Behavior Of Magnetic Field Research Articles

Phenomenological theory of the magnetic 90∘ helical state

We explore a phenomenological phase diagram for the magnetic helical state with ${90}^{\ensuremath{\circ}}$ turn angle between neighboring spins in the external magnetic field. Such a state is formed by the Eu spin layers in the superconducting iron arsenide ${\mathrm{RbEuFe}}_{4}{\mathrm{As}}_{4}$. The peculiarity of this spin configuration is that it is not realized in the standard Heisenberg model with bilinear exchange interactions. A minimum model allowing for such a state requires the biquadratic nearest-neighbor interaction term. In addition, in tetragonal materials, the ${90}^{\ensuremath{\circ}}$ helix state may be stabilized by the in-plane fourfold anisotropy term, which also fixes helix orientation with respect to the crystal lattice. Such a system has a very rich behavior in the external magnetic field. The magnetic field induces the metamagnetic transition to the double-periodic state with the moment angles ($\ensuremath{\alpha}$, $\ensuremath{\alpha}$, $\ensuremath{-}\ensuremath{\alpha}$, $\ensuremath{-}\ensuremath{\alpha}$) with respect to the field for the four subsequent spins. The transition field to this state from the deformed helix is determined by the strength of biquadratic interaction. The transition is second-order for small biquadratic coupling and becomes first-order when this coupling exceeds the critical value. On the other hand, the aligned state at high magnetic field becomes unstable with respect to the formation of an incommensurate fan state, which transforms into the double-periodic state with decreasing magnetic field. The range of this incommensurate state near the saturation field is proportional to square of the biquadratic coupling. In addition, when the magnetic field is applied along one of four the equilibrium moment directions, the deformed helix state experience the first-order rotation transition at the field determined by the fourfold anisotropy.

Numerical simulation of coupled fluid flow and solidification in a curved round bloom continuous caster with a combined rotary electromagnetic stirring

ABSTRACT The magnetically driven fluid flow and solidification process in a curved round bloom continuous caster for production of a highstrength low-alloy steel grade (Q345) with the diameter of ø600mm by a combined rotary electromagnetic stirring including the mold, the strand and the final electromagnetic stirring (M+S+F-EMS) is numerically described. The strand surface temperature and the solidified shell thickness are compared and validated by the available measured data. The magnetic field, fluid flow, temperature field and macrosolidification behaviors under the combined electromagnetic stirring are analyzed. The effects of casting speed and the position of the FEMS on the solidification and electromagnetic stirring are focused. Results show that the variation of casting speed is very important to control the length of liquid core and the solidification pool thickness for a large-size round bloom. The pool thickness and liquid fraction of the melt determines the range of the optimum stirring location and the effectiveness of the final electromagnetic stirring.

Zeeman and Davydov splitting of Frenkel excitons in the antiferromagnet CuB2O4

The optical spectra of antiferromagnetic copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ are characterized by an exceptionally rich structure of narrow absorption lines due to electronic transitions within the magnetic ${\mathrm{Cu}}^{2+}$ ions, but their unambiguous identification and behavior in magnetic fields have remained far from being fully understood. We study the polarized magnetoabsorption spectra of this tetragonal antiferromagnet with high spectral resolution across the energy range of $1.4055--1.4065$ eV in magnetic fields up to 9.5 T for temperatures from 1.6 up to the N\'eel temperature ${T}_{N}=20$ K. We observe a set of eight absorption lines at $T=1.6$ K in magnetic fields exceeding 1.4 T, which we identify as arising from Frenkel excitons related to the ground and first excited states of the ${\mathrm{Cu}}^{2+}$ ions. The number of these excitons is defined by the presence of the four ${\mathrm{Cu}}^{2+}$ ions with doubly degenerate spin state $S=1/2$ at the $4b$ positions in the crystallographic unit cell. The energies of these excitons are determined by the exchange interaction of 0.5 meV of the ${\mathrm{Cu}}^{2+}$ ions in the excited state with the surrounding ions and by the Davydov splitting of 0.12 meV. In large magnetic field the observed Zeeman splitting is controlled by the anisotropic $g$-factors of both the ground and excited states. We develop a theoretical model of Frenkel excitons in the magnetic field that accounts for specific features of the spin structure and exchange interactions in ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$. The model is used for fitting the experimental data and evaluation of the Frenkel exciton parameters, such as the Davydov splitting, the molecular exchange energy, and the $g$-factors of the ground and excited states of the ${\mathrm{Cu}}^{2+}$ ions.

MHD Flow around Circular Cylinder UnderControl through three Different magnetic Permeability Status

Through the current work, the flow around an insulating cylinder of various magnetic permeability, submitted to a flow of an incompressible electrically conducting fluid has been investigated. The homogeneous fluid flow and the homogeneous applied magnetic field are aligned at infinity. When the cylinder has the similar magnetic permeability as the fluid, the applied field is not influenced by the cylinder. However, when the cylinder has a different magnetic permeability than that of the fluid, the applied field is agitated by the cylinder and then the flow structures will become different from those of the previous case. Consequently, two cases can be distinguished following the last state whereas the cylinder magnetic permeability may have less or greater value than the fluid one’s. The best agreed results that have been obtained following the comparison to the literature magnetic field behaviors were selected .The event has been occurred, where the convergence and divergence of the magnetic stream lines were realized to the previous both cases of magnetic permeability situations, respectively. In fact, an important influence of magnetic field on the fluid has been occurred. Consequently, it can be concluded that the fluid has significant influences on the cylinder structure, through the impact of the fluid structure phenomenon, the shedding vortex process and its suppression. Accordingly, good results of drag aerodynamic coefficient evolution were obtained at the fixed value of the Reynolds number Re=550, for different Stuart numbers, greater than and lessen than a critical value adopting at N=0.8.

Влияние магнитных и электрических полей на динамику образования плазмоидов в гатчинском разряде

The behavior in magnetic and electric fields of the Gatchina discharge, which is used mainly to create an analog of ball lightning in the laboratory in a normal atmosphere, is analyzed. Shown that in these studies it is possible to determine the sign of an uncompensated electric charge as in the active phase of the discharge, and in the forming long-lived luminous formations. Also shown that electric and magnetic fields can change the direction of movement of the forming luminous formation and even completely block its formation. The type and mechanism of existence firework ball lightning are considered, photos of which are presented in widely known monographs on the ball lightning.

Possible superconductivity at ∼70 K in tin hydride SnHx under high pressure

Hydrides of air non-sensitive IVA-group elements were predicted to be possible high temperature superconductors (HTS) by N.W. Ashcroft (PRL 92, 187002 (2004)), which inspired the exploration of hydrogen dominated metallic alloy-based HTS. Although the IVA-group hydrides have been investigated theoretically by several groups, the experimental realization of any superconducting IVA-group hydrides is still lacking. Here, we report on the observation of the possible superconducting transition at ∼70 K in a SnHx sample synthesized by laser heating Sn and ammonia borane inside a diamond anvil cell at ∼200 GPa. The main phase of the obtained sample can be indexed with the monoclinic C2/m SnH12 via comparison with the theoretical structural modes, which have distinct H-network with respect to the clathrate-type structure. A sudden drop of resistance and the systematic downward shift under external magnetic fields signal the occurrence of superconductivity in SnHx with an upper critical field μ0Hc2(0) ≈ 11.2 T, and the resistive broadening behavior in magnetic fields follows the trend of traditional standard superconductors. This work provides an alternative way to obtain metal hydrides HTS via p electron-metals rather than the active s or f electron-metals (such as Mg, Ca, La, Y, Ce), which will stimulate the experimental exploration of hydrides HTS in different element groups.

Model for nuclear spin product-state distributions of ultracold chemical reactions in magnetic fields

Based on a theoretical model where the nuclear spins remain unchanged during a collision, we provide an analytical and general expression for the nuclear spin state-to-state distribution of an ultracold chemical reaction in a magnetic field, for given rotational transitions of the molecules. It simply requires knowledge of the field-dependent eigenfunctions of the molecular reactants and products of the chemical reaction. The final state-to-state distribution drastically changes with the magnetic field. When the distribution is summed over all the final products, a simplified expression is found where only the knowledge of the eigenfunctions of the molecular reactants is required. The present theoretical formalism has been successfully used to explain the magnetic field behavior of the product-state distribution in chemical reactions of ultracold KRb molecules [Hu et al., Nat. Chem. 13, 435 (2021)].

Magnetic field behavior in s+is and s+id superconductors: Twisting of applied and spontaneous fields

We consider magnetic field screening and spontaneous magnetic fields in $s+is$ and $s+id$ superconductors both analytically and numerically. We show that in general, the linearized model couples the moduli of order parameters to the magnetic modes. This causes magnetic field screening that does not follow the standard exponential law and hence cannot be characterized by a single length scale: the London penetration length. We also demonstrate that the resulting linear mixed modes, correctly predict spontaneous fields and their orientation. We show that these mixed modes cause external fields to decay non-monotonically in the bulk. This is observed as the magnetic field twisting direction, up to an angle of $\pi/2$, as it decays in the nonlinear model. Finally, we demonstrate that there are two non-degenerate domain wall solutions for any given parameter set. These are distinguished by either clockwise or anti-clockwise interpolation of the inter-component phase difference, each producing a different solution for the other fields. However, only domain wall solutions in $s+id$ systems exhibit magnetic field twisting.

Contactless Fault Detection of a DC Motor Direction of Rotation Using Its Stray Magnetic Field

In large-scale manufacturing and assembly applications, especially when trying to automate most steps, implementing quality control as early in the process as possible is the key to prevent expenses later. We deal mainly with the production of DC motor powered fuel pumps, which are commonly used in the automotive industry. The goal of this paper is to present a newly developed technique for non-invasive fault detection of a DC motor’s direction of rotation using a stray magnetic field out of the motor chassis. The results presented in this paper show that it is possible to detect faults even on low-power motors while the algorithm is kept as simple as possible to allow for large-scale deployment on a production line. It also gives new insight into the behavior of the stray magnetic field of electric motors, which may benefit other applications and future research.

Temperature Dependence of Magnetic Properties and Magnetic Field Behavior of Co/Gd/Co Thin-Film Three-Layer Systems

Temperature Dependence of Magnetic Properties and Magnetic Field Behavior of Co/Gd/Co Thin-Film Three-Layer Systems

INFLUENCE OF MATERIAL STRUCTURE ON THE MAGNETOSTRICTIVE PROPERTIES OF A RADIATOR FOR DEFROSTING HEAT EXCHANGERS OF VENTILATION EQUIPMENT

the article deals with the properties of ferromagnetics and their behaviour in an external magnetic field. The conditions under which magnetism occurs in materials are shown and the choice of material for a magneto-strictive emitter is justified. The composition and properties of permendur as the most suitable material for the manufacture of magnetostrictive radiators are presented. It is shown that for the manufacture of the magnetostrictor it is feasible to use electro-erosion equipment for cutting packages from permendur com-pared to the costly and cumbersome method of stamping in a matrix of a particular shape. Tests were carried out on a duralumin heat exchanger with artificial frostbite. The evaporator was fed with refrigerant at 0.22 MPa, which corresponds to the boiling point of R 410a refrigerant at 35°C, by means of a refrigerant line made of aluminium pipes. Frostbite was then produced by applying moist air using an ultrasonic steam gen-erator. Frost on the evaporator surface is discharged by means of a magnetostrictor mounted on the heat ex-changer. The proposed method allows for the most effective cleaning of the surfaces of heat exchangers of ventilation equipment from scale, fouling and other mechanical deposits by means of mechanical vibrations.

Effect of Liquid Crystalline Host on Structural Changes in Magnetosomes Based Ferronematics.

The effect of the liquid crystalline host on structural changes in magnetosomes based on ferronematics is studied using the surface acoustic wave (SAW) technique supported by some capacitance and light transmission measurements. The measurement of the attenuation response of SAW propagating along the interface between LC and the piezoelectric substrate is used to study processes of structural changes under magnetic field. The magnetosome nanoparticles of the same volume concentration were added to three different nematic LCs, 5CB, 6CB, and E7. Unlike to undoped LCs, the different responses of SAW attenuation under the influence of magnetic and electric fields in LCs doped with magnetosomes were observed due to characteristic structural changes. The decrease of the threshold field for doped LCs as compared with pure LCs and slight effects on structural changes were registered. The threshold magnetic fields of LCs and composites were determined from capacitance measurements, and the slight shift to lower values was registered for doped LCs. The shift of nematic-isotropic transition was registered from dependencies of SAW attenuation on temperature. The acoustic anisotropy measurement approved the previous supposition about the role of bulk viscosity in used SAW measurements. In addition, capacitance and light transmition investigations supported SAW results and pointed out conclusions about their magnetic field behavior. Obtained results are discussed and confronted with previous ones and coincide well with those observed using acoustic, optical, or dielectric techniques.

Effect of multi-ion species on collisionless plasma driven by a compact plasma focus device in perpendicular magnetic field

We investigated the behavior of a mixed-gas plasma flow driven by a compact plasma focus device into a perpendicular magnetic field, by using a one-dimensional hybrid particle-in-cell simulation. When the plasma flowed into the perpendicular magnetic field, some accelerated ions were generated in front of the plasma flow owing to the compression of the magnetic field. The behavior of the compressed magnetic field changed by mixing heavy ions with the collisionless plasma. The maximum velocity of the accelerated light ions was proportional to the square root of the average ion mass of the plasma. These results indicate that the effect of the multi-ion species strongly affects the injection of light ions.

Entropy analysis for radiative inclined MHD slip flow with heat source in porous medium for two different fluids

The motive of the current investigation is to present a comparative study of entropy generation on Newtonian and Non-Newtonian fluids. Heat transfer of the slip flow over a melting expending surface, is investigated with an imposed heat source and non-uniform radiation. Uniform inclined magnetic field is also applied and medium is considered porous. A further non-linear chemical reaction is also studied. Non-Newtonian Williamson fluid is taken and compared with viscous Newtonian fluid. The developed dimensionless governing equations by using proper similarity transforms are solved with Runge-Kutta method of order four with shooting technique and numerical outcomes are described for various non-dimensional physical parameters by the graphs. From the results after comparing with available literature, it is concluded that the entropy generation can be increased with higher magnetic field parameter, parameter of porosity and inclination angle α of magnetic field and opposite behavior is seen with decreasing values of slip parameter for both fluids. With increasing the melting surface parameter, the thickness of the temperature profile get cut down.

Surrogate approximation of the Grad–Shafranov free boundary problem via stochastic collocation on sparse grids

In magnetic confinement fusion devices, the equilibrium configuration of a plasma is determined by the balance between the hydrostatic pressure in the fluid and the magnetic forces generated by an array of external coils and the plasma itself. The location of the plasma is not known a priori and must be obtained as the solution to a free boundary problem. The partial differential equation that determines the behavior of the combined magnetic field depends on a set of physical parameters (location of the coils, intensity of the electric currents going through them, magnetic permeability, etc.) that are subject to uncertainty and variability. The confinement region is in turn a function of these stochastic parameters as well. In this work, we consider variations on the current intensities running through the external coils as the dominant source of uncertainty. This leads to a parameter space of dimension equal to the number of coils in the reactor. With the aid of a surrogate function built on a sparse grid in parameter space, a Monte Carlo strategy is used to explore the effect that stochasticity in the parameters has on important features of the plasma boundary such as the location of the x-point, the strike points, and shaping attributes such as triangularity and elongation. The use of the surrogate function reduces the time required for the Monte Carlo simulations by factors that range between 7 and over 30.

Ion beam etching dependence of spin–orbit torque memory devices with switching current densities reduced by Hf interlayers

We report on the fabrication of nanoscale, three-terminal in-plane spin–orbit torque switching devices with low switching current densities. Critical parameters in the fabrication process, including the ion beam etching angle and time, were optimized to avoid fabrication defects and improve device yield. Measurements of the magnetic field and current-induced switching behavior of the tunnel junctions demonstrate a sensitivity to the nanopillar aspect ratio, which dictates the nanopillars’ anisotropy and thermal stability. Additionally, we show that the current density required for switching can be reduced and the device thermal stability increased by inserting Hf interlayers into the heterostructure. Micromagnetic simulations are generally consistent with the experimentally observed switching behavior, suggesting an increase in the interfacial perpendicular anisotropy at the CoFeB/MgO interface and the reduction in the Dzyaloshinskii–Moriya interaction at the W/CoFeB interface by the Hf interlayers.

Element-specific contributions to improved magnetic heating of theranostic CoFe2O4 nanoparticles decorated with Pd

Decoration with Pd clusters increases the magnetic heating ability of cobalt ferrite (CFO) nanoparticles by a factor of two. The origin of this previous finding is unraveled by element-specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) combined with atomic multiplet simulations and density functional theory (DFT) calculations. While the comparison of XAS spectra with atomic multiplet simulations show that the inversion degree is not affected by Pd decoration and, thus, can be excluded as a reason for the improved heating performance, XMCD reveals two interrelated responsible sources: significantly larger Fe and Co magnetic moments verify an increased total magnetization which enhances the magnetic heating ability. This is accompanied by a remarkable change in the field-dependent magnetization particularly for Co ions which exhibit an increased low-field susceptibility and a reduced spin canting behavior in higher magnetic fields. Using DFT calculations, these findings are explained by reduced superexchange between ions on octahedral lattice sites via oxygen in close vicinity of Pd, which reinforces the dominating antiparallel superexchange interaction between ions on octahedral and tetrahedral lattice sites and thus reduces spin canting. The influence of the delocalized nature of Pd 4d electrons on the neighboring ions is discussed and the conclusions are illustrated with spin density isosurfaces of the involved ions. The presented results pave the way to design nanohybrids with tailored electronic structure and magnetic properties.

Effects of Ferromagnetism Particles on Electrical Tree Growth in Epoxy/Fe3O4 Composites in Magnetic Field

In this paper, Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> particles with different diameters are added to epoxy resin. Dielectric properties and the electrical tree growth characteristics of the specimens are tested in magnetic field. Experimental results show that magnetic field leads to higher charge carrier mobility and more serious electrical tree degradation. The addition of ferromagnetism particles with a diameter of 20 nm generates more deep traps and causes the decrease of conductivity. Moreover, nanocomposites show great inhibition effect to electrical tree whether high magnetic field is applied. Particles with a diameter of 500 nm play a role as scattering center of electrical tree and cause larger damage area. Ferromagnetism particles with a diameter of 30 μm induce local magnetic field and electric field concentration, accelerating the growth of electrical tree. Quantum chemical and electrodynamics calculation are combined to analyze the energy level distribution and the charge carrier dynamic behavior in high magnetic field. Changes in electrical tree growth result from the size effects of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> particles, including changing relative permeability, as well as acting as exciton killer, scattering center and weak points.

Magnetic Field Draping of the Heliopause and Its Consequences for Radio Emission in the Very Local Interstellar Medium

Abstract We discuss the observations and simulations related to the interaction of the solar wind (SW) and local interstellar medium (LISM), and the interstellar magnetic field draping around the heliopause (HP). This Letter sheds light on some processes that are not directly seen in the Voyager data. Special attention is paid to the magnetic field behavior at the HP crossing, penetration of shocks, and compression waves across the HP, and their merging in the LISM surrounding it. Modeling identifies forward and reverse shocks propagating through the heliosheath. Voyager data shows that the magnetic field strength experiences a jump at the HP, while the elevation and azimuthal angles are continuous across it. We show that our prior numerical results are in agreement with the Voyager data, if the heliospheric magnetic field is not assumed unipolar. The simulations confirm the importance of taking into account time dependencies of the SW flow, including the presence of transient structures and magnetohydrodynamic instabilities. For the first time, we provide the heliospheric community with the Alfvén speed distribution observed by Voyagers, which shows that it is unexpectedly small and decreases with distance from the HP. This is of critical importance for the identification of physical mechanisms responsible for the Langmuir wave and radio emission generation behind the HP. The data shows that outward-propagating, subcritical shocks traversing the LISM have a rather wide dissipation structure, which raises questions about their ability to reflect electrons as collisionless shocks can do.

Point-of-need detection of pathogen-specific nucleic acid targets using magnetic particle spectroscopy

The ongoing COVID-19 pandemic stresses the need for widely available diagnostic tests for the presence of SARS-CoV-2 in individuals. Due to the limited availability of vaccines, diagnostic assays which are cheap, easy-to-use at the point-of-need, reliable and fast, are currently the only way to control the pandemic situation.Here we present a diagnostic assay for the detection of pathogen-specific nucleic acids based on changes of the magnetic response of magnetic nanoparticles: The target-mediated hybridization of modified nanoparticles leads to an increase in the hydrodynamic radius. This resulting change in the magnetic behaviour in an ac magnetic field can be measured via magnetic particle spectroscopy (MPS), providing a viable tool for the accurate detection of target nucleic acids.In this work we show that single stranded DNA can be detected in a concentration-dependent manner by these means. In addition to detecting synthetic DNA with an arbitrary sequence in a concentration down to 500 pM, we show that RNA and SARS-CoV-2-specific DNA as well as saliva as a sample medium can be used for an accurate assay. These proof-of-principle experiments show the potential of MPS based assays for the reliable and fast diagnostics of pathogens like SARS-CoV-2 in a point-of-need fashion without the need of complex sample preparation.