Effect Of External Magnetic Field Research Articles

Thermo-hydraulic characteristics of magnetic nanofluid in opposing and assisting minichannel under the influence of external magnetic field

The present numerical study investigates the effect of external magnetic field on a magnetic nanofluid flow in an inclined channel. A uniform magnetic field is used to generate vortex in the channel for heat transfer enhancement. Fe3O4–water nanofluid of 2 vol. % is flowing in an inclined two-dimensional channel with a heated bottom wall. Numerical simulations are carried out for different inclination angles varying from −90° < θ < 90° at low Reynolds numbers, in the presence of external magnetic field of intensities varying from 0–2000 G. The heat sink has dimensions of 40 × 4 mm2, with a magnet pair placed at 15 mm from the origin. Different thermo-hydraulic properties, like Nusselt number, friction factor, pressure drop and thermal enhancement factor (TEF), are calculated for all the cases. There is an average increase in the Nusselt number by 4.95% and 19.27% when a magnetic field of 1500 and 2000 G is applied, respectively. This heat transfer enhancement comes with a penalty of 32.95% and 89.23% increase in the friction factor for the respective magnetic fields. Magnetic field decreases the pressure drop by reducing the contact area for positive inclinations, while increases pressure drop by increasing turbulence for negative inclinations. TEF for the flow increases by 9.53% and 12.50% when the magnetic field of 1500 and 2000 G is applied, respectively. The TEF value is observed to be greater than one for very large ranges of inclinations when magnetic fields are applied, as compared to the flow without magnetic field.

Effects of flash annealing and external magnetic field on magnetic properties of relatively high Nd content (37% by weight) NdFeB alloy

Effects of flash annealing and external magnetic field on magnetic properties of relatively high Nd content (37% by weight) NdFeB alloy

The study of high-speed MIG welding assisted by compound external magnetic fields for 6N01-T6 aluminum alloy

The magnetic field generator was designed and assembled on the welding torch to produce perpendicular and parallel magnetic fields periodically in the arc to improve the weld formation of 6N01-T6 aluminum alloy at high-speed welding. The mechanism of this hybrid weld process was studied based on the analysis of arc behavior and bead formation at different excitation parameters. The microstructure, tensile strength and impact toughness of joints were tested and analyzed. Results showed that the compound external magnetic fields suppressed the weld-bead defects effectively and increased the welding speed from 0.75 m/min to 2.1 m/min. The stirring effect of compound external magnetic fields had the advantage of refining grains in the weld zone, which improved the tensile strength and impact toughness of joint at high-speed welding. It proved that the compound external magnetic fields assisted melt inert-gas welding was a high-speed and low-cost welding technology for aluminum alloy.

The nonlinear second harmonic generation and third harmonic generation in GaAs/GaAlAs exponentially confined quantum well: Electric and magnetic field effects

The second harmonic generation(SHG) and third harmonic generation(THG) of a GaAs/GaAlAs exponential quantum well(QW) are numerically investigated in the presence of applied electric and magnetic field, which are parallel to the QW growth direction. The eigenvalues and corresponding eigenfunctions of the system are obtained by using the finite difference technique . The expression for the second- and third-order susceptibility is given by the density matrix and iterative approach. It is showed that, the SHG and THG not only depend strongly on quantum confinement, but also the intensity of external fields. We have systematically studied the SHG and THG of different confinement QWs by adjusting the structural parameters, as well as the effects of external electric and magnetic fields. We give the optimal system parameters in which the external field effects play important role.

Geomagnetic storms in the context of spacecraft attitude estimation under different noise levels

This study examines geomagnetic storm conditions for attitude estimation of a spacecraft having magnetometers with various measurement noise levels at low Earth orbit. The geomagnetic field models are introduced by taking the external field effects into account. It is important to discuss the limitations of the models and the measurements that are being used. Therefore, the external effects are evaluated under different noise levels of magnetometer to determine and discuss the suppression level caused by the noise levels in a general framework. External magnetic field effects are examined by the ratio to noise under various magnetometer standard deviations up to 300 nT. An analysis is carried out for attitude estimation using Kalman-type filter in order to determine what noise level is acceptable on a particular sensor with a specified attitude requirement, magnetic field model, and space weather conditions.This study indicates that external magnetic fields are vital for establishing attitude processes based on magnetometers with varying noise on which the spacecraft relies for accuracy.

Kendi Kendini Konfigüre Edebilen Robotik Bir Sistem için Mikro Ölçekte Elektromanyetik Dış Eyleyici Tabanlı Hareket Modeli Geliştirilmesi

Self-reconfigurable modular robots (SRMRs) are considered as autonomous kinematic machines that can change their own shape by rearranging the connectivity of their parts to perform new tasks, adapt to new circumstances or recover from damage. One of the main goals in SRMRs field is to reach to a million modules integrated self-reconfigurable systems. However, miniaturization efforts in self-reconfigurable modular robots bring some challenges such as storage and packaging. Developing externally actuated micro-robots can be a good alternative for miniaturization of SRMRs with the help of rapid enhancements in micro-manufacturing technologies encountered in the last decades. In this study a novel self-reconfiguration mechanism for micro-robots that are externally actuated by magnetic actuators is developed. In the study the motion of the magnets inside the microtubes under the effect of external magnetic field is investigated by using finite elements method. Dynamic model of the mechanism is obtained by using finite elements method and its applicability is exhibited by simulations. The results are compared with the theoretical values. It is envisioned that the study will contribute to micro-robotic systems in industry, defense industry and space missions as well as biomedical applications and medical robots.

GeV Electron Acceleration by Trapezoidal Laser Pulse Envelope Under Combined Effect of Frequency Chirp and Axial Magnetic Field in Vacuum

In the present article, we have investigated efficient electron acceleration by employing trapezoidal laser pulse envelope under the combined effect of linear chirp and external axial magnetic field in vacuum. In this envelope, the electric signal linearly rises to a certain level, stay there, and then falls back to its initial level. So, there will be an opportunity to increase the interaction time between laser pulse and electron for efficient energy transfer, which has been reflected in this study. A few megaelectronvolt electrons have been injected axially to the front of a trapezoidal short laser pulse. In all calculations, the front end of each laser pulse captures electron at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t= 0$ </tex-math></inline-formula> . The dynamics arise from analytical and numerical solutions of Newton–Lorentz force equations of motion. Electron energy gain has been calculated for initial laser phase varying from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> . For the optimum set of laser parameters, we have observed electro’s energy gain of the order of GeV.

Linear and nonlinear optical properties in wurtzite MgZnO/ZnO double quantum wells: Roles of external electric and magnetic fields

In this work, the modulation effects of external magnetic and electric fields on the inter-subband optical absorption coefficients and refractive index changes in asymmetric wurtzite MgZnO/ZnO double quantum wells are investigated considerably. The linear and nonlinear characteristics among the three lowest confined states are calculated here. The obtained results show that the optical properties of the structure are very sensitive to the external field. The optical characteristics related to higher energy levels which have been ignored in past works are as important as the transition related to the ground state. The transition from the ground state to the second excited state is the most sensitive to the external fields. The optical parameters can be risen by two orders of magnitude through the manipulation of the applied electric and magnetic fields.

Greatly Enhanced Methanol Oxidation Reaction of CoPt Truncated Octahedral Nanoparticles by External Magnetic Fields

Tunable behavior in electrocatalysis by external multifields, such as magnetic field, thermal field, and electric field, is the most promising strategy to expand the theory, design, and synthesis of state‐of‐the‐art catalysts and the cell in the near future. Here, a systematic investigation for the effect of external magnetic field and thermal field on methanol oxidation reactions (MOR) in magnetic nanoparticles is reported. For Co42Pt58 truncated octahedral nanoparticles (TONPs), the catalytic performance in MOR is greatly increased to the maximum of 14.1% by applying a magnetic field up to 3000 Oe, and it shows a monotonical increase with increasing working temperature. The magnetic enhanced effect is closely related to the Co content of CoxPt100‐x TONPs. Furthermore, the enhancement effect under a magnetic field is more obvious for Co42Pt58 TONPs annealed at 650 °C. First‐principle calculation points out that the magnetic fields can facilitate the dehydrogenation of both methanol and water by suppression of entropy of the electron spin and lowering of the activation barrier, where OHad intermediates on Co sites play a more important role. The application of magnetic fields together with thermal fields in MOR provides a new prospect to manipulate the performance of direct methanol fuel cells, which will accelerate their potential applications.

Control and tunability of magnetic bubble states in multilayers with strong perpendicular magnetic anisotropy at ambient conditions

The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional degree of freedom that promises the development of multidimensional magnetic memories. A key requirement for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates. In the present work, we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy ${\text{[Co(0.44 nm)/Pt(0.7 nm)]}}_{X},$ $X=48$, 100, and 150 multilayers on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ substrates by exploring the effect of external magnetic fields $({H}_{\mathrm{m}})$ of different strength and angles $(\ensuremath{\theta})$ with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at ${H}_{\mathrm{m}}=0.87{H}_{\mathrm{s}}$ (${H}_{\mathrm{s}}$ is the magnetic saturation field) and $\ensuremath{\theta}={60}^{\ensuremath{\circ}}$--${75}^{\ensuremath{\circ}}$ for all $X$ with a maximum of 3700 domains/100 $\ensuremath{\mu}{\mathrm{m}}^{2}$ for the $X=48$ sample. The experimental observations are supported by micromagnetic simulations, taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping to understand the different densities of the bubble states found in these systems.

INFLUENCE OF TEMPERATURE ON THE BEARING CAPACITY OF PASSIVE MAGNETIC BEARINGS

When magnetic-passive bearings are heated, the magnetic properties of permanent magnets change and, therefore, the force of interaction between them changes. Depending on the temperature and exposure time, irreversible changes in the strength of the interaction of magnets occur, caused by changes in the magnetic structure, phase composition, as well as reversible changes due to the temperature dependence of the magnetization and coercive force of the magnet material. A distinctive feature of the operation of magnets in bearings is the effect of large external magnetic fields on them. For this reason, it is impossible to predict in advance the properties of bearings at elevated temperatures using only the data given in the literature for magnetically rigid materials or individual magnets. The influence of temperature on the interaction of two coaxially arranged magnets modeling a part of a bearing is theoretically and experimentally considered. Numerical values of temperature coefficients of changes in bearing capacity and stiffness of bearings are determined, as well as the theoretical conclusion about the constancy of these coefficients for bearings with optimal magnet sizes is confirmed.

Magnetic field assisted nucleation dynamics of laser induced manganese oxide nanoparticles in water

Pulsed laser ablation at manganese (paramagnetic)–water interface led to the formation of cubic bixbyite α-Mn2O3 nano-particles. The effect of external magnetic field on to the size of the nano-particles was investigated. Nucleation modelling were carried out to validate the experimental results. To study the affect of the external magnetic field on to the nucleation dynamics, two different models were employed—model A: influence of the magnetic pressure, and model B: influence of the magnetic energy, that affects the laser-induced nucleation dynamics when an external magnetic field is applied. It was observed that the nucleation modelling using model A gives more agreeable results to the experimental observation than model B. A similar investigation was also carried out using ferromagnetic: nickel target, which shows significant influence exhibiting a decrease in nano-particle sizes using both the models. The fluid dynamical counterpart: cavitation bubbles formed at laser interaction with solid targets immersed in liquids, are also probed. Cavitation bubbles formed at the manganese–water interface seem impervious to the external magnetic field; however, for targets such as nickel, energy dispensed to ferromagnetic interactions is translated to cavitation bubbles which exhibit larger bubble radius.

Boosting photocatalytic activity through tuning electron spin states and external magnetic fields

Photocatalysis is considered as one of the most promising technologies to generate renewable energy and degrade environmental pollutants. Tremendous efforts have been made to improve photocatalytic efficiency. Among these, tuning spin polarization and introducing an external magnetic field are considered two promising strategies to boost photocatalytic performance. Herein this review highlights the recent breakthroughs through manipulating spin states and applying external magnetic fields for enhancing photocatalytic reactions. The relevant characterization techniques and fundamental mechanisms are summarized. Spin polarization states of photocatalysts have received considerable attention due to their unique roles, including inhibiting the recombination of photoexcited carriers owing to spin orientation constraint, enhancing the reaction product selectivity, and reducing the reaction barriers via optimizing the absorption energy and binding strength. As for the effects of external magnetic field on photocatalytic performance, we mainly discuss the separation enhancement of photoinduced carriers under static and time-varying magnetic fields and the magneto-hydrodynamic effect of charged particles. Lastly, the negative magnetoresistance effect is discussed due to the synergistic effects of the electron spin state and an external magnetic field. These discussions in this review may provide new insights into designing new semiconductors for boosting photocatalytic performance in internal and external magnetic fields.

Size-dependent and nonlinear magneto-mechanical coupling characteristics analysis for extensional vibration of composite multiferroic piezoelectric semiconductor nanoharvester with surface effect

We study the extensional vibration of composite multiferroic piezoelectric semiconductor (PS) nanoharvester driven via a time-harmonic magnetic field. A theoretical analysis about working performances of device is performed based on zero-order plate equations with surface and nonlocal effects, consisting of a nonlinear magneto-mechanical coupling constitutive model for giant magnetostrictive material Terfenol-D and a linear phenomenological theory of PS material ZnO. Numerical results indicate that the basic bahaviors (including resonant frequency, bandwidth characteristic, magneto-electric (ME) coupling effect, output power and energy conversion efficiency) of the nanoharvester, can be dramatically improved through circuit types, semiconduction, external magnetic field, pre-stress and surface effect. This work is essential and crucial for understanding the size-dependent and nonlinear mechanical behaviors of multiferroic PS nanodevices under the extremely complex magnetic field and pre-stress field environments.

Enhancement of resistance element welding of AA6061 to DP600 steel by using external magnetic field

The application of aluminum (Al)/steel hybrid structure has a broad prospect in automobile weight reduction. In this work, AA6061/DP600 steel was joined using a resistance element welding (REW) method to achieve high strength and plasticity. To modify the deviation of weld nugget and increase the diameter of nugget, annular permanent magnet was designed and used. The effect of external magnetic field on microstructure evolution and mechanical properties was investigated. An electromagnetic stirring effect was generated inside weld nugget, resulting in a larger bonding area between Q235 rivet and DP600 steel. Convection in weld nugget contributed to restraint of dendrite growth and changes in heat distribution, leading to grain refinement at weld nugget. The microstructure of weld nugget and heat affected zone (HAZ) was bainite in magnetically assisted resistance element welding (MA-REW) specimens. The microhardness distribution presented a symmetrical distribution in both horizontal and vertical direction of cross section while the highest microhardness value occurred in weld nugget. Compared to REW specimens, MA-REW specimens achieved improvement of 10.2 % in peak load and 26.8 % in displacement during the tensile test. When the welding current reached 11 kA, the fracture mode of MA-REW specimen transformed from interfacial fracture to button fracture. Enlarged weld nugget size and refined grains were responsible for the improved performance.

Lie symmetry analysis of heat transfer in a liquid film over an unsteady stretching surface with viscous dissipation and external magnetic field

Fluid flow and heat transfer in thin films over unsteady stretching sheets are studied and analyzed by employing similarity transformations and analytical or numerical solution schemes. We derive Lie point symmetries for system of partial differential equations describing the flow and heat transfer in a thin liquid film on an unsteady stretching surface with viscous dissipation in the presence of external magnetic field. We deduce a 5-dimensional Lie point symmetry algebra. We use a couple of the admitted Lie point symmetry generators to construct new similarity transformations for the considered model, through associated invariants. These transformations enable reductions of the model to nonlinear ordinary differential equations. We construct analytic solutions for this system using the Homotopy analysis method to study the considered MHD flow and heat transfer. These results are presented in the form of tables and figures to reflect variations in fluid velocity, film thickness and heat transfer with the magnetic parameter, Prandtl number, Eckert number and unsteadiness parameter. We show that these variations in the velocity and temperature profiles are different from those reported earlier for the flow and heat transfer within a thin film under the effects of viscous dissipation and external magnetic field.

The magnetic properties of mixed spin-(52,12) anisotropic Heisenberg model in the Oguchi approximation

We have used the Oguchi approximation to study the effects of longitudinal crystal (D) and longitudinal external magnetic (h) fields on the mixed spin-(52,12) anisotropic Heisenberg model. The thermal variations of sublattice and total magnetizations are exploited in detail for given values of D, exchange anisotropy parameter (Δ) and coordination numbers z=3,4 and 6 when h=0.0. Thus, the phase diagrams are constructed on the planes of (D/J,kBT/J) and (Δ,kBT/J). In this case, it is found that the model presents either first- or second-order phase transitions as well as tricritical points. In the presence of magnetic field, the hysteresis loops of the model are also analyzed and it was found that the coercive field and remanent magnetization decrease as the temperature increases and increase as the crystal field or the exchange anisotropy increases.

The effect of external pressure & magnetic field in star formation: The critical mass model

Theoretical work addressing the role of external pressure with magnetic fields in collapsing molecular clouds is important in building a comprehensive theory of star formation(SF). In many SF studies turbulence, magnetic fields, and self-gravity are described as the key dynamical processes involved in SF. However, the importance of external pressure in collapsing strongly magnetized clouds has not yet been particularly explained. Magnetic fields transport excess angular momentum from the central core while external pressure compresses the cloud. Thus the outflow of angular momentum, and on the other hand, the compression made by external pressure are the cause for matter falling onto the central core from the envelope. Therefore, external pressure facilitates the collapse of gas and the inflow of matter. In this work, we show theoretically how the strong magnetic field is dragged inward by the external pressure during the core collapse and formulate the critical mass of the core in the presence of external pressure.

Study of coupling effects on the three-dimensional magnetohydrodynamic flows around single and tandem cylinders in duct

This work focuses on magnetohydrodynamic (MHD) flow around either single or two identical tandem circular cylinders confined in a duct under external transverse magnetic field. The influences of transverse magnetic field, inlet velocity of conducting fluid, and the distance between two cylinders arranged along the flow direction on the flow modes, vortex shedding frequency, separation angle, and drag coefficient are investigated. The simulations are conducted with the Hartmann number Ha and Reynolds number Re and are varied from 0 to 1.35 and from 100 to 200, respectively. For the MHD flow around single cylinder, the map of flow instability is set up and the relation between critical Hartmann number Hacr and Re is concluded as The coupling effect of geometry confinement and external transverse magnetic field can increase the separation angles and drag coefficients and decrease vortex shedding frequencies and lengths of reverse flow. For the tandem double cylinder cases, the spacing ratio between two cylinders L/d changes from 2 to 8. The boundary between steady regime and unsteady regime is described as, The three wake patterns are found under various magnetic fields. The nonlinear and nonmonotonous characteristics of coupling effects of Ha, Re, and L/d are summarized. The physical mechanisms of influences of MHD flow field in gap region between tandem cylinders and the existence of upstream cylinder on flow instability and wake pattern are revealed. Finally, the critical conditions corresponding to the transition from steady flow to unsteady flow are achieved with formulas.

The dynamics of current-driven vortex in two-band superconductor with s+d wave pairing

Two-band mesoscopic superconductors are known to exhibit specific vortex states. In this paper, finite element method (FEM) is used to numerically solve the time-dependent Ginzburg-Landau (TDGL) equation with s+d wave coupling for a 3D mesoscopic superconducting strip. We discuss the effect of external magnetic field H and applied current I on s+d wave coupling at the temperatures Ts<T<Td, where Ts and Td are the critical temperatures of s- and d-bands, respectively. The obtained results show that this coupling is enhanced by the applied current and reflected by the vortex dynamics in two bands. A small amount of Cooper pairs in the s-band form a series of different vortex and antivortex states. Mutual attraction of vortex and antivortex induces simultaneous motion of s- and d-vortices at applied current I. This work provides theoretical background for the design of mesoscopic superconducting devices.