Direction Of External Magnetic Field Research Articles

Numerical study of magneto-convection flows in a complex prototypical liquid-metal fusion blanket geometry

Abstract Numerical simulation of time-dependent magneto-convection flows in a liquid metal breeding blanket unit cell prototypical to the geometry of Water-Cooled Lithium-Lead (WCLL) breeding blanket is performed by COMSOL Multiphysics. Its modeling capability for fully coupled magnetohydrodynamic (MHD)/heat transfer flows in a complex geometry under fusion relevant conditions is tested with the demonstration of the main flow features and temperature profiles. Here, the magneto-convection flow in the bulk region is dominated by the effect of natural convection, leading to two columnar-like, counter-rotating circulations which occupy the whole plenum region of the breeding zone along the external magnetic field direction. High velocity flow jets with the magnitude of 1–2 orders greater than inlet velocity are formed near structural walls that are parallel to the applied magnetic field and break into unsteady sidewall vortices that are mainly confined in boundary layers. Due to the buoyancy effect, the hot fluid particles are stratified near the top structure wall where the cooling effect is the least. The calculated maximum temperature at the wall is lower than the structure’s temperature limit but still exceeds the corrosion temperature limit.

Cooling a Spin Relaxation Hot Spot

The rate at which electron spins relax in silicon quantum dots is controlled by the strength and direction of external magnetic fields.

Attachment characteristics of charged magnetic cubic particles to two parallel electrodes (3D Monte Carlo simulations)

ABSTRACT The cubic particles magnetised along a diagonal direction are negatively charged and confined between two parallel electrode walls. These particles have a tendency to migrate toward and attach to the positive electrode together with the formation of chain-like clusters. We here have focused on three cases of external magnetic field direction. In the case of an external magnetic field applied in the oblique direction to the electrode wall, almost all the constituent particles of the chain-like clusters attach to the positive electrode wall in a face-to-face contact manner. In the case of an external magnetic field applied along the surface of the electrode wall, the chain-like clusters are formed in the space of the two parallel walls. In the case of an external magnetic field applied normal to the electrode wall, the tendency for the particles to form chain-like clusters along the electrode wall is the weaker among the three field directions. In this situation, the cubic particles attach to the wall surface with a vertex of the cube contacting with the surface, and from the anchored particle, a chain-like cluster is formed in the space between the two parallel walls.

Non-classically Controlled Sign in a 1.6 Tesla Magnetic Circularly Polarized Luminescence of Three Pyrenes in a Chloroform and a PMMA Film

Molecular pyrene, 1-hydroxylpyrene, and 2-hydroxypyrene in chloroform and embedded in poly(methyl methacrylate) film reveal mirror-image magnetic circularly polarized luminescence (MCPL) when south (S)-up or north (N)-up direction of the longitudinal external magnetic field of 1.6 T to unpolarized incident light (Faraday geometry) is employed. The |gMCPL| magnitude is on the order of 10−3 T−1 in 374–416 nm. The sign of MCPL is controlled by the S-up and N-up geometry and by the position (1- or 2-) of the hydroxy group.

Anomalous magnetoresistance in detwinned EuFe2As2**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. 2014JJCB27).

The in-plane magnetotransport of detwinned EuFe2As2 single crystal has been investigated. In the antiferromagnetic phase of Eu2+ spins, very different magnetoresistance results are observed upon the change of the external magnetic field direction and the current direction. This could be attributed to the tunable orientation of the Eu2+ spins under magnetic field. Electron scattering by spin fluctuation, superzone boundary effect, and cyclotron motion of charge carriers are used to interpret the observed anomalous magnetoresistance which is measured by using a current along a direction. The remarkable features of magnetoresistance suggest that itinerant electrons strongly couple with the spin configuration of Eu2+, which has a huge influence on the transport properties of EuFe2As2.

Neutron diffraction evidence for local spin canting, weak Jahn–Teller distortion, and magnetic compensation in Ti1−xMnxCo2O4 spinel

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1−xMnxCo2O4 in the temperature interval 1.6 K T 300 K. Our neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1−xMnxCo2O4 system for all the Jahn–Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d1, 3d4 and 3d6, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet–Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length dc(TB-O) is longer than the equatorial bond length dab(TB-O) due to the splitting of the eg level of Mn3+ ions into and . For the composition x = 0.4, the distortion in the TBO6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at TCOMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below TCOMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.

Low-frequency plasma waves in a radiative dusty magnetoplasma

The impact of electromagnetic (thermal) radiation on the dispersive low-frequency waves is examined in a radiative dusty magnetoplasma. For this purpose, a magneto-hydrodynamic model along with the Maxwell equations is employed to describe the three-component radiative dusty plasma that contains inertialess electrons, dynamical positive ions and negatively charged static dust particulates. The behavior of the electrostatic and electromagnetic waves significantly changes in a plasma medium when a radiation pressure is taken into consideration, as compared to the waves in vacuum. After seeking a plane wave solution, a general dispersion relation is obtained to investigate different limiting cases of the low-frequency modes propagating parallel, perpendicular and oblique to the external magnetic field direction both analytically and numerically. The calculations reveal that the usual thermal and acoustic speeds (cT, ca) do not remain constant even if the temperature is kept constant, because of the radiation pressure which strongly depends on the equilibrium number density as well. The present results may prove a useful understanding for the new features of the dispersive dust-ion-acoustic and compressional Alfven waves in astrophysical dusty magnetoplasmas.

Theoretical Modeling and Exact Solution for Extreme Bending Deformation of Hard-Magnetic Soft Beams

Abstract Hard-magnetic soft materials (HMSMs) manufactured by embedding hard-magnetic particles in soft materials belong to a new type of soft active materials. The abilities of fast and complicated transformations of hard-magnetic soft structures provide a promising technology for soft robotics, flexible electronics, and biomedical devices. It is significant to investigate the mechanical behaviors of hard-magnetic soft structures for their better applications. In this work, a hard-magnetic soft beam under an external magnetic field is theoretically modeled and the exact solutions for its mechanical responses are presented. First, the governing equations and boundary conditions are derived based on the principle of minimum potential energy. To solve the derived governing equations analytically, a new polynomial fitting model for hyperelastic materials is proposed for the hard-magnetic soft beam. Then, the exact solutions of a cantilevered hard-magnetic soft beam actuated by a uniform magnetic field in any direction are obtained. The newly derived exact solutions are further verified by comparing current results with those from recent simulations and experiments. For large bending angles up to 90 deg and extreme bending angle up to 180 deg, quite consistent agreement among exact solutions, numerical simulations, and experimental observations can be achieved. Finally, using our theoretical model, the deformation of the hard-magnetic soft beam actuated by magnetic fields in an arbitrary direction with non-zero magnetic declination is explored. When the magnetic actuation is increased from a small level gradually, the hard-magnetic soft beam deflects and it would undergo small, large, and extreme bending deformations in sequence. It is very interesting that, when the magnetic actuation is sufficiently large, the hard-magnetic soft beam is stretched and its centerline tends to align with the external magnetic field direction, implying that the hard-magnetic soft beam undergoes a uniaxial tension. The theoretical modeling and exact solutions for hard-magnetic soft beams are expected to be useful in the analysis and design of soft materials and structures.

Geometry-dependent effects in Majorana nanowires

Starting from the Bogolubov - de Gennes theory describing the induced p-wave superconductivity in the Majorana wire of an arbitrary shape, we predict a number of intriguing phenomena such as the geometry-dependent phase battery (or a phi-Josephson junction with the spontaneous superconducting phase difference) and generation of additional quasiparticle modes at the Fermi level with the spatial position tuned by the external magnetic field direction. This tuning can be used to extend the capabilities of the braiding protocols in Majorana networks.

Zeeman-splitting-induced topological nodal structure and anomalous Hall conductivity in ZrTe5

We investigate the topological nodal structure of three-dimensional (3D) ZrTe$_5$ driven by Zeeman splitting as a function of the direction of external magnetic (B) field by using a Wannier-function-based tight-binding (WFTB) model obtained from first-principles calculations. It is known that small external stimuli can drive 3D ZrTe$_5$ into different topological phases including Dirac semimetal. In order to emphasize the effect of Zeeman splitting, we consider 3D ZrTe$_5$ in a strong TI phase with a small band gap. With Zeeman splitting greater than the band gap, the WFTB model suggests that a type-I nodal ring protected by (glide) mirror symmetry is formed when the B field aligns with the crystal $a$ or $b$ axes, and that a pair of type-I Weyl nodes are formed otherwise, when conduction and valence bands touch. We show that a pair of Weyl nodes can disappear through formation of a nodal ring, rather than requiring two Weyl nodes with opposite chirality to come together. Interestingly, a type-II nodal ring appears from crossings of the top two valence bands when the B field is applied along the $c$ axis. This nodal ring gaps out to form type-II Weyl nodes when the B field rotates in the $bc$ plane. Comparing the WFTB and linearized $k \cdot p$ model, we find inadequacy of the latter at some B field directions. Further, using the WFTB model, we numerically compute the intrinsic anomalous Hall conductivity ${\sigma_{ac}}$ induced by Berry curvature as a function of chemical potential and B field direction. We find that ${\sigma_{ac}}$ increases abruptly when the B field is tilted from the $a$ axis within the $ab$ plane. Our WFTB model also shows significant anomalous Hall conductivity induced by avoided level crossings even in the absence of Weyl nodes.

FEM solution of MHD flow in an array of electromagnetically coupled rectangular ducts

The magnetohydrodynamic (MHD) flow of an electrically conducting fluid is studied in an array of identical parallel ducts stacked in the direction of external magnetic field and are separated by conducting walls of arbitrary thickness. Such arrangement of electromagnetically coupled ducts arises in fusion blanket applications in which a liquid metal is used both as coolant and tritium generation. The finite element method (FEM) with SUPG stabilisation is used for solving the set of coupled MHD equations. Numerical results show that, there is a significant effect of coupling the ducts with conducting walls of varying thickness, on the flow and induced current behaviours especially near the walls and for increasing values of Hartmann number. The results are presented for one, two and three coupled ducts in both coand counter-flow configurations which induce reversal and counter-current flows.

Terahertz emission generated from Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/CoFeB heterostructures grown by magnetron sputtering

High-performance terahertz emitters, which convert the femtosecond laser pulses into terahertz pulses, are essential for terahertz spectroscopy technology and terahertz wireless communication. Spintronic terahertz emitters based on ferromagnet/nonmagnet bilayers have attracted tremendous attention due to their high efficiency, ultra-broadband, low cost and high flexibility. Here, we systematically investigate the terahertz emission from polycrystalline topological insulator Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/ferromagnetic CoFeB heterostructure grown by magnetron sputtering. The Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/CoFeB heterostructure exhibits high efficiency of terahertz emission, and the polarization of terahertz waves can be controlled by the external magnetic field direction. The performance of Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/CoFeB heterostructure is almost comparable to that of the Pt/CoFeB bilayer. In contrast, no terahertz emission is observed in the pure Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt; or CoFeB film driven by femtosecond laser pulses, probably because the Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt; prepared by sputtering is polycrystalline and the thickness of CoFeB is too thin. We also compare the performances of Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/CoFeB grown on MgO, glass and high-resistivity silicon substrates, and find that the samples grown on MgO substrates exhibit the best emission performances. The glass substrate absorbs more terahertz waves than MgO substrate, resulting in a slightly weaker terahertz signal emitted from the Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/CoFeB grown on the glass substrate. Although the absorption coefficient of high-resistivity silicon to terahertz waves is very small, the residual pump light excites the high-resistivity silicon to generate the photo-generated carriers, which change the conductivity of the high-resistivity silicon and reduce the transmittance of terahertz wave. We attribute the mechanism of the terahertz emission to the spin-charge conversion at the interface of Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;/CoFeB. The terahertz emission efficiency of our sample is expected to be able to be further improved by optimizing the samples. Moreover, with the sputtering method, it is possible to fabricate large area samples at low cost, which is critical for commercial applications.

Microlinear Halbach Array of Thick-Film Nd–Fe–B Magnets Utilizing Local Laser Irradiation and a Direction-Changeable External Magnetic Field

The Halbach array magnetization of thick-film Nd–Fe–B magnets using local laser irradiation is examined. A 75 µm thick isotropic Nd–Fe–B magnet is deposited on a Si substrate by pulsed laser deposition and used to form microlinear Halbach arrays. The micromagnetization patterning of the Halbach arrays is realized by a combination of pulsed magnetization and laser-assisted heating with changing the external magnetic field direction. Halbach magnetic patterns with a pitch of 200 µm are obtained over a surface area of 1.7 mm width and 2.8 mm length. Their magnetic density over the top surface was measured by a scanning Hall probe. The vertical magnetic field value peak-to-peak at 100 µm from the magnet surface was 65 mT for the Halbach pattern, 2.7 times that of 24 mT for an alternating pattern.

Low magnetic field effects on triplet pair annihilations at canonical orientations

Using the density operator formalism, a simple analytical model is developed to study low magnetic field effects on triplet pair annihilations in organic solids. Analysis is restricted to canonical orientations where two identical triplet molecules have the same orientation and the direction of the external magnetic field is parallel to one of the principle axes of the dipolar coupling tensor for a triplet. The analytical solution reveals that the low magnetic field effect in the triplet pair arises from the anisotropic dipole-dipole coupling in a triplet. In the presence of the dipole-dipole coupling, the spin quantization axis for each triplet gradually changes with the increase of the external magnetic field from zero field to high field. The low magnetic field effect reaches a maximum when the Zeeman splitting between the spin states matches a dipole-dipole coupling component orthogonal to the external magnetic field direction. The result is also discussed with the low magnetic field effect in the radical pair with one isotropic hyperfine coupling.

Electric readout of magnetic stripes in insulators

In superconductors, a topological configuration of the superconducting order parameter called a superconducting vortex carries magnetization. Such a magnetic topological object behaves like a minute particle generating a magnetic flux. Since the flux is localized with a nanometer scale, the vortex provides a nano-scale probe for local magnetic fields. Here we show that information of magnetic stripes in insulators can be read out by using vortices in an adjacent superconductor film as a probe. The orientation and width of magnetic micro stripes are both transcribed into resistance change of the superconductor through the modulation of vortex mobility affected by local magnetization. By changing the direction of external magnetic fields, zero-field resistance changes continuously according to the stripe orientation, and its modulation magnitude reaches up to 100%. The width of the stripes can also be estimated from the oscillatory magnetoresistance. Our results demonstrate a new possibility for non-volatile analog memory devices based on topological objects.

Dynamic Model of Magnetic Flux Ropes

Developing ideas of the paper Zaitsev and Stepanov (2018) with regards to activation of a current-carrying filament with an increase in electric current, in which the Ampere force is opposed by gravity and medium viscosity, we take into account the influence of an external magnetic field. It is shown that, depending on mutual directions of the external magnetic field and electric current in the flux rope, the additional Ampere force can lead to the deceleration or to additional eruption of the flux rope. A phase diagram describing the peculiarities of flux rope dynamics has been constructed. The maximum height and velocity of the flare loop are determined for the event on March 30, 2001.

Spin-wave excitation and critical angles in a hybrid photon-magnon-coupled system

We studied “strongly enhanced” excitations of magnetostatic spin-waves in a hybrid magnon-photon-coupled system consisting of an inverted split-ring resonator and an yttrium iron garnet film. The backward-volume magnetostatic wave, magnetostatic surface wave, forward-volume magnetostatic wave modes, as well as multiple mixed modes were observed according to the direction of externally applied magnetic fields with respect to the spin-wave propagation direction. From experimental measurements, we found that there exist certain characteristic angles, which is to say, critical angles (θc, ϕc), whereby the group velocity of the excited spin-wave modes reaches zero, along with a single ferromagnetic resonance peak. Furthermore, we analytically derived the critical angles as sinθc=(Heff/H){2−(1+μ0Ms/Heff)sin2ϕc}−1/2, where Heff is the effective magnetic-field strength, H the externally applied field strength, and μ0Ms is the saturation magnetization. This analytical form is in quantitative agreement with estimations from experimentally observed spin-wave modes. This work not only provides a better understanding of multiple-mode spin-wave excitations in given field directions with the help of photon-magnon coupling, but also offers better coupling controllability based on the change of the external magnetic field direction.

Linear Stability Analysis of Liquid Metal Flow in an Insulating Rectangular Duct under External Uniform Magnetic Field

Linear stability analysis of liquid metal flow driven by a constant pressure gradient in an insulating rectangular duct under an external uniform magnetic field was carried out. In the present analysis, since the Joule heating and induced magnetic field were neglected, the governing equations consisted of the continuity of mass, momentum equation, Ohm’s law, and conservation of electric charge. A set of linearized disturbance equations for the complex amplitude was decomposed into real and imaginary parts and solved numerically with a finite difference method using the highly simplified marker and cell (HSMAC) algorithm on a two-dimensional staggered mesh system. The difficulty of the complex eigenvalue problem was circumvented with a Newton—Raphson method during which its corresponding eigenfunction was simultaneously obtained by using an iterative procedure. The relation among the Reynolds number, the wavenumber, the growth rate, and the angular frequency was successfully obtained for a given value of the Hartmann number as well as for a direction of external uniform magnetic field.

An integrated three-dimensional micro-solenoid giant magnetoimpedance sensing system based on MEMS technology

In this paper, we proposed an integrated three-dimensional (3D) micro-solenoid giant magnetoimpedance (GMI) sensing system. The sensing system is composed by a 3D micro-solenoid inside which patterned Cobalt-based ribbon is kept as magnetic core and this system was fabricated by Micro-electromechanical Systems (MEMS) technology. The system was measured by non-contact method, which has the advantages of high sensitivity, low power consumption and widespread application prospect. The influence of external magnetic field direction and excitation frequency on micro-solenoid GMI sensing system has been discussed respectively. The fabricated system exhibits an optimum magnetoimpedance (MI) ratio of 4360±40 % at the excitation frequency of 13 MHz under longitudinal external magnetic field. In consequence, 3D micro-solenoid GMI sensing system offers great magnetic performance and this integrated sensing system has potential applications in the field of weak magnetic field detection.

Berichtigung: Bioinspired, Size‐Tunable Self‐Assembly of Polymer—Lipid Bilayer Nanodiscs

The University of Michigan found through its internal investigation1 that Sudheer Kumar Ramadugu, one of the co-authors on the paper, had falsified data in the spectra in Figures 4E and 4F in this Communication. The experiments have been repeated and the correct 14N NMR spectra (Fig. 4E and Fig. 1F) demonstrate the magnetic alignment of the macro-nanodiscs as originally reported. The 14N quadrupole couplings in Figure 4E and 4F have been corrected from 8 kHz and 16 kHz to 9 kHz and 18 kHz, respectively. 31P NMR spectra (Fig. 4B and 4C) for the new samples were also recorded. The chemical shift values have been corrected from −17.9 ppm to −13 ppm in Figure 4B and from 20 ppm to 22 ppm in Figure 4C. The corrected Figure 4 and associated caption are as follows. Magnetic-alignment of SMA-EA polymer based macro-nanodiscs: Static 31P (A–C) and 14N (D–F) NMR spectra demonstrate the presence of isotropic nanodiscs (ca. 10 nm diameter) (A,D), the magnetic-alignment of macro-nanodiscs (ca. 50 nm) with the lipid bilayer perpendicular to the external magnetic field (B,E), and flipped macro-nanodiscs (95:5 DMPC:DMPE-DTPA) in the presence of Yb3+ ions with the lipid bilayer parallel to the external magnetic field (C,F). G) 2D 15N–1H TROSY sQC spectrum of isotropic SMA-EA nanodiscs containing 15N-cytochrome b5 reconstituted in DMPC lipid bilayers. All spectra were recorded at 35 °C, where the DMPC lipids are in a fluid lamellar phase bilayer. H) 2D 1H–15N PISEMA spectrum of SMA-EA macro-nanodiscs containing 15N-cytochrome b5 reconstituted in DMPC lipid bilayers with the bilayer normal perpendicular to the external magnetic field. The new data have also resulted in a few minor changes to the text: On page 11626 the sentence should read: The observation of a peak at −13 ppm in the 31P NMR spectrum (Figure 4B) and approximately 9 kHz 14N quadrupole splitting (Figure 4E) suggest that the lipid bilayer normal is oriented perpendicular to the external magnetic field direction. On page 11627 the sentence should read: the 31P peak appeared at around 22 ppm (Figure 4C) and the 14N quadrupole coupling splitting increased by a factor of 2 to become about 18 kHz (Figure 4F). On page 11627 Figure 4I is no longer relevant and thus the sentence “The ability to control the extent of flip of macro-nanodiscs is also demonstrated by recording 31P spectra for various concentrations of Yb3+ (Figure 4I).” is no longer applicable. An updated Supporting Information for this paper is made available online along with this Corrigendum. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.