Magnetic Regime Research Articles

Mass transfer intensification between fluidizing gas and Geldart‐B nonmagnetizable particles in magnetized fluidized bed

AbstractThis work aimed to intensify the mass transfer between fluidizing gas and Geldart‐B nonmagnetizable particles by simultaneously introducing magnetizable particles and applying the magnetic field. The mere addition of magnetizable particles hardly affected the interphase mass transfer. Moreover, such mass transfer was not improved by the subsequent application of the magnetic field under the magnetization‐LAST operation mode. However, the mass transfer was significantly enhanced by the magnetic field in the magnetic stabilization and transition flow regimes under the magnetization‐FIRST operation mode. Apparently, the mass transfer intensification depended not only on the magnetic field intensify (H) but also on the operation mode. The gas–solid contact performance in the magnetic stabilization flow regime was always comparable to that in fixed beds. Hence, further increases in H could not raise the magnitude of intensification but only increase the operating range and stability of the magnetic stabilization, which was in metastable equilibrium.

A Computational Model for the Radiated Kinetic Molecular Postulate of Fluid-Originated Nanomaterial Liquid Flow in the Induced Magnetic Flux Regime

The performance of mass transfer rate, friction drag, and heat transfer rate is illustrated in the boundary layer flow region via induced magnetic flux. In this recent analysis, the Buongiorno model is introduced to inspect the induced magnetic flux and radiative and convective kinetic molecular theory of liquid-initiated nanoliquid flow near the stagnant point. The energy equation is modified by radiation efficacy using the application of the Rosseland approximation. Through similarity variables, the available formulated partial differential equations are promoted into the nondimensional structure. The variation of the induced magnetic field near the wall goes up, and very far away, it decays when the size of the radiation characteristic ascends. The velocity amplitude expands by enlargement in the amount of the magnetic parameter, mixed convection, thermophoresis parameter, and fluid characteristic. The nanoparticle concentration reduces if the reciprocal of the magnetic Prandtl number expands. The temperature spectrum declines by enhancing the amount of the magnetic parameter. Drag friction decreases by the increment in the values of radiation and thermophoresis parameters. Heat transport rate increases when there is an increase in the values of Brownian and magnetic parameters. Mass transfer rate increases when there is incline in the values of the magnetic Prandtl and fluid parameter.

Magnetic Field Dynamic Strategies for the Improved Control of the Angiogenic Effect of Mesenchymal Stromal Cells.

This work shows the ability to remotely control the paracrine performance of mesenchymal stromal cells (MSCs) in producing an angiogenesis key molecule, vascular endothelial growth factor (VEGF-A), by modulation of an external magnetic field. This work compares for the first time the application of static and dynamic magnetic fields in angiogenesis in vitro model, exploring the effect of magnetic field intensity and dynamic regimes on the VEGF-A secretion potential of MSCs. Tissue scaffolds of gelatin doped with iron oxide nanoparticles (MNPs) were used as a platform for MSC proliferation. Dynamic magnetic field regimes were imposed by cyclic variation of the magnetic field intensity in different frequencies. The effect of the magnetic field intensity on cell behavior showed that higher intensity of 0.45 T was associated with increased cell death and a poor angiogenic effect. It was observed that static and dynamic magnetic stimulation with higher frequencies led to improved angiogenic performance on endothelial cells in comparison with a lower frequency regime. This work showed the possibility to control VEGF-A secretion by MSCs through modulation of the magnetic field, offering attractive perspectives of a non-invasive therapeutic option for several diseases by revascularizing damaged tissues or inhibiting metastasis formation during cancer progression.

Magnetic anisotropy in the van der Waals ferromagnet VI3

A comprehensive study of magnetocrystalline anisotropy of a layered van-der-Waals ferromagnet VI3 was performed. We measured angular dependences of the torque and magnetization with respect to the direction of the applied magnetic field within the "ac" plane perpendicular to and within the basal ab plane, respectively. A two-fold butterfly-like signal was detected by magnetization in the perpendicular "ac" plane. This signal symmetry remains conserved throughout all magnetic regimes as well as through the known structural transition down to the lowest temperatures. The maximum of the magnetization signal and the resulting magnetization easy axis is significantly tilted from the principal c axis by ~40{\deg}. The close relation of the magnetocrystalline anisotropy to the crystal structure was documented. In contrast, a two-fold-like angular signal was detected in the paramagnetic region within the ab plane in the monoclinic phase, which transforms into a six-fold-like signal below the Curie temperature TC. With further cooling, another six-fold-like signal with an angular shift of ~30{\deg} grows approaching TFM. Below TFM, in the triclinic phase, the original six-fold-like signal vanishes, being replaced by a secondary six-fold-like signal with an angular shift of ~30{\deg}.

Eigenmode characterizations of slab ion-temperature-gradient instabilities in various magnetic shear configurations

Comprehensive eigenmode characterizations of ion-temperature-gradient (ITG) instabilities in slab geometries with different magnetic shear profiles are investigated using an eigenvalue method. The results in the uniform magnetic shear configuration are verified via the Hamaguchi–Horton theory [S. Hamaguchi and W. Horton, Phys. Fluids B 2, 1833 (1990)]. However, it is interestingly found that the linear growth rate and mode frequency change non-monotonically as the magnetic shear at the half simulation domain s(x < x0) changes continuously from the positive value to the negative value. There are multiple peaks in the dependence curve of the linear growth rate on s(x < x0) in the weak magnetic shear regime. The variation of magnetic shear, which can produce an additional potential well to excite instability, is identified to play an important role in the maximization of the growth rate of slab ITG modes. In the configuration with a moderate separation between two potential wells, multiple ITG modes with higher radial wave numbers l become unstable simultaneously. While |s(x < x0)| is weak compared to the local magnetic shear s(x = x0) at the center mode rational surface, asymmetric structures of low-order eigenmodes are obtained and high-order eigenmodes tend to be localized between two potential wells. Additionally, as the separation between two rational surfaces in the negative shear configuration further decreases, the high-order-l eigenmode would be stabilized. The mode structures of the low-order-l unstable eigenmode are present between two rational surfaces.

Unified gas-kinetic wave-particle methods IV: multi-species gas mixture and plasma transport

In this paper, we extend the unified gas-kinetic wave-particle (UGKWP) methods to the multi-species gas mixture and multiscale plasma transport. The construction of the scheme is based on the direct modeling on the mesh size and time step scales, and the local cell’s Knudsen number determines the flow physics. The proposed scheme has the multiscale and asymptotic complexity diminishing properties. The multiscale property means that according to the cell’s Knudsen number the scheme can capture the non-equilibrium flow physics when the cell size is on the kinetic mean free path scale, and preserve the asymptotic Euler, Navier-Stokes, and magnetohydrodynamics (MHD) when the cell size is on the hydrodynamic scale and is much larger than the particle mean free path. The asymptotic complexity diminishing property means that the total degrees of freedom of the scheme reduce automatically with the decreasing of the cell’s Knudsen number. In the continuum regime, the scheme automatically degenerates from a kinetic solver to a hydrodynamic solver. In the UGKWP, the evolution of microscopic velocity distribution is coupled with the evolution of macroscopic variables, and the particle evolution as well as the macroscopic fluxes is modeled from a time accumulating solution of kinetic scale particle transport and collision up to a time step scale. For plasma transport, the current scheme provides a smooth transition from particle-in-cell (PIC) method in the rarefied regime to the magnetohydrodynamic solver in the continuum regime. In the continuum limit, the cell size and time step of the UGKWP method are not restricted by the particle mean free path and mean collision time. In the highly magnetized regime, the cell size and time step are not restricted by the Debye length and plasma cyclotron period. The multiscale and asymptotic complexity diminishing properties of the scheme are verified by numerical tests in multiple flow regimes.

Operating range of magnetic stabilization flow regime for magnetized fluidized bed with Geldart-B magnetizable and nonmagnetizable particles

The magnetic stabilization flow regime could also be created for Geldart-B nonmagnetizable particles provided some magnetizable particles are introduced and the magnetic field is applied. This study aimed to explore the size (dpM) and density (ϿpM) effects of magnetizable particles on its operating range. The upper limit (UmbH) could not be determined from the οPb⿿Ug⿿ curve but could from analyzing the variation of οPb-fluctuation with increasing Ug. Due to the variation of UmfH (lower limit) with dpM and ϿpM, both UmbH⿿UmfH and (UmbH⿿UmfH)/UmfH were used to quantify the operating range of magnetic stabilization. UmbH⿿UmfH varied hardly with dpM but increased significantly with decreasing ϿpM. (UmbH⿿UmfH)/UmfH increased as dpM or ϿpM decreased. It was more difficult for the nonmagnetizable particles to escape from the network formed by the smaller/lighter magnetizable particles. For the same magnitude of change, dpM had a stronger effect than ϿpM on (UmbH⿿UmfH)/UmfH. Neither UmbH⿿UmfH nor (UmbH⿿UmfH)/UmfH varied monotonously with the minimum fluidization velocity of the magnetizable particles, indicating that no straightforward criterion for matching the magnetizable particles to the given nonmagnetizable particles could be established based on their minimum fluidization velocities to maximize the operating range of magnetic stabilization.

Magnetic reversal and pinning in a perpendicular zero-moment half-metal

Compensated ferrimagnets are promising materials for fast spintronic applications based on domain wall motion as they combine the favourable properties of ferromagnets and antiferromagnets. They inherit from antiferromagnets immunity to external fields, fast spin dynamics and rapid domain wall motion. From ferromagnets they inherit straightforward ways to read out the magnetic state, especially in compensated half metals, where electrons flow in only one spin channel. Here, we investigate domain structure in compensated half-metallic Mn2Ru0.5Ga films and assess their potential in domain wall motion-based spin-electronic devices. Our focus is on understanding and reducing domain wall pinning in unpatterned epitaxial thin films. Two modes of magnetic reversal, driven by nucleation or domain wall motion, are identified for different thin film deposition temperatures $(T_{dep})$. The magnetic aftereffect is analysed to extract activation volumes $(V^*)$, activation energies $(E_A)$, and their variation $({\Delta}E_A)$. The latter is decisive for the magnetic reversal regime, where domain wall motion dominated reversal (weak pinning) is found for ${\Delta}E_A<0.2$ eV and nucleation dominated reversal (strong pinning) for ${\Delta}E_A>0.5$ eV. A minimum ${\Delta}E_A=28$ meV is found for $T_{dep}=290{\deg}$C. Prominent pinning sites are visualized by analysing virgin domain patterns after thermal demagnetization. In the sample investigated they have spacings of order 300 nm, which gives an upper limit of the track-width of spin-torque domain-wall motion-based devices.

Small-scale Dynamo in Stably Stratified Turbulence

Abstract We present numerical investigations into three principal properties of a small-scale dynamo in stably stratified turbulence: the onset criterion, the growth rate, and the nature of the magnetic field anisotropy in the kinematic regime. The results suggest that all three dynamo properties are controlled by the scale separation between the Ozmidov scale and the viscous or resistive scale. In addition to the critical magnetic Reynolds number, this allows for the definition of critical buoyancy and magnetic buoyancy Reynolds numbers for stratified small-scale dynamo onset in the high and low magnetic Prandtl number regimes, respectively. The presence of a small-scale dynamo in stellar radiative zones could affect dynamics through the resulting Maxwell stresses or its influence on large-scale dynamo mechanisms in regions of differential rotation. Taking the solar radiative zone as a representative example and applying the onset criterion, we find that the stratification is strong enough to make the small-scale dynamo marginally active in the stably stratified turbulence of the solar tachocline.

Temperature and field dependences of transverse relaxivity of Co–Zn ferrite nanoparticles coated with silica: The role of magnetic properties and different regimes

The transverse relaxivity r2, characterizing the performance of negative contrast agents like magnetic nanoparticles in T2-weighted MRI, depends on multiple factors, most importantly on magnetic properties and the size of the particles, including both the size of magnetic cores and the effective size of whole particles present as individual entities in the suspension. The current study probes the transverse relaxivity of ferrimagnetic CoxZnyFe3-x-yO4 nanoparticles whose behavior varies from the blocked state, across the superparamagnetic regime to the vicinity of the paramagnetic state, while other parameters, namely the size of the magnetic cores (≈10 nm), the size of whole colloidally stable particles, and their geometry (≈30 nm-sized clusters of crystallites uniformly coated with ≈12 nm thick silica shell) are kept the same. Among the samples studied, the silica-coated clusters of Co0·66Zn0·47Fe1·87O4 crystallites show high values of r2 = 534 s−1 mmol(Me3O4)−1 L at room temperature in the magnetic field of 1.5 T. Importantly, the field and temperature dependences of transverse relaxivity are discussed in relation to relevant theoretical models, i.e., motional averaging and static dephasing regimes. The increase in r2 in higher magnetic fields is related to the notable paraprocess in the employed cores, whereas the gradual decrease of r2 with increasing temperature is dominated by the significant increase of the self-diffusion coefficient of water.

Dynamical Signatures of Quasiparticle Interactions in Quantum Spin Chains.

We study the transverse dynamical susceptibility of an antiferromagnetic spin-1/2 chain in the presence of a longitudinal Zeeman field. In the low magnetization regime in the gapless phase, we show that the marginally irrelevant backscattering interaction between the spinons creates a nonzero gap between two branches of excitations at small momentum. We further demonstrate how this gap varies upon introducing a second neighbor antiferromagnetic interaction, vanishing in the limit of a noninteracting "spinon gas." In the high magnetization regime, as the Zeeman field approaches the saturation value, we uncover the appearance of two-magnon bound states in the transverse susceptibility. This bound state feature generalizes the one arising from string states in the Bethe ansatz solution of the integrable case. Our results are based on numerically accurate, unbiased matrix-product-state techniques as well as analytic approximations.

Solar Wind Turbulence Around Mars: Relation between the Energy Cascade Rate and the Proton Cyclotron Waves Activity

Abstract The first estimation of the incompressible energy cascade rate at magnetohydrodynamic (MHD) scales in the plasma upstream of the Martian bow shock is obtained, making use of magnetic field and plasma observations provided by Mars Atmosphere and Volatile EvolutioN (MAVEN) over 600 orbits. In particular, the energy cascade rate is computed for events with and without proton cyclotron wave (PCW) activity, for time intervals when MAVEN was in the solar wind with no magnetic connection to the bow shock. It is shown that the nonlinear cascade of energy at the MHD scales is slightly amplified when PCWs are present in the plasma, around the Martian perihelion. In addition, the analysis of the normalized cross helicity and residual energy for the turbulent fluctuations shows the presence of Alfvénic and non-Alfvénic fluctuations in a magnetic dominant regime for the majority of the cases.

Modeling the dynamic behavior of laminated steels using a Fourier-based approach

A new magneto-dynamic model is proposed to approximate the dynamic hysteresis effect in laminated steels considering the static hysteresis, eddy-current field, and excess field. An accurate congruency-based hysteresis model is used to predict the static hysteresis field. The eddy-current is determined from the 1D diffusion equation and the well-known Bertotti empirical equation is utilized to model the excess-field effect. The dynamic lamination model obtained from coupling three field components is solved using a Fourier-based approach. In this approach, the flux density across the lamination thickness is approximated by a cosine-based Fourier series. The coefficients of the Fourier series are determined by solving a system of nonlinear equations through an iterative procedure. Owing to the employed congruency-based static hysteresis model, the proposed magneto-dynamic model offers high accuracy for arbitrary magnetization regimes. To validate the model accuracy, the model results are compared with sinusoidal and multi-harmonic measurements. The comparison shows that the proposed model predicts the dynamic hysteresis phenomenon in laminated steels with a relative energy error of less than 7%.

Relativistic magnetic reconnection in laser laboratory for testing an emission mechanism of hard-state black hole system.

Magnetic reconnection in a relativistic electron magnetization regime was observed in a laboratory plasma produced by a high-intensity, large energy, picoseconds laser pulse. Magnetic reconnection conditions realized with a laser-driven several kilotesla magnetic field is comparable to that in the accretion disk corona of black hole systems, i.e., Cygnus X-1. We observed particle energy distributions of reconnection outflow jets, which possess a power-law component in a high-energy range. The hardness of the observed spectra could explain the hard-state x-ray emission from accreting black hole systems.

Inverse and direct magnetoelectric effects in orthorhombic DyMnO3 manganite single crystals

Correlations between the direct and inverse magnetoelectric effects in orthorhombic DyMnO3 single crystals have been investigated. In the inverse magnetoelectric effect, the magnetic moment of the crystal appears to have a contribution that is sinusoidal oscillating in ac electric fields below the temperature TFE of ferroelectric phase transition. The first and the second harmonics of the inverse magnetoelectric effect are clearly detected. The magnetoelectric susceptibilities α and β, corresponding to first and second harmonics, are found to correlate with the derivative of polarization with respect to the magnetic field. The influence of the magnetic and electric regimes during cooling on magnetoelectric effects has also been studied. The maximum change in the magnetic moment of a sample under the action of electric fields is observed under the same (T, H) conditions as the rotation of the spontaneous polarization vector from the crystallographic c direction to the a axis in the magnetic field H || b.

The influence of metallic Bi in BiVO4 semiconductor for artificial photosynthesis

BiVO4 is a non-titania inorganic photocatalyst recognized as an effective visible-light-driven semiconductor that has been shown to be effective for CO2 reduction. However, some characteristics, such as a low separation rate of photogenerated electron-hole pairs and low mobility of electron-hole carriers, are still challenges to the widespread use of this semiconductor. In this paper, the influence of metallic Bi on the CO2 photoreduction activity was evaluated for the BiVO4 semiconductor. Bi–BiVO4 catalysts were prepared by microwave heating at 240 °C, employing different reaction times and magnetic stirring regimes. Metallic Bi improved the catalytic activity of BiVO4 for CO2 reduction, enhancing the absorption of visible light and promoting internal photoemission of electrons in the metal-semiconductor interface, which improves the electron density in the surface of the catalyst. This resulted in an astonishing concentration of methanol; Bi–BiVO4 prepared at 240 °C, for 5 min, and without magnetic stirring, produces around 5.0 mmol L−1 g−1catalyst of methanol and 40 μmol L−1 g−1catalyst of acetone after 240 min of reaction. The mechanism of charge transfer between the BiVO4 and the metallic Bi is influenced by the size of the microsphere crystallites, moreover, the production of methanol increased as the Bi grain size decreased.

On magnetic helicity generation and transport in a nonlinear dynamo driven by a helical flow

The relationship between nonlinear large-scale dynamo action and the generation and transport of magnetic helicity is investigated at moderate values of the magnetic Reynolds number ( $Rm$ ). The model consists of a helically forced, sheared flow in a Cartesian domain. The boundary conditions are periodic in the horizontal and impenetrable for the vertical. The magnetic field is required to be vertical at the upper and lower boundaries. There are two consequences of this choice; one is that the magnetic helicity is not gauge invariant, the second is that fluxes of magnetic helicity are allowed in and out of the domain. We select the winding gauge, define all the contributions to the evolution of the helicity in this gauge and measure these contributions for various solutions of the dynamo equations. We vary $Rm$ and the shear strength, and find a rich landscape of dynamo solutions including travelling waves, pulsating waves and non-wave-like solutions. We find that, at the $Rm$ considered, the main contribution to the growth of magnetic helicity comes from processes throughout the volume of the fluid and that boundary terms respond by limiting the growth. We find that, in this magnetic Reynolds number regime, helicity conservation is not a strong constraint on large-scale dynamo action. We speculate on what may happen at higher $Rm$ .

Magnetic Kondo regimes in a frustrated half-filled trimer

We analyze theoretically the phase diagram of a triangular triple quantum dot with strong onsite repulsion coupled to ferromagnetic leads. This model includes the competition of magnetic ordering of local or itinerant magnetic moments, geometric frustration and Kondo screening. We identify all the phases resulting from this competition. We find that three Kondo phases -- the conventional one, the two-stage underscreened one, and the one resulting from the ferromagnetic Kondo effect -- can be realized at zero temperature, and all are very susceptible to the proximity of ferromagnetic leads. In particular, we find that the quantum dots are spin-polarized in each of these phases. Further, we discuss the fate of the phases at non-zero temperatures, where a plethora of competing energy scales gives rise to complex landscape of crossovers. Each Kondo regime splits into a pair of phases, one not magnetized and one comprising magnetically polarized quantum dots. We discuss our results in the context of heavy-fermion physics in frustrated Kondo lattices.

Accessing new magnetic regimes by tuning the ligand spin-orbit coupling in van der Waals magnets.

Van der Waals (VdW) materials have opened new directions in the study of low dimensional magnetism. A largely unexplored arena is the intrinsic tuning of VdW magnets toward new ground states. Chromium trihalides provided the first such example with a change of interlayer magnetic coupling emerging upon exfoliation. Here, we take a different approach to engineer previously unknown ground states, not by exfoliation, but by tuning the spin-orbit coupling (SOC) of the nonmagnetic ligand atoms (Cl, Br, I). We synthesize a three-halide series, CrCl3 - x - y Br x I y , and map their magnetic properties as a function of Cl, Br, and I content. The resulting triangular phase diagrams unveil a frustrated regime near CrCl3. First-principles calculations confirm that the frustration is driven by a competition between the chromium and halide SOCs. Furthermore, we reveal a field-induced change of interlayer coupling in the bulk of CrCl3 - x - y Br x I y crystals at the same field as in the exfoliation experiments.

In-plane Néel wall chirality and orientation of interfacial Dzyaloshinskii-Moriya vector in magnetic films

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is of great interest as it can stabilize chiral spin structures in thin films. Experiments verifying the orientation of the interfacial DMI vector remain rare, in part due to the difficulty of separating vector components of DMI. In this study, Fe/Ni bilayers and Co/Ni multilayers were deposited epitaxially onto Cu(001) and Pt(111) substrates, respectively. By tailoring the effective anisotropy, spin reorientation transitions (SRTs) are employed to probe the orientation of the DMI vector by measuring the spin structure of domain walls on both sides of the SRTs. The interfacial DMI is found to be sufficiently strong to stabilize chiral N\'eel walls in the out-of-plane magnetized regimes, while achiral N\'eel walls are observed in the in-plane magnetized regimes. These findings experimentally confirm that the out-of-plane component of the DMI vector is insignificant in these fcc(001) and fcc(111) oriented interfaces, even in the presence of atomic steps.