Nonuniform Magnetic Field Research Articles

Magnetically modulated superconductor-graphene-superconductor (SGS) Josephson junctions and their tunability

Abstract Graphene-based Josephson junctions \cite{Heersche2007, Du2008, Popinciuc2012, Mizuno2013, Efetov2016, bretheau2017, Borzenets2016} played an important role in various quantum devices from their inception. 
 Magnetic tunnel junctions or vertical devices \cite{Cobas2012, Meng2013,Chen2013,Huang2024} were also made out of graphene by exposing the graphene layer to localised pattern of strong magnetic field created by hard ferromagnetic material.
 By combining the essence of these different methods for constructing graphene based junctions, in this work we propose that the temperature-dependent Josephson current in such junctions can be tuned 
 by exposing the graphene regions to a combination of highly localised non-uniform magnetic field, dubbed as magnetic barrier, and spatially modulated gate voltage. Within the framework of Dirac-Bogoliubov-de-Gennes (DBDG) theory, we show by explicit calculation 
 that in such magnetically modulated Josephson Junctions, the band structure of graphene gets significantly altered, which results in the change of the Andreev reflections in such junctions. This leads to a significant modulation of the Josephson current. 
 We numerically evaluated the Josephson current as a function of the strength of the magnetic barrier and the gate voltage and discussed the practical consequences of such controlling of Josephson currents

Synthesis of ferrofluid using magnetite (Fe3O4), citric acid and deionized water and its characterizations for the application of a non-uniform magnetic thermosyphon

The performance improvement of a thermosyphon for disseminating heat is significant for the thermal management of a system. Furthermore, the water as the working liquid in a thermosyphon has an operating limit. Therefore, adding particles in the nanofluids as a working liquid could improve the thermal properties of thermosyphon and boiling temperature. Additionally, the effects of a non-uniform magnetic field on the ferrofluid thermosyphon at three locations: evaporator, adiabatic, and condenser were analysed. In this investigation, a two-step method was utilized, the ferrofluid was synthesized using magnetite (Fe3O4), citric acid, and deionized water. First, the Fe3O4 nanoparticles and associated ferrofluid were fully characterized from X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), magnetic properties analysis, zeta potential, and thermal properties, respectively. Then, the performance of thermosyphon heat pipes filled using high filling ratios of 70, 80, 90, and 100% of stable synthesized ferrofluid at 0.5wt% were examined. The results were compared to water using similar filling ratios that have been experimentally evaluated. The thermosyphon heat pipe (copper container) has a diameter of 12.7mm and a length of 200mm. The thermal performance test of the thermosyphon filled using the ferrofluid suggested a thermal resistance decrease from 1.95 to 1.20 °C/W with filling ratios of 70 to 90% of evaporator volume. The effective thermal conductivity of the thermosyphon is improved from 685 to 1160W/m°C at similar filling ratios. On the other hand, higher thermal resistance and lower effective thermal conductivity of thermosyphon filled using water were obtained at similar filling ratios of 70 to 90%. The values are 2.2 to 1.3 °C/W and 600 to 1000W/m°C, respectively. Moreover, the thermosyphon filled using the ferrofluid with a filling ratio of 90% and a non-uniform magnetic field (two sides) on the evaporator section has a thermal resistance and an effective thermal conductivity of 1.9 °C/W and 690W/m°C. The result has a lower thermal conductivity value compared to the thermosyphon filled with ferrofluid without a non-uniform magnetic field at a similar filling ratio of 90% of 1160W/m°C. This occurs due to less particle deposition on the inner surface of the thermosyphon as an effect of the magnetic field. A non-uniform magnetic field effect to the several sections of the thermosyphon (e.g., evaporator, adiabatic, and condenser) as the novelty of the research required to be modified to optimize the results.

Heat Transfer and Pressure Drop Analysis of a Confined Nanofluid Jet Flow Under a Non-uniform Magnetic Field

Heat Transfer and Pressure Drop Analysis of a Confined Nanofluid Jet Flow Under a Non-uniform Magnetic Field

Numerical analysis of turbulent natural convection in the presence of wire-induced non-uniform magnetic field inside a porous medium

Turbulent natural convection of Fe3O4-water ferrofluid with Reynolds Averaged Navier-Stokes (RANS) based turbulence model of k−ω in the presence of wire-induced non-uniform magnetic field inside a porous medium is simulated, numerically. To discretize and solve the related equations the FVM method and SIMPLE algorithm are implemented. For applying the non-uniform magnetic field, two wires carrying electric currents have been installed below and above the enclosure. Simulations are implemented for different Rayleigh numbers (106≤Ra≤108), porosity number of (η=0.5and0.9), volume fractions of nanoparticles 0≤ϕ≤4%, magnetic field numbers 0≤MFN≤109. According to the results, in low Rayleigh number and high MFN, at the high-volume fraction of nanoparticles, applying a magnetic field optimally influenced transfer and Nusselt number. At high porosity numbers, low Ra numbers and ϕ=4%, the heat transfer rate improved by up to 17 %. However, at high Ra numbers and highϕ, applying the magnetic field reduces the Nusselt number by almost 12 %.

Closed-form expressions for magnetic polarizabilities of circular apertures excited by nonuniform magnetic fields

Abstract The problem of electromagnetic penetration through an aperture in a perfectly electrically conducting plane is a classical problem in electromagnetic theory and applications. This work aims to investigate the behavior of an electrically small aperture when excited by nonuniform magnetic fields. Based on the Taylor expansion of the exact solution in integral form of the corresponding mixed boundary problem, we derive the equivalent magnetic dipole moment of a circular aperture. Closed-form expressions of dipole moments are derived. Also, for the special case of axisymmetric excitation where the dipole moment vanishes, we develop a quadrupole model to describe the aperture behavior. The effectiveness of the obtained expressions is verified by comparisons with accurate analytical solutions in special cases and the exact solutions in integral form.

Retraction Note: The application of non-uniform magnetic field for thermal enhancement of the nanofluid flow inside the U-turn pipe at solar collectors

Retraction Note: The application of non-uniform magnetic field for thermal enhancement of the nanofluid flow inside the U-turn pipe at solar collectors

A magnetic pendulum for demonstrating Mathieu-type parametric resonance

Abstract We describe a magnetic pendulum apparatus designed for studying and demonstrating Mathieu-type parametric resonance. The apparatus features a T-shaped rigid arm with a neodymium (NdFeB) magnet bob, suspended by double V-shaped thread suspensions. It is parametrically driven by an alternating uniform magnetic field produced by Helmholtz coils. The pendulum is structurally simple and easy to implement, and its parameter-driven mechanism is transparent in terms of physics. The use of electromagnetic driving allows for precise control over both the frequency and amplitude of parameter modulation, enabling quantitative studying on parametric resonance. We have observed the first and second instability region of parametric resonance. We also theoretically analyze the impact of non-uniformity in the Helmholtz coils' magnetic field on the pendulum motion, revealing that the non-uniformity can significantly alter the pendulum's nonlinearity, influencing both the magnitude and the sign of the coefficients of the nonlinear terms. However, under the parameters of our experimental setup, the nonlinearity effect is mainly manifested in large swing angle motions. For small swing angle within 5\textsuperscript{$\circ$}, the impact of magnetic field non-uniformity on parametric resonance is negligible. The experiment reported here is appropriate for undergraduates to carry out experimental research, helping them develop a more intuitive and quantitative understanding of parametric resonance.

Electron series resonance excited in the 27.12 MHz magnetron sputtering discharge

Abstract The electron series resonance (ESR) excited in the 27.12 MHz magnetron sputtering discharge was investigated. By analyzing the discharge impedances, the imaginary part of impedance was found to undergo a transition from capacitive to inductive on varying RF power, and the conditions for ESR excited was satisfied at the 27.12 MHz magnetron sputtering. By analyzing the discharge current and its higher order harmonics, the near-sinusoidal current waveform and the weak 2nd order harmonic were obtained, showing a weak nonlinear effect of RF current. However, for the magnetron sputtering discharge, the non-uniform magnetic field has a significant effect on the sheath width and the transverse current, making the sheath thinner and the transverse current smaller. As a result, the small capacitive reactance was obtained and the inductive reactance was easily cancelled. Therefore, the electron series resonance excited in the 27.12 MHz magnetron sputtering was caused together by the strong effect of non-uniform magnetic field and the weak 2nd (H2) order current harmonic. By estimating the ESR frequency res,B, the 2nd order current harmonic (54.24 Hz) was found to be responsible for the ESR excitation.

Depolarization and Faraday effects in AGN Jets

ABSTRACT Radio interferometric observations of active galactic nuclei (AGN) jets reveal the significant linear polarization of their synchrotron radiation that changes with frequency due to the Faraday rotation. It is generally assumed that such depolarization could be a powerful tool for studying the magnetized plasma in the vicinity of the jet. However, depolarization could also occur within the jet if the emitting and rotating plasma are cospatial (i.e. the internal Faraday rotation). Burn obtained very simple dependence of the polarization on the wavelength squared for the discrete source and resolved slab that is widely used for interpreting the depolarization of AGN jets. However, it ignores the influence of the non-uniform large-scale magnetic field of the jet on the depolarization. Under the simple assumptions about the possible jet magnetic field structures, we obtain the corresponding generalizations of Burn’s relation widely used for galaxies analysis. We show that the frequency dependences of the Faraday rotation measure and polarization angle in some cases allow to estimate the structures of the jets magnetic fields.

Droplet generation in a co-flowing microchannel under the non-uniform magnetic fields produced by an electric current loop

Micro-magnetofluidics, the study of fluid behavior under magnetic fields in microscale systems, is vital for applications like drug delivery, chemical synthesis, and lab-on-a-chip technologies. Controlling droplet size and formation frequency in these systems is challenging due to the complex interplay of magnetic forces and fluid dynamics. This study introduces a novel approach to control droplet generation in a co-flowing microchannel under a non-uniform magnetic field generated by an electric current loop. Critical parameters such as electric current intensity, continuous phase flow rate, and current loop position are systematically examined for their impact on droplet behavior. The results highlight the unique influence of the magnetic field configuration, specifically the electric current loop, in inducing a transition from dripping to jetting flow patterns with increasing current intensity, leading to larger droplets and reduced generation frequency. Additionally, a distinct behavior of droplet coalescence near the current loop, followed by re-separation, is observed when the loop is positioned downstream of the inlet. Moreover, increasing the continuous phase flow rate consistently reduced droplet size and increased generation frequency, regardless of the current loop’s position.

The oscillation of a falling ferrofluid droplet induced by a nonuniform magnetic field

Understanding the droplet size and shape control mechanism in a magnetic field is critical for precisely manipulating ferrofluid droplets. Here, we conducted an experimental investigation on the dynamic behavior of a falling ferrofluid droplet under a nonuniform magnetic field produced by current coils. We observed an interesting phenomenon: the uneven distribution of the magnetic field and the jump in magnetic properties at fluid interfaces will cause the Laplace pressure difference on the droplet surface, stimulating the droplet’s oscillation. We also use the Laplace pressure difference equation and the interfacial tension coefficient correlation to model the deformation of ferrofluid droplets and determine the oscillation frequencies and deflection angles. The droplets’ oscillation frequency is related to the magnetic Bond number: f∼Bom-0.523∼-0.501. The deflection angle of the droplet is further diminished by the superposition of a viscous shear moment and a magnetic moment (7.41°∼12.90°). Our research lays the groundwork for precise ferrofluid droplet manipulation in drug delivery and soft robots.

True pinch mode of magnetorheological fluids

Abstract Magnetorheological (MR) fluids are representatives of smart materials. They react to magnetic fields by developing a yield stress. The effect has been employed in real-world applications such as automotive chassis systems or optical finishing. By convention, MR devices can be operated in at least one of the fundamental modes: flow, shear, squeeze, gradient pinch of which the former has been the least studied and understood. In pinch mode, the material in the flow channel is exposed to non-uniform magnetic fields in the direction parallel to fluid flow. As a result, only the volume of MR fluid near the channel walls are energized to modify the particular material property (yield stress). The result is the channel’s effective diameter change. The behavior of the material in pinch mode is unique and unseen in the other controllable fluids. To study the material’s characteristics in the specific mode, the authors developed a novel circuit concept for energizing the material in an effort to achieve the ‘true-zero’ pinch mode magnetic behavior. Contrary to the existing pinch mode valve concepts, the concept valve allows to achieve zero magnetic flux density in the center of the flow channel regardless of the current level. To test the hypothesis a prototype valve was modeled, manufactured and tested across a range of external (flow rate, current/magnetic flux) stimuli. The obtained results yield sufficient evidence proven by results of magnetic simulations to support the underlying hypothesis. The experimental results illustrate the pinch mode type behaviour, i.e. the slope change in the pressure vs flow rate characteristics.

The Impact of a Non-Uniform and Transient Magnetic Field on Natural Convective Jeffrey Fluid Flowing over a Heated Porous Plate: A Comparitive Study

Examining the effects of varying transient magnetic fields and their orientations on boundary layers aids in understanding the flow in complex surface geometries, turbulence control, drag reduction, aircraft and ship design, groundwater management, and the study of construction and coastal engineering. The study introduces a novel dynamics of the boundary layer of a natural convective Jeffrey fluid flowing over an erect porous moving plate under the influence a non-uniform and transient magnetic field M ˜ that grows exponentially over time with α as an accelerating parameter. The governing partial differential equations (PDEs) were non-dimensionalized using similarity variables and then solved analytically using the perturbation technique. The impact of various thermosphysical properties including chemical reactions, thermal source, Jeffrey fluid parameter and plate permeability on the fluid flow were presented graphically utilizing finite difference approximation(FDA) technique in MATLAB ODE15s solver. The study highlighted a notable decrease in shear stress (skin friction) by introducing M ˜ to the vertical plate results in the reduction of momentum boundary layer, accompanied by an increment in thermal boundary layer. Also, the results shows that increasing the magnitude of α leads to decrease the velocity and increase the temperature distribution curve. Additionally, the concentration profile was also found to decrease with stronger chemical reactions. Furthermore, key parameters like heat and mass flux were estimated and discussed through comparison tables. Understanding the effects of unsteady and externally applied increasing magnetic field M ˜ on viscous fluid flow have potential applications in biomedical engineering, such as the design of magnetic drug delivery systems and the analysis of blood flow in human body.

Single-layer iron network microstructure magnetorheological elastomer for transparent soft actuator

The motion of a soft magnetic magnetorheological elastomer, a composite material consisting of soft magnetic fillers and an elastomeric matrix, is limited by the gradient of the applied magnetic field, making its application as an out-of-plane soft actuator difficult. In this study, an anisotropic soft magnetic magnetorheological elastomer-based transparent soft actuator with large out-of-plane deformation was fabricated using a very low concentration (1 vol%) of carbonyl iron particles as the soft magnetic fillers. An applied AC electric field treatment method was used to manipulate iron particles in silicone oil, assembling and depositing them onto a glass substrate to form a single-layer iron-network microstructure. A single-layer iron-network microstructure-based magnetorheological elastomer (SL-sMRE) film with initial self-buckling was fabricated by exploiting the swelling properties of the residual silicone oil and polydimethylsiloxane. The SL-sMRE film exhibited out-of-plane deformation with a very short reaction time (228 ms) under a uniform magnetic field. When stimulated by a non-uniform magnetic field, the SL-sMRE film deformed away from the permanent magnet, which was not possible with an isotropic film. An optically transparent SL-sMRE film was applied to a transparent flexible gripping device that could detect the surface of a gripped object in real time.

Kinetic study of dry magnetic separation based on Gauss-Maxwell magnetic stress tensor: A 3D finite element method (FEM)

In this paper, two artificial magnetic particles (1#, 2#) with known magnetic parameters were taken as the objects, a finite element model based on Gauss’s law was established for the calculation of the transient Maxwell magnetic stress tensor on the surfaces of particles and kinetic study, a high-speed camera was used to obtain the motion behaviors of magnetic particles in comparison with the simulation. According to the results of multiple simulation-experiment comparisons, the motion behaviors of magnetic particles in the finite element simulation were consistent with experimental phenomena under the identical conditions, indicated that the accuracy of the model is reliable. In the comparison of two kinds of magnetic force calculations, the magnetic force FM based on the Gaussian formula had a similar tendency to FD based on the kinetic calculations, and since the FM was obtained by converting the surface tension of particles, it more accurately reflected the overall magnetic force and magnetic torque acting on the particles. Kinetic analysis showed that the magnetic force acting on a particle was strictly dependent on its magnetization, dynamic and non-uniform magnetization caused the magnetic particle to be subjected to magnetic force and magnetic torque in the non-uniform magnetic field, resulting in displacements and flips. In addition, compared to the particle release attitude, the influence of the distribution of magnetic substance and the particle’s release position on the displacement was particularly significant. The 3D finite element model established for dry magnetic separation can be further used for the study of dynamic, non-uniform magnetization and the force of magnetic particle or grain groups, which is of certain significance for the kinetic study of magnetic separation and improving research of magnetic separation equipment.

Fission Dynamics of a Ferrofluid Droplet on a Smooth Plate under a Nonuniform Magnetic Field.

A ferrofluid droplet on a plate undergoes interesting fission under a magnetic field. In this article, a recently developed simplified multiphase lattice Boltzmann method coupled with a self-correction solution for the magnetic potential equation is employed to explore this complex flow phenomenon. It is found that the droplet fission follows a four-pattern process under a nonuniform magnetic field. The mechanisms behind this behavior are influenced by the strength and gradient of the magnetic field, surface tension, droplet size, and the wetting boundary condition. These findings provide valuable insights into the application of ferrofluids in droplet microfluidics and contribute to advancements in this field.

The induced non-uniformity of magnetic field enables to enhance heat transfer through the annular gap in a uniform magnetic field three – five times

We have investigated the influence of the induced non-uniformity on thermomagnetic convection when the inner cylinder of an annular gap has high magnetic permeability and a special wave-like shape. The outer uniform magnetic field induces a non-uniformity, which causes thermomagnetic convection. The problem is analyzed numerically. The in-house code uses the finite volume method with exponential interpolation functions for vorticity and temperature and linear functions for the stream function and magnetic field potential. It was found that increasing the periodicity and amplitude of the wave-like structure enhances heat transfer by 30–50 % compared to a circular annular gap filled with magnetic fluid, and 3–5 times compared to an annular gap filled with ordinary fluid. Such a way of creating induced non-uniformity of magnetic field and convection enhancement is especially effective for microfluidics, as the gravitational Rayleigh number is low in this case, and it is a hard technical problem to create a non-uniform magnetic field in small channels by directly using permanent magnets.

Features of 2D mapping technique of non-uniform magnetic fields using self-biased magnetoelectric composites based on “bidomain LiNbO3/Ni/Metglas” structures

Magnetoelectric (ME) composites are extensively researched for their ability to detect magnetic fields but are typically characterized by their interactions with homogeneous magnetic fields. However, practical applications often involve non-uniform magnetic fields (NMFs). In this study, we developed and validated a technique for 2D mapping of NMFs using sensor based on ME composite. The primary focus was on mapping the magnetic field generated by a single wire carrying an alternating current (AC). The experimental setup included a “bidomain LiNbO3/Ni/Metglas” ME structure, which demonstrated a ME coefficient of 0.83 V/(cm⋅Oe) without external biasing at 117 Hz. The ME structure was characterized using both quasi-static and dynamic methods, with the mapping performed at a frequency of 232 Hz, chosen to ensure a linear response and minimize resonance effects. Our measurements revealed that the position of maximum magnitude of the ME signal was consistently shifted from the position of the maximum integral magnetic field intensity of the wire. This shift was attributed to the deformation of the ME structure and the redistribution of the magnetic flux along the sample due to the magnetic layers, as confirmed through modeling. Further, the measured 2D mapping of the NMF from the wire closely matched the theoretical distribution, displaying axial symmetry but with a persistent shift of 2–3 mm along the length of the ME structure. These effects were linked to the linear dimensions and clamping methods of the ME structure. Overall, our experimental results closely correlated with theoretical data and the FEM model, demonstrating that ME composites are effective for NMF detection and mapping. Future work will aim to reduce the dimensions of the ME structure using MEMS technology to minimize the observed shift effects in the measurements.

Impact of induced electric field on the motion of a charged particle in a uniform time-dependent magnetic field that is linearly increasing with time

The classical and quantum motion of a charged particle in a uniform constant magnetic field is widely studied in the scientific literature since it is a problem that allows an exact mathematical analytical solution. On the other hand the cases of a non-uniform, time-dependent or both non-uniform and time-dependent magnetic field lead to mathematical problems that do not have general exact analytical solutions except for a few very specific cases. We present in this work, the results for the two-dimensional motion of a charged particle in a uniform time-dependent magnetic field that is linearly increasing with time for a certain finite interval and then stabilizes to a constant value afterwards. The work draws attention to certain interesting features observed for the two-dimensional motion of the charged particle as a result of the interplay between the effects of an induced electric field and the time varying magnetic field.

Non-uniform dynamic magnetic field compensation for a lightweight magnetically shielded room based on distributed differential coils

Abstract Due to the influence of material characteristics and engineering imperfections, the residual magnetic fields including static and dynamic field in a lightweight magnetically shielded room (MSR) present non-uniform distribution characteristic. To effectively reduce the residual static field, a non-uniform field compensation method based on distributed coils has been proposed. This method takes the L2-norm of the magnetic field at the target array points as the objective function, to optimize the compensation current in each of the designed distributed coils, achieving non-uniform field compensation. To further achieve compensation for non-uniform residual dynamic field, a closed-loop control system containing a voltage divider and multi-channel amplifiers is designed in this paper. Additionally, the coil constant of the distributed differential coils and the initial magnetic field distribution vector for dynamic compensation are redefined. In conjunction with a PI controller, dynamic compensation is achieved using a single optically pumped magnetometer (OPM) as the feedback component. The experimental results indicate that the average dynamic field decreased from 376.75 pT to 27.80 pT over the central volume of (300 mm)³ in MSR, achieving a near-zero magnetic environment. This method has universal applicability for compensating non-uniform residual field under different working conditions.