Absence Of Magnetic Field Research Articles

CFD analysis and optimization of nano-enhanced phase change process in multiple port vented cavity equipped with encapsulated PCM under the combined effects of triple rotating cylinders and inclined magnetic field

In this study, phase change process dynamics in a multiple vented cavity system equipped with encapsulated phase change materials (PCM) are analyzed under the combined utilization of multiple rotating cylinders and magnetic field. The applications can be found in electronic cooling, rotating type heat exchangers where PCMs are immersed in between and for control purpose of the thermal management. The numerical work is conducted for different values of rotational Reynolds number (Rew, between 0 and 10000), distance between the cylinders (sn between 0.05H and 0.2H), strength of magnetic field (Hartmann number-Ha between 0 and 50) and size of the inlet port (w between 0.075H and 0.25H, H as the cavity size). The phase change process is found to be slower for Rew up to Rew=6000 after which it becomes fast. The full transition time (tF) rises by about 170% at Rew=6000 and then reduces 62% at Rew=10000. At the highest magnetic field, the process intensifies and up to Ha=20, it becomes slower. When magnetic field is active up to Ha=20, the value of tF rises by about 46.5% as compared to case without magnetic field while a reduction of the value by 43.8% is achieved by using magnetic field at the highest strength (Ha=50). Increasing the distance between the triple cylinders and port size of the vented cavity have negative impact on the process both with and without magnetic field. The phase completion time increases by about 124% in the absence of magnetic field when port size is increased from w=0.075H to w=0.25H while the value is 33% with magnetic field at Ha=50. Optimization studies are conducted to find the optimum set of parameter at two different port sizes (w). At w=0.1H, the optimum values are found as (Ha, Rew, sn)=(49.24,4073,0.094H) and at w=0.25H, the values are (Ha, Rew, sn)=(50,2296,0.068H).

Effect of polydispersity in concentrated magnetorheological fluids

Magnetorheological fluids (MRF) are smart materials of increasing interest due to their great versatility in mechanical and mechatronic systems. As main rheological features, MRFs must present low viscosity in the absence of magnetic field (0.1–1.0 Pa.s) and high yield stress (50–100 kPa) when magnetized, in order to optimize the magnetorheological effect. Such properties, in turn, are directly influenced by the composition, volume fraction, size, and size distribution (polydispersity) of the particles, the latter being an important piece in the improvement of these main properties. In this context, the present work aims to analyze, through experiments and simulations, the influence of polydispersity on the maximum packing fraction, on the yield stress under field (on-state) and on the plastic viscosity in the absence of field (off-state) of concentrated MRF (φ = 48.5 vol.%). Three blends of carbonyl iron powder (CIP) in polyalphaolefin oil were prepared. These blends have the same mode, but different polydispersity indexes (α), ranging from 0.46 to 1.44. Separate simulations show that the random close packing fraction increases from about 68% to 80% as the polydispersity indexes increase over this range. The on-state yield stress, in turn, is raised from 30 ± 0.5 kPa to 42 ± 2 kPa (B ≈ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8 Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the literature, increases packing efficiency and reduces the viscosity of concentrated dispersions, but beyond that, it proved to be a viable way to increase the magnetorheological effect of concentrated MRF. The Brouwers model, which considers the void fraction in suspensions of particles with lognormal distribution, was proposed as a possible hypothesis to explain the increase in yield stress under magnetic field.

Nano-jet impingement cooling of a sinusoidal wavy hot surface by using an oriented L-shaped object under magnetic field with two phase mixture formulation

In this study, slot nano-jet impingement cooling of a wavy sinusoidal surface is analyzed by combined utilization of an oriented L-shaped object and magnetic field. A two phase mixture model of nanofluid is adopted. It is observed that the presence of the L-shaped object and its orientation are very influential on the local and average heat transfer performance. Magnetic field suppresses the vortices near the object and on the bottom all. The average Nusselt number (Nu) rises by 17.5% and 8.6% when its strength is varied for object orientations of te=−90 and te=90. When magnetic field inclination is concerned, the cooling performance improvements are in the range of 25.6%–53.9% depending upon the orientation of the object. The orientation of te=0 and te=45 are the most favorable cases in terms of thermal performance while significant variations in the average Nu are seen in the absence of magnetic field when varying the L-shaped object orientation. When compared to reference case of slot jet impingement (without magnetic field using pure fluid and without object), using nanofluid improves the cooling performance by about 8.4%. When object is installed at Ha=0, it rises by 27% at orientation of te=45. When magnetic field is used, additional rise of thermal performance with nanofluid is obtained up to 10% when compared to case by using only pure fluid.

Three-dimensional core-collapse supernovae with complex magnetic structures – II. Rotational instabilities and multimessenger signatures

ABSTRACT The gravitational collapse of rapidly rotating massive stars can lead to the onset of the low T/|W| instability within the central proto-neutron star (PNS), which leaves strong signatures in both the gravitational wave (GW) and neutrino emission. Strong large-scale magnetic fields are usually invoked to explain outstanding stellar explosions of rapidly rotating progenitors, but their impact on the growth of such instability has not yet been cleared. We analyse a series of three-dimensional magnetohydrodynamic models to characterize the effects of different magnetic configurations on the development of the low T/|W| and the related multimessenger features. In the absence of magnetic fields, we observe the growth on dynamical time-scales of the low T/|W|, associated with a strong burst of GW and a correlated modulation of the neutrino emission. However, models with a strong magnetic field show a quenching of the low T/|W|, due to a flattening of the rotation profile in the first ∼100 ms after shock formation caused by the magnetic transport of angular momentum. The associated GW emission is weakened by an order of magnitude, exhibits a broader spectral shape, and has no dominant feature associated with the PNS large-scale oscillation modes. Neutrino luminosities are damped along the equatorial plane due to a more oblate PNS, and the only clear modulation in the signal is due to Standing Accretion Shock Instability activity. Finally, magnetized models produce lower luminosities for νe than for $\bar{\nu }_e$, which is connected to a higher concentration of neutron-rich material in the PNS surroundings.

Enhancement of Out-of-Plane Spin-Orbit Torque by Interfacial Modification.

Spin-orbit torque (SOT)-induced switching of perpendicular magnetization in the absence of magnetic field is crucial for the application of SOT-based spintronic devices. Recent works have demonstrated that the low-symmetry crystal structure in CuPt/CoPt can give rise to an out-of-plane (OOP) spin torque and lead to deterministic magnetization switching without an external field. However, it is essential to improve OOP effective field for the efficient switching. In this work, the impact of interface oxidation on the generation of OOP effective field in a CuPt/ferromagnet heterostructure is systematically studied. By introducing an oxidized CuPt surface, it is found that the field-free switching performance shows remarkable improvement. OOP effective field measurement indicates that the oxidation treatment can enhance the OOP effective field by more than two times. It is also demonstrated that this oxidation-induced OOP SOT efficiency enhancement is independent of the device shapes, magnetic materials, or magnetization easy axis. This work contributes to improve the performance of SOT devices and provides an effective fabrication guidance for future spintronic devices that utilize OOP SOT.

Laser‐Assisted Growth of Fe3O4 Nanoparticle Films on Silicon Substrate in Open Air

This work presents a growth of Fe3O4 nanoparticle films on silicon substrate. The iron oxide is deposited applying a pulsed laser deposition technique. The process is performed in open air in the absence and presence of external magnetic field. In fact, the morphologies of the obtained Fe3O4–Si samples are similar. The Fe3O4 nanoparticles are spherical with average diameters of 30 nm and are densely agglomerated on the Si substrate. The Fe3O4–Si material prepared in the absence of magnetic field has revealed more intense signals during X‐ray diffraction and Raman measurements. The magnetic investigations indicate that the Fe3O4 nanoparticles are significantly coupled with the Si substrate and do not exhibit superparamagnetic behavior. Moreover, the Verwey transition is 98 K for both investigated Fe3O4–Si samples.

Influence of grinding, pressing and sintering of the powder 50.0 wt% Fe, 47.0 wt% BaTiO3 and 3.0 wt% TiO2 on morphology, microstructure, magnetic and electric properties

A mixture of 50.0 wt% Fe, 47.0 wt% BaTiO3, and 3.0 wt% TiO2 powders is milled and then pressed and sintered. Samples sintered from ground powders have a structure consisting of plates. They consist of the crystalline phase BaFe12O19 and crystalline and amorphous phases Fe, BaTiO3, and TiO2. Samples sintered from powders ground for more than 180 min are compact and composed of BaFe12O19 crystals embedded in an amorphous matrix consisting of amorphous BaTiO3 and TiO2 phases. Magnetization of a sample ground for 220 min gradually decreases with increasing temperature up to 440?C. This decrease result from the transition of ordered domains into chaotically directed domains. Above 440?C, the thermal energy is sufficiently large to destroy ordered paramagnetic ferromagnetic and weakly ferromagnetic domains. During sample cooling in the absence of an external magnetic field, the temperature dependence of magnetization is the same as that obtained during heating in the magnetic field. However, during the sample cooling in the magnetic field, substantially higher values of magnetization are obtained. Considerably higher magnetization of the cooled sample remains even after switching off the external magnetic field. The sintered sample, previously pressed in the magnetic field, has a higher dielectric constant than the sample pressed in the absence of the magnetic field. The dielectric constant decreases with increasing frequency.

MAGNETOHYDRODYNAMIC FLOW AND HEAT TRANSFER THROUGH A CIRCULAR TUBE FILLED WITH HOMOGENEOUS AND HETEROGENEOUS POROUS MEDIUM

In this paper, we study the laminar steady flow of incompressible viscous magnetohydrodynamic (MHD) fluid inside a circular tube filled with homogeneous/heterogeneous porous medium. A transverse uniform/nonuniform magnetic field is applied, along with a uniform heat flux boundary condition. The Brinkman equation is used to model the flow through the saturated porous medium. Exact solutions of velocity and temperature fields, friction factor, and Nusselt number (<i>Nu</i>) in terms of the shape parameter (σ) and Hartmann number (<i>M</i>) are presented. In the limit of the zero shape parameter and the absence of magnetic field, the Brinkman model results converge to the Hagen-Poiseuille flow solution of circular pipe flow. In the opposite limit of the infinite shape parameter and the absence of magnetic field, the solutions approach the Darcy model results. In terms of the hydro-thermal characteristics of the flow, the Poiseuille flow results and the Darcy model results are shown to serve as the lower and upper bounds, respectively. A significant increase in the heat transfer in heterogeneous porous medium with the nonuniform magnetic field (highest <i>Nu</i> = 12) compared to homogeneous porous medium with uniform magnetic field is identified (highest <i>Nu</i> = 8). We also validate the results of homogeneous and heterogeneous porous medium with uniform/nonuniform magnetic field using numerical solutions.

Field-like torque-induced tunable zero-field spin-torque nano-oscillator

The spin-torque nano-oscillator (STNO), which is a novel type of nano-sized microwave oscillator driven by direct current, is considered as a promising candidate for future radio frequency (RF) transceivers owing to its scalability, nanoscale size and high frequency tunability. However, the potential application of STNO is limited because its stable oscillation requires an external magnetic field. In this work, the influences of the field-like torque and applied current intensity on the stable oscillation of STNO with a perpendicularly magnetized free layer are studied theoretically based on the macrospin model (also known as the single-spin or single-domain model) and the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation in the absence of magnetic field. It is demonstrated numerically that a stable oscillation of STNO can be observed when the ratio between the field-like torque and the spin torque is a negative value and larger than a certain value that depends on the damping coefficient and the current intensity, whose physical mechanism can be understood by the energy balance equation. Moreover, the frequency of stable oscillation of STNO can be modulated by the ratio between the field-like torque and the spin torque and also by the current intensity. Particularly, the larger the absolute value of the ratio between the field-like torque and the spin torque and the smaller the applied current intensity (above the critical current intensity), the more conducive it is to suppressing the formation of second and third oscillation frequencies, thereby enhancing the STNO’s “single-frequency” feature. Our findings provide a theoretical scheme for realizing a frequency tunable zero-field STNO, which may be useful for designing future RF transceivers.

Terahertz magnetic excitation in antiferromagnets: atomistic spin simulations versus a coupled pendulum model

Understanding and manipulating of the antiferromagnetic (AF) ultrafast spin dynamics in antiferromagnets (AFMs) is a crucial importance issue because of the promising applications in terahertz spintronic devices. In this study, an analytical theory extended from the classic coupled pendulum model has been developed to describe the intrinsic magnetic excitation of AFMs. The derived frequency dispersion of the AF resonances has been further checked by using the atomistic-level Landau–Lifshitz–Gilbert simulations. We show that the rutile crystalline AFM MnF2 possess two separate resonance modes at low magnetic fields: high frequency mode with right-handed polarization and low frequency mode with left-handed polarization. In the absence of magnetic field, these two resonance modes could degenerate into a single resonance state. When the applied magnetic field is higher than the spin-flip field, the system behaves a quasi-ferromagnetic mode. Both quantitative and qualitative agreement with atomistic simulation results confirm the theoretical picture of the AF resonance dynamics. This study provides a simple but physical understanding of the ultrafast dynamics of AF excitations.

Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers. II. Zero-field topological superconductivity

It has been proposed that band-inverted electron-hole bilayers support a phase transition from an insulating phase with spontaneously broken time-reversal symmetry to a quantum spin Hall insulator phase as a function of increasing electron and hole densities. Here we show that in the presence of proximity-induced superconductivity, it is possible to realize Majorana zero modes in the time-reversal symmetry broken phase in the absence of magnetic field. We develop an effective low-energy theory for the system in the presence of a time-reversal symmetry-breaking order parameter to obtain analytically the Majorana zero modes and we find good agreement between the numerical and analytical results in the limit of weakly broken time-reversal symmetry. We show that the Majorana zero modes can be detected in superconductor/time-reversal symmetry broken insulator/superconductor Josephson junctions through the measurement of a $4\ensuremath{\pi}$ Josephson current. Finally, we demonstrate that the Majorana fusion-rule detection is feasible by utilizing the gate voltage dependence of the spontaneous time-reversal symmetry breaking order parameter.

Magnetic reversal stability of spin textures in synthetic antiferromagnetic nanodots

In this work, we demonstrate the nucleation and stability of spin textures in synthetic antiferromagnetic (SAF) nanodot-shaped systems. By employing micromagnetic simulations at 300 K and zero-field, we obtain phase diagrams distinguishing antiferromagnetic (AFM) single domain, skyrmion and skyrmionium states, among other relevant spin textures, depending on the magnetic and geometric parameters of the circular disk. Furthermore, we investigate the magnetization reversal stability under out-of-plane magnetic fields, revealing that, only under specific conditions, AFM skyrmionic textures are preserved in the absence of magnetic fields. The conservation of the skyrmionic structures along the hysteresis cycle is directly linked with an energy barrier reduction for the nucleation of the spin textures. Our findings demonstrate a very promising route in stabilizing AFM chiral skyrmionic structures in nanodots, at ambient conditions, which is a step towards their use in future antiferromagnetic devices.

Anomalous Transport Signatures in Weyl Semimetals with Point Defects.

We present the first theoretical study of transport properties of Weyl semimetals with point defects. Focusing on a class of time-reversal symmetric Weyl lattice models, we show that dilute lattice vacancies induce a finite density of quasilocalized states at and near the nodal energy, causing strong modifications to the low-energy spectrum. This generates novel transport effects, namely, (i)an oscillatory behavior of the dc conductivity with the charge carrier density in the absence of magnetic fields, and (ii) a plateau-shaped dissipative optical response for photon frequencies below the interband threshold, E_{F}≲ℏω≲2E_{F}. Our results provide a path to engineer unconventional quantum transport effects in Weyl semimetals by means of common point defects.

Experimental study on effect of magnetic field on flow dynamics of iron (III) oxide (Fe2O3) - water based nanofluid using Taylor Couette flow apparatus

Experimental investigation of magnetic field effect on Fe3O4-Water based nanofluid undergoing Taylor Couette flow is of great importance since some lubricating systems are replicas of Taylor Couette flow. In this study several experiments were carried out to investigate the influence of nanoparticle concentration on the vortex dynamics of a water based Newtonian nanofluid formulated from Fe2O3 in the presence and absence of magnetic field. Flow dynamics was studied in relation to flow transition, stability, hysteresis and effect of magnetism on these properties. Flow regimes covered in this study include Circular Couette flow, Taylor vortex flow, wavy Vortex flow and Modulated Wavy vortex flow. Bifurcation parameters are nanoparticle volume fraction, inner cylinder rotating frequency and Reynolds number. Experiments were performed on distilled water and Fe3O4-Water based nanofluid to understand its behavior in Taylor Couette apparatus. Critical Reynolds number, Azimuthal wavenumber and travelling waves frequency was recorded for each nanoparticle volume fraction. Power Spectral Analysis of nanofluid flow was carried out using video data from the experiment. It was observed that critical frequency for various transitions decrease with nanoparticle volume fraction for nanofluid in the presence of magnetic field but an inconsistent pattern was observed for nanofluid in the absence of magnetic field. These results show that magnetic field and nanoparticle volume fraction greatly influence the behavior of the flow.

Gaseous slip flow in a porous two-dimensional rectangular microchannel subjected to inclined magnetic field

In this study, two-dimensional, steady, laminar, and compressible gaseous flow of Newtonian fluid in a rectangular porous microchannel affected by an electromagnetic field, and under local thermal equilibrium condition was analytically investigated in the slip flow region (0.001<Kn < 0.1). The porous microchannel is entirely affected by uniform inclined magnetic field with an angle α from the positive x-axis, the range of this angle is 0 ≤ α ≤ 90°. Also, the porous microchannel effected by uniform electric field which points in the positive z-direction. Thermophysical properties of the fluid were assumed to be constant. The normalized governing equations were asymptotically solved and expressions of the flow velocity components, the pressure, and the temperature were provided using first-order velocity slip and temperature jump boundary conditions. The effects of various parameters on the flow were studied, such as Darcy number, the porosity, the thermal conductivity ratio, the electrical conductivity ratio, Hartmann number, Knudsen number, the electric field to magnetic field ratio (0 ≤ K ≤ 1), and the magnetic field inclination angle. It was found that increasing the permeability of the porous medium increases the flow velocity but decreases its temperature. Further, the results reveal that applying a stronger magnetic field in absence of electric field decreases the flow velocity but increases its temperature. On the other hand, the stronger the electric field, the higher the flow velocity and its temperature. Also, as the magnetic field inclination angle increases the flow velocity decreases. The effects of Darcy number and the electrical conductivity ratio on the heat flux were studied as well. A comparison of the analytical solutions with the numerical results were presented as a validation for this study. The comparison showed that the analytical results were in good agreement with the numerical results.

Stefan flow of MHD chemically reactive nanofluid past a plate with Navier slip and convective boundary condition

This article plans to report the consequences of Navier slip and Stefan blowing on forced convection nanofluid flow past a plate in presence of a magnetic field with zero nanoparticle flux at the boundary which has not yet been addressed by any researcher and indicates the novelty of our work Nanofluid is considered to be chemically reactive and convective boundary condition has also been considered in this study. By the proper conversion by using similarity transformations, the leading PDEs (Partial Differential Equations) are shortened to ODEs (Ordinary Differential Equations) and those nonlinear equations are then numerically solved by shooting technique. Due to increase in magnetic parameter, fluid velocity and concentration augment (initially) but the temperature reduces. Compared to the result for in absence of magnetic field it is noted that, shear stress at the wall becomes almost double for half strength of the magnetic field. Moreover, in this case rate of heat transfer at the wall becomes almost double when the convective parameter becomes double. The concentration of nanoliquid declines by the rise in the speed of compound reaction. The outcome of this examination uncover a variety of inspiring distinctiveness which insist further study of the problem.

Quantum anomalous Hall effect in M2X3 honeycomb Kagome lattice

The quantum anomalous Hall (QAH) effect has recently drawn great attention in spintronics with extraordinary property of chiral edge states without dissipation in absence of magnetic field. In M2X3 honeycomb Kagome lattice, numerous two-dimensional materials are predicted to be QAH insulators including metal oxides/sulfides and metal organic lattice. In this work, we proposed a general model to explain the mechanism of Dirac half metal with absence of spin orbital coupling and the nontrivial topological property with spin orbital coupling, which could be induced by combination of electron counting rule, crystal field effect and orbitals hybridization. Based on the mechanism, we further predict that triphenyl-metal lattice M2(C6H4)3 (M = V, Nb, Ta) are all QAH insulators with high Curie temperature and large nontrivial band gap for triphenyl-Nb and triphenyl-Ta lattice.

Magnetic-field-controlled multitude of spin excitation modes in magnetoelectric Co4Nb2O9 as investigated by magnetoterahertz spectroscopy

Gapped spin orders induce electric polarization in multiferroics with strong magnetoelectric (ME) coupling. The sensitivity of terahertz (THz) radiation to the spin gaps and the dielectric medium can uniquely access this technologically relevant ME effect. Here, we implement magneto-THz time-domain spectroscopy to demonstrate the manifestation of spin-wave excitations responsible for the ME coupling in multiferroic compound ${\mathrm{Co}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$. We observed two sharp and a weak spin-gap resonance mode at 0.77, 1.58, and 0.91 THz at 5 K below the antiferromagnetic ordering in the absence of magnetic field. In an applied magnetic field, the strength of the 0.77 THz mode decreases while two field-induced modes, namely, the Goldstone mode and magnon excitation, appear at 0.27 and 1.50 THz, respectively. This THz optical evidence of a rich manifestation of field-controlled magnetic resonances, a mutual transfer of optical spectral weight between the zero-field and field-induced excitations, and associated change of complex refractive index in magnetic fields unravel a unique ME coupling in ${\mathrm{Co}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$.

Hybrid nano-jet impingement cooling of a curved elastic hot surface under the combined effects of non-uniform magnetic field and upper plate inclination

In this study, slot hybrid nano-jet impingement cooling application of a curved elastic surface is considered under the combined effects of non-uniform magnetic field and upper plate inclinations. When magnetic field is present, the local Nusselt number (Nu) peaks are suppressed, but in absence of magnetic field, profound rise of stagnation point Nu is seen with higher Re. The increment amount of the local peak Nu becomes 155.5% and 50.5% at Ha=0 and Ha=20. Significant variations of the local peak Nu locations are obtained when elastic modulus are varied with magnetic field at the highest strength. There is 141% variation of average Nu for the curved elastic part when walls with the lowest and highest elastic modulus cases are compared. There is 7% variations in the average Nu when the amplitude of the magnetic field is varied. The left and right plate inclinations contributed differently to the overall convective cooling performance due to the non-uniform magnetic field effects. Non-uniform magnetic field parameters along with the plate inclinations can be used for cooling performance of confined slot jet impingement system for a surface with curved elastic part.

Electrochemical dealloying in a magnetic field – Tapping the potential for catalyst and material design

Nanocatalyst optimisation through electrochemical dealloying has been employed as a successful strategy to increase catalytic activity, while reducing the need for precious metals. We present here a new pathway to influence the electrochemical dealloying, through external homogeneous magnetic fields. A homogeneous magnetic field with a flux density of 450 mT in two orientations, parallel or perpendicular to the current direction, was used during electrochemical dealloying using cyclic voltammetry of AgAu nanoparticles. We found increased porosity for low dealloying cycle numbers and improved catalytic properties after longer cycling, compared to nanoparticles dealloyed in the absence of magnetic fields. These findings demonstrate that magnetic fields applied during electrochemical dealloying have currently untapped potential that can be used to influence material properties in a new way and give researchers another powerful tool for material design.