droplet is transformed into the reciprocating transport behavior. The influencing factors of such behavior have also been analyzed. The reported reprogrammable manipulation of the droplet on one surface provides a versatile prototype for the actuation of droplets in microfluidic and biological analysis devices.

Automated Fabrication of 3D Printed Magnetic Soft Robots With Programmable 3D Magnetizations

Magnetic soft robots (MSRs) are a viable tool for many biomedical applications, such as targeted drug delivery and minimally invasive surgery, since they can be actuated remotely using external magnetic fields. These robots are developed by programming ferromagnetic domains with specific magnetizations using magnetic particles embedded in a flexible substrate. Existing fabrication methods rely on partially automated or manual processes, which limit production rates and realistic design iterations. To address these challenges, a fully automated workflow that translates robot simulations into an instruction set for a stereolithography 3D printer is presented. In this process, a rotating permanent magnet is used to program 3D magnetizations by reorienting hard magnetic particles within a photosensitive resin. Geometric resolutions of 1.6 mm are achieved with a layer height of 0.1 mm, enabling the creation of structures 14 layers thick. Beam bending tests identify an optimal 6:1 resin‐to‐magnetic particle mass ratio, yielding a maximum deflection angle of 80°. Demonstrated applications include rolling and climbing locomotion in a maze and independent control of each arm in a multiarmed robot. By enabling fast, repeatable production of MSRs within 30 min, this automated system shortens the feedback loop from design to application, advancing their potential as a biomedical tool.

A Method for Sensing the Internal Current Distribution State of Press-Pack IGBT Devices Based on Hall Sensors

Flexible DC transmission technology is an important support for constructing new power systems. The press-pack IGBT device has the advantages of double-sided heat dissipation and high power density in series and is the core device in the flexible HVDC project. However, the compact parallel arrangement of multiple chips in the press-pack IGBT (PPI) device easily causes uneven current distribution in the device, which affects its reliability. In this paper, a noninvasive sensing method for the internal current distribution of the PPI device is proposed based on the measurement of the external magnetic field and the numerical analysis of the magnetic field. According to the correlation between the device current distribution and the external magnetic field, a noninvasive current distribution sensing method based on the measurement and numerical analysis of the external magnetic field is proposed, and the placement strategy of the magnetic field measurement points is given. The error of the perception method under ideal conditions is analyzed, the error size and rule are summarized, and the correction methods are given, respectively. Finally, a magnetic field measuring device is designed and verified by experiments. The results can accurately indicate the direction of pressure deviation.

Numerical Simulation of Natural Convection in a Chamfered Square Cavity with Fe3O4-Water Nanofluid and Magnetic Excitation

This study delves into the numerical exploration of the MagnetoHydroDynamic (MHD) characteristics of an Fe3O4-Water nanofluid contained within a chamfered square enclosure under the influence of an external magnetic field. The enclosure, characterized by distinct hot and cold imposed temperatures on its side walls, features both straight and chamfered sections. The orientation of magnetic field lines was manipulated by varying the angular placement of the magnetic source. The computational framework for nanofluid dynamics is mathematically formalized through a dimensionless formulation of the Navier-Stokes equations derived from their dimensional counterparts. A comprehensive numerical analysis was conducted employing the Finite Element (FE) method, a. The interaction between the Hartmann number and the angular placement of the magnetic source was analyzed, with a specific focus on nanofluid isotherms, temperature profiles, and velocity magnitude distributions. The results were thoroughly investigated and extensively discussed.

N-Alkylation of Amines Utilizing Magnetic Nano Chitosan Functionalized with EDTA/Co(II)

This research work has explored the application of a magnetic catalyst composed of coreshell nanoparticles [Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>@CS@EDTA/Co(II)], known as NCM@EDTA/Co(II), in the conversion of various alcohols containing electron-donating or electron-withdrawing groups into their respective secondary or primary amine derivatives. The investigation has focused on optimizing reaction conditions by considering factors, such as the inclusion of a base, duration of reaction time, reaction temperature, catalyst quantity, and choice of transition metal, in order to determine the optimal parameters. The most favorable outcomes have been achieved by using 0.2 mmol of catalyst per 1 mmol of substrate under reflux conditions for a duration ranging from 3 to 24 hours. The reaction has demonstrated high efficiency, with the catalyst's easy separation via an external magnetic field, stability, and recyclability, highlighting its potential applications in chemistry and industrial environments.

Non-Keplerian Charged Accretion Disk Orbiting a Black Hole Pulsar

Recent studies have focused on how spinning black holes (BHs) within a binary system containing a strongly magnetized neutron star, then immersed in external magnetic fields, can acquire charge through mechanisms like the Wald process and how this charge could power pulsar-like electromagnetic radiation. Those objects called “Black hole pulsar” mimic the behaviour of a traditional pulsar, and they can generate electromagnetic fields, such as magnetic dipoles. Charged particles within an accretion disk around the black hole would then be influenced not only by the gravitational forces but also by electromagnetic forces, leading to different geometries and dynamics. In this context, we focus here on the interplay of the magnetic dipole and the accretion disk. We construct the equilibrium structures of non-conducting charged perfect fluids orbiting Kerr black holes under the influence of a dipole magnetic field aligned with the rotation axis of the BH. The dynamics of the accretion disk in such a system are shaped by a complex interplay between the non-uniform, non-Keplerian angular momentum distribution, the black hole’s induced magnetic dipole, and the fluid’s charge. We show how these factors jointly influence key properties of the disk, such as its geometry, aspect ratio, size, and rest mass density.

Recent progress in magnetic covalent organic framework materials for the enrichment and detection of typical organic pollutants

Trace contaminants are toxic and their widespread presence in the environment potentially threatens human health. The levels of these pollutants are often difficult to determine directly using instruments owing to the complexities of environment matrices. Hence, pretreatment steps, such as sample purification and concentration, are key along with various processes that enhance the accuracy and sensitivity of the detection method. To date, researchers have successfully developed a variety of efficient and reliable sample-pretreatment techniques that are based on different principles. Among these, magnetic solid-phase extraction (MSPE) is a rapid and efficient sample-pretreatment technology that is based on the similar solid-phase-extraction (SPE) principle, which mainly enriches target analytes by exploiting their interactions with functional groups on the surfaces of magnetic materials, thereby achieving rapid separation when an external magnetic field is applied. MSPE has been a focus of attention in the environmental-sample-pretreatment, food-analysis, and other fields owing to advantages that include ease of operation, low cost, and high enrichment efficiency. The selection of the magnetic material is key to MSPE process because traditional magnetic materials exhibit certain functionality limitations. Accordingly, designing and synthesizing green and efficient functionalized magnetic materials have become a research focus in this field, with researchers having extensively explored multiple ways of preparing functionalized modified magnetic materials through the introduction of a variety of emerging materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), carbon nanotubes (CNTs), graphene oxide (GO), and other specific functional groups to modify magnetic materials and effectively expanded the applications scope of MSPE. Among these, COFs are porous crystalline materials consisting of light elements (C, N, H, O and B, etc.) linked through covalent bonds. COFs are mainly classified as imine COFs, boronic-acid COFs, triazine COFs, and ketenimine COFs according to bonding type. Moreover, it is worth mentioning that COFs can be synthesized from a number of monomers, and the functional groups exposed on the COF surface can also be used for further modification purposes. COFs are versatile and modifiable; consequently, they have attracted significant research attention, with a series of COF-functionalized magnetic materials having been designed and synthesized. The magnetic COFs (MCOFs) combine the advantages of COFs and magnetic materials. MCOFs are not only endowed with the large specific surface areas, high porosities, and good stabilities that are characteristic of COFs, but also exhibit the rapid separation and reusability characteristics of magnetic material, thereby quickly and efficiently enriching targets through hydrogen bonding, hydrophobicity, π-π stacking, and van der Waals forces, making them ideal sample-pretreatment materials. MCOFs have also been converted into more-versatile functional materials using post-modification strategies. Combining MCOFs with advanced analytical techniques, such as high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) has effectively improved the limits of detection (LODs) for various analytes as well as method accuracy; these techniques have been widely used to enrich and detect trace pollutants. However, some material design and synthesis challenges remain and need to be overcome, despite the promising potential of MCOFs. Future research needs to focus on exploring novel synthetic strategies that reduce experimental costs and improve the functionalities of MCOFs while expanding their applicabilities to broader sample matrices. In this review, we first introduce and discuss the construction and functionalized designs of various MCOF composites, and then summarize their use in applications that include the enrichment and detection of pesticides, endocrine disruptors (EDCs), pharmaceuticals, and personal care products (PPCPs), and finally provide an outlook on future developmental prospects.

Migration of magnetic microparticles through a liquid–liquid interface under an external magnetic field

Liquid–liquid interfaces play a pivotal role in various microfluidic processes involving microparticles, including coating, dissolution, controlled release of polyelectrolytes or drugs, and self-assembly processes. In all of these cases, noninvasive techniques to manipulate the microparticle transport are essential. Magnetic manipulation offers an accessible and straightforward means of controlling the motion of magnetic particles within microfluidic devices. Magnetic microparticles are commonly used for conformal polyelectrolyte coating and drug encapsulation by passing them through a liquid–liquid interface due to their high saturation magnetization, stability, and low toxicity. In this work, we draw inspiration from the lack of studies on the behavior of magnetic particles near a liquid–liquid interface under conditions of low Reynolds numbers and high capillary action, despite its engineering relevance in microfluidic systems. We consider a canonical flow configuration in which particle motion is driven by the stagnation-point flow that is generated when two different liquids flow toward one another. We show how the operating conditions dictate whether the particle will pierce the interface and become coated or not and illustrate this via parameter-space plots. We use the results of this analysis to understand how the operating conditions influence the fraction of particles that pass through the liquid–liquid interface and are conformally coated, which may be used to guide a variety of industrial processes.

Enhancing the Combustion Performance of Solid Propellants by External Magnetic Field

Enhancing the Combustion Performance of Solid Propellants by External Magnetic Field

Dynamics and energy dissipation of collisional blast waves in a perpendicular magnetic field

We experimentally investigate the evolution and dynamics of laser-produced collisional blast waves (BW) under the influence of a perpendicular magnetic field up to 20 T. We show that an external magnetic field causes the BW to diverge from the Taylor–Sedov solution while also impacting its structural morphology. We notably explore the significance of various magnetohydrodynamic (MHD) processes occurring on scales similar to the width of the BW front by comparing their characteristic lengths to it and demonstrate that the downstream plasma's transition from being super- to sub-magnetosonic plays a pivotal role in the overall structure. Our results show that multiple MHD effects can contribute to shaping a magnetized BW, illustrating the complexity of the underlying physics.

Anisotropy of free-surface wave turbulence induced by a horizontal magnetic (electric) field

This work investigates the effects of anisotropy of free-surface wave turbulence under the influence of an external horizontal magnetic (electric) field based on direct numerical simulation technique. As the field increases, the surface wave system passes into the magnetohydrodynamic (MHD) turbulence regime with the formation of anisotropic condensate of capillary waves slowly propagating perpendicular to the external field. In the high field regime, the effect of the formation of discrete levels of frequency broadening in the space–time spectrum of capillary waves is reported for the first time. The performed correlation analysis showed that the resonance curve of the three-wave interaction is compressed with increasing field. This process leads to a non-local energy transfer from large-scale MHD waves to the small-scale capillary waves. Thus, discrete frequency levels in the spectrum of capillary waves arise as a result of the resonant decay of a MHD wave into secondary harmonics. Despite the anisotropy, the angle-averaged spectrum of surface elevations agrees well with the analytical estimate obtained under the assumption of isotropic wave propagation. We explain this contradiction by the fact that magnetic energy fluctuations on the liquid surface are distributed quasi-isotropically, and the anisotropic coherent condensate of capillary waves has a weak effect on the turbulence spectrum.

Interaction of upper hybrid waves with dust-ion-magnetoacoustic waves and stable two-dimensional solitons in dusty plasmas

We obtain a two-dimensional (2D) nonlinear system of equations for the electrostatic potential envelope and the low-frequency magnetic field perturbation to describe the interaction of the upper hybrid wave propagating perpendicular to an external magnetic field with the dust-ion-magnetoacoustic (DIMA) wave in a magnetized dusty plasma. The equations contain both scalar and vector nonlinearities. A nonlinear dispersion relation is derived, and the decay and modulation instability thresholds and growth rates are obtained. Numerical estimates show that instability thresholds can easily be exceeded in real dusty plasmas. In the static (subsonic) approximation, a 2D soliton solution (ground state) is found numerically by the generalized Petviashvili relaxation method. The perturbations of the magnetic field and plasma density in the soliton are nonmonotonic in space, and, along with the perturbation in the form of a well, there are also perturbation humps. Such peculiar radial soliton profiles differ significantly from previously known results on 2D solitons. The key point is that the presence of a gap in the DIMA wave dispersion due to the Rao cutoff frequency causes the nonlinearity to be nonlocal. We show that due to nonlocal nonlinearity the Hamiltonian is bounded below at fixed energy, proving the stability of the ground state.