Magnetostatic Research Articles

Plasma-Based Intelligent Reflecting Surface for Beam-Steering and Polarization Conversion

The design of a reflective surface operated in the GHz range is proposed to enable beam-steering and polarization conversion simultaneously. The concept presented in this work relies on plasma-based Intelligent Reflecting Surfaces (IRS) in which the plasma is magnetized. Plasma-based IRSs have been introduced recently and consist of rectangular plasma discharges placed on top of a metallic ground plane. The reflected signal can be reconfigured electronically by varying the plasma parameters (e.g., density). First, a theoretical model is exploited to evaluate the capability of a plasma-based IRS to implement beam-steering and polarization conversion simultaneously. Second, the preliminary design of two plasma-based IRSs is presented to combine beam-steering with 1) cross-polarization or 2) linear-to-circular polarization conversion. According to the numerical results, the proposed concepts are feasible assuming the plasma density can be reconfigured in the range <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.9\times 10^{17}$ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13.7\times 10^{17}\,\,\text{m}^{-3}$ </tex-math></inline-formula> and the intensity of the magnetostatic field in the range 60–183 mT; these values are consistent with the plasma technology at the state-of-the-art. The operation frequency is 10 GHz, and the bandwidth is between 0.5-0.8 GHz for the two plasma-based IRSs presented in this work.

Magnetoelectric‐Field Electrodynamics: Search for Magnetoelectric Point Scatterers

AbstractResonant scattering of electromagnetic (EM) waves by small particles is considered as one of the basic problems in metamaterial science. At present, special subwavelength resonators are considered as structural elements in chiral and bianisotropic metamaterials. There is a general consensus that these small scatterers behave like “artificial atoms” with strong electrical and magnetic responses and an interconnection between these responses. However, the observed effect of magnetoelectric (ME) coupling in these meta‐atoms is not associated with the near‐field manipulation properties caused by intrinsic magnetoelectricity. This arises the question whether ME point scatterers of EM radiation really exist. In this paper, we show that there are mesoscopic structures with electric and magnetic dipole‐carrying excitations that behave like point scatterers with their inherent magnetoelectricity. In such subwavelength resonators, coherent oscillations of the electric polarization and magnetization can be considered as quasistatic oscillations described by electrostatic (ES) and magnetostatic (MS) scalar wave functions. The ME resonance effect arises from the coupling of two, ES and MS, oscillations. The near fields of these resonators, called the ME near fields, are characterized by simultaneous violation of time reversal and inversion symmetry. In study of ME fields and EM problems associated with these fields, we put forward the concept of ME‐field electrodynamics.

Numerical study of convective heat transfer and particle distribution subject to magneto-static field in a square cavity

Magnetic nanoparticle suspension has been applied in a variety of energy transport systems due to its excellent flowing characteristics and extraordinary thermal conductivity, and it has proven to significantly influence the sustainable development of energy and the environment. This work aims to focus on the thermal, fluidic properties of the magnetic nanofluid while also considering the coupling factors of magnetic field, thermal exchange, fluidity and nanoparticle concentration. Firstly, the simulation results of velocity and temperature distribution will be validated using the finite element method, which is in accordance with previous results of natural convection in a square cavity. The analysis model of developed magneto-hydrodynamics heat transfer will then be constructed with the explanation of entropy and concentration distribution. In comparison to natural convection, the magnetic field-enhanced method is able to break through the limit of thermal exchange efficiency. In addition, when the magnetic field is increased to 30 kA/m, the thermodynamic force is replaced by magnetic volumetric force and this plays a role in the heat transfer of the cavity, while the average Nusselt numbers increase by 17% in comparison to natural convection performance (Ra = 105). Entropy generation increases as magnetic field strength increases as a result of the enhancement to circulation and temperature gradient in the nanofluid. This work utilises an advanced strategy for the convection performance of magnetic nanofluid and analyses the multi-physical field.

Magnetic-field-induced Wigner crystallization of charged interlayer excitons in van der Waals heterostructures

Ever since its inception, coherent excited states of semiconductors have been the focus of semiconductor materials research to evolve into a vibrant field of low-dimensional solid-state physics. The field is gaining new momentum these days due to emerging transdimensional semiconductors such as van der Waals bound layers of transition metal dichalcogenides (TMDs) of controlled thickness. Here, we develop the theory of magnetic-field-induced Wigner crystallization for charged interlayer excitons (CIE) discovered recently in TMD heterobilayers. We derive the ratio of the potential interaction energy to the kinetic energy for the many-particle CIE system in the perpendicular magnetostatic field of an arbitrary strength and predict the crystallization effect in the strong field regime. We show that magnetic-field-induced Wigner crystallization and melting of CIEs can be observed in magneto-photoluminescence experiments with TMD bilayers of systematically varied electron-hole doping concentrations. Our results advance the capabilities of this new generation of transdimensional quantum materials.

Field-Driven Magnetic Phase Diagram and Vortex Stability in Fe Nanometric Square Prisms

In this work, we deal with the zero temperature hysteretic properties of iron (Fe) quadrangular nanoprisms and the size conditions underlying magnetic vortex states formation. Different aspect ratios of a square base prism of thickness t with free boundary conditions were considered in order to summarize our results in a proposal of a field-driven magnetic phase diagram where such vortex states are stable along the hysteresis loops. To do that, a Hamiltonian consisting of exchange, magnetostatic, Zeeman and cubic anisotropy energies was considered. The time dynamics at each magnetic field step was performed by solving the time-dependent Landau-Lifshitz-Gilbert differential equation. The micromagnetic simulations were performed using the Ubermag package based on the Object Oriented Micromagnetic Framework (OOMMF). Circular magnetic textures were also characterized by means of topological charge calculations. The aspect ratio dependencies of the coercive force, nucleation and annihilation fields are also analyzed. Computations agree with related experimental observations and other micromagnetic calculations.

Crystal size dependence of dipolar ferromagnetic order between Mn6 molecular nanomagnets

We study how crystal size influences magnetic ordering in arrays of molecular nanomagnets coupled by dipolar interactions. Compressed fluid techniques have been applied to synthesize crystals of ${\mathrm{Mn}}_{6}$ molecules (spin $S=12$) with sizes ranging from $28\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ down to 220 nm. The onset of ferromagnetic order and the spin thermalization rates have been studied by means of ac susceptibility measurements. We find that the ordered phase remains ferromagnetic, as in the bulk, but the critical temperature ${T}_{\mathrm{c}}$ decreases with crystal size. Simple magnetostatic energy calculations, supported by Monte Carlo simulations, account for the observed drop in ${T}_{\mathrm{c}}$ in terms of the minimum attainable energy for finite-sized magnetic domains limited by the crystal boundaries. Frequency-dependent susceptibility measurements give access to the spin dynamics. Although magnetic relaxation remains dominated by individual spin flips, the onset of magnetic order leads to very long spin thermalization time scales. The results show that size influences the magnetism of dipolar systems with as many as ${10}^{11}$ spins and are relevant for the interpretation of quantum simulations performed on finite lattices.

Magnetically derived fabrication of aligned porous poly(sulfonated polystyrene-co.-chitosan) monolith as a high efficient adsorbent for metal ions separation

Designing and tuning towards the porous structure of adsorbents is of great importance for promoting the adsorptive performance. Herein, a magnetic-induced copolymerization method is applied to generate an aligned porous structure in the aligned porous poly(SPS-co.-CTS) monolith. Typically, the aligned macropores primarily derives from the equilibrium of the magnetic dipolar force to the thermodynamic Brown force by tuning the applied magnetostatic field intensity to 25 mT. Due to the aligned macroporous structure, the monolith features with low back pressure, low mass transfer resistance and high adsorptive capacity. The hydrophilicity of the monolith surface is further improved by capping chitosan activated by amino onto its porous surface via the Hinsber reaction. Detail analyses are performed by a series of physical characterizations, by which exhibit the aligned macropores centering at 5.18 μm, specific surface reaching 54 m2 g−1 and large porosity. Finally, batch adsorption experiments confirm that the adorptive behaviors to Cu2+ ions and Co2+ ions submit to the Langmuir model and Pseudo-second order adsorption kinetics. Most intriguing, the film mass transfer ([kLa]f) has been stabilized due to the aligned macropores, thus facilitating the porous diffusion ([kLa]d) and improving the chromatographic performance of the monolith. All results demonstrate that the aligned porous poly(SPS-co.-CTS) monolith promises broad applications in the field of wastewater remediation.

High repetition granular Co/Pt multilayers with improved perpendicular remanent magnetization for high-density magnetic recording

Thanks to the strong perpendicular magnetic anisotropy (PMA), excellent processing compatibility as well as novel spintronic phenomenon, Co/Pt multilayers have been attracting massive attention and widely used in magnetic storage. However, reversed magnetic domains come into being with the increasing layer repetition ‘N’ to reduce magneto-static energy, resulting in the remarkable diminishment of the remanent magnetization (M r). As a result, the product of M r and thickness (i.e., the remanent moment-thickness product, M r t), a key parameter in magnetic recording for reliable data storing and reading, also decreases dramatically. To overcome this issue, we deposit an ultra-thick granular [Co/Pt]80 multilayer with a total thickness of 68 nm on granular SiN x buffer layer. The M r t value, M r to saturation magnetization (M s) ratio as well as out of plane (OOP) coercivity (H coop) are high up to 2.97 memu/cm2, 67%, and 1940 Oe (1 Oe = 79.5775 A⋅m−1), respectively, which is remarkably improved compared with that of continuous [Co/Pt]80 multilayers. That is because large amounts of grain boundaries in the granular multilayers can efficiently impede the propagation and expansion of reversed magnetic domains, which is verified by experimental investigations and micromagnetic simulation results. The simulation results also indicate that the value of M r t, M r/M s ratio, and H coop can be further improved through optimizing the granule size, which can be experimentally realized by manipulating the process parameter of SiN x buffer layer. This work provides an alternative solution for achieving high M r t value in ultra-thick Co/Pt multilayers, which is of unneglectable potential in applications of high-density magnetic recording.

The meaning of maxima and minima in first order reversal curves: Determining the interaction between species in a sample

First-order reversal curves (FORCs) are a characterization technique for magnetic materials used in a wide range of research fields. Since their first application in the Earth Sciences two decades ago, their importance in science has been continuously growing and new experimental techniques have been subsequently designed based on the original idea of FORCs. Nonetheless, very recent experimental works on very well designed and simple magnetic structures demonstrate that even for the most simple cases the interpretation of FORC data lacks understanding. In this work, we address this problem analytically, explaining the meaning of maxima, minima and noisy tails and set a strategy to extract the interaction field between magnetic structures. The origin of this interaction field is often the magnetostatic energy, however, we propose that this strategy could be applied for estimating exchange interactions too.

Permanent Magnet Selections for AFPM Disc Generators

In this article, the field (FEM) and analytical analyses were used for the optimal selection of magnets material for the Axial Flux Permanent Magnet Generator (AFPMG), without building the prototype before. The tested generator is an axial flux machine which consists of a single stator and two rotor discs with Permanent Magnets (PM). Three-dimensional (3D) ANSYS Maxwell package was used for magnetostatic and transient field (FEM) simulations. Two types of PM were selected for the analysis: Ceramic (also known as “Ferrite”) magnets made from Strontium Ferrite powder and Neodymium Iron Boron magnets (NdFeB). The authors compared obtained electromotive forces (EMF) and generator powers for selected magnets materials, performed FFT analyses of voltages and currents and indicated the optimal solutions. In addition to the operational properties of the AFPMG, the magnet and manufacturing costs were compared.

Thermo-fluid flow behavior of the IN718 molten pool in the laser directed energy deposition process under magnetic field

PurposeThis paper aims to understand the magnetohydrodynamics (MHD) mechanism in the molten pool under different modes of magnetic field. The comparison focuses on the Lorenz force excitation and its effect on the melt flow and solidification parameters, intending to obtain practical references for the design of magnetic field-assisted laser directed energy deposition (L-DED) equipment.Design/methodology/approachA three-dimensional transient multi-physical model, coupled with MHD and thermodynamic, was established. The dimension and microstructure of the molten pool under a 0T magnetic field was used as a benchmark for accuracy verification. The interaction between the melt flow and the Lorenz force is compared under a static magnetic field in the X-, Y- and Z-directions, and also an oscillating and alternating magnetic field.FindingsThe numerical results indicate that the chaotic fluctuation of melt flow trends to stable under the magnetostatic field, while a periodically oscillating melt flow could be obtained by applying a nonstatic magnetic field. The Y and Z directional applied magnetostatic field shows the effective damping effect, while the two nonstatic magnetic fields discussed in this paper have almost the same effect on melt flow. Since the heat transfer inside the molten pool is dominated by convection, the application of a magnetic field has a limited effect on the temperature gradient and solidification rate at the solidification interface due to the convection mode of melt flow is still Marangoni convection.Originality/valueThis work provided a deeper understanding of the interaction mechanism between the magnetic field and melt flow inside the molten pool, and provided practical references for magnetic field-assisted L-DED equipment design.

Sensing Model for Detecting Ferromagnetic Debris Based on a High-Gradient Magnetostatic Field

Wear debris sensing is an important approach for machine condition monitoring. To date, different types of debris sensors have been developed, among which the magnetostatic inductive debris sensor is preferable for monitoring ferromagnetic debris, but the existing induced voltage model cannot be used to quantify wear debris in lube oil. Accordingly, a modeling method was proposed for the debris sensor based on a high-gradient magnetostatic field. First, a Gaussian curve is used to fit the magnetic field along the axis of the sensor. Then, time-harmonic electromagnetic field analysis and Fourier series decomposition are combined to establish an induced voltage model. Furthermore, the influence of the relative permeability, size, and velocity of wear debris and induction coil parameters on the output voltage is analyzed. The results show a linear relationship between the theoretical calculations and experimental results. Therefore, the proposed model can be used to realize quantitative analysis of wear debris, laying a foundation for optimizing the sensor structure, and extracting features from output voltage signals.

Improvement of CNN-Based Anisotropic Magnetostatic Field Computation via Adaptive Data Subset Selection

A numerical issue arises when we extend the convolutional neural network (CNN) U-net to the anisotropic magnetostatic field computation. The output magnetic field has a significant gradient with respect to the input geometry parameter, which introduces inevitable errors in the training process to degrade the performance of deep learning (DL). To address this issue, the subset selection approach is utilized to divide the whole database into several subsets, where the samples are assigned according to the gradient between the input and output. Then these subsets with different sample densities are combined into a global one. Taking the uniform dataset with the same sample size as a comparison, the influence of subset selection on DL is investigated by comparing the performance of CNN on different datasets. Numerical experiments illustrate that the adaptive subset selection can be employed to improve the accuracy and efficiency of the CNN network.

Piezoelectric Contributions to Parametric Amplification of Acoustical Phonons in Magnetized n-InSb Crystal

Abstract: Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, we develop a theoretical formulation to study piezoelectric contributions to parametric amplification of acoustical phonons in magnetized n-InSb crystal. The origin of nonlinear interaction is assumed to lie in effective second-order optical susceptibility arising due to nonlinear induced current density and electrostrictive polarization of the medium. Expressions are obtained for threshold pump amplitude for the onset of parametric process and parametric gain coefficient (well above the threshold pump field) in the presence and absence of piezoelectricity and/or externally applied magnetostatic field. Numerical analysis is made for n-InSb crystal irradiated by 10.6 m pulsed CO2 laser. The piezoelectric contributions to three-wave parametric amplification process are only in the presence of magnetostatic field. The parametric gain coefficient is independent of doping concentration for the cases when either/both piezoelectricity and magnetostatic field are absent. Around resonance (electron-cyclotron frequency ~ pump frequency), the parametric gain coefficient in the presence of piezoelectricity is 102 times higher than that in the absence of piezoelectricity. The analysis establishes the technological potentiality of transversely magnetized n-InSb as the host for parametric devices, like parametric amplifiers and oscillators. Keywords: Parametric amplification, Acoustical phonons, Piezoelectricity, Electrostriction, n-InSb crystal. PACS numbers: 42.65.-k, 78.55.Cr, 71.38.-k, 52.35.Mw.

Temperature-Gradient-Driven Magnetic Skyrmion Motion

The static and dynamic properties of skyrmions have recently received increased attention due to the potential application of skyrmions as information carriers and for unconventional computing. While the current-driven dynamics has been explored deeply, both theoretically and experimentally, the theory of temperature gradient-induced dynamics---skyrmion caloritronics---is still at its early stages of development. Here, we move the topic forward by identifying the role of entropic torques due to the temperature dependence of magnetic parameters. Our results show that skyrmions move towards higher temperatures in single-layer ferromagnets with interfacial Dzyaloshinski-Moriya interactions, whereas, in multilayers, they move to lower temperatures. We analytically and numerically demonstrate that the opposite behaviors are due to different scaling relations of the material parameters as well as a non-negligible magnetostatic field gradient in multilayers. We also find a spatially dependent skyrmion Hall angle in multilayers hosting hybrid skyrmions due to variations of the thickness-dependent chirality as the skyrmion moves along the temperature gradient.

Thickness‐Dependent Reconfigurable Spin‐Wave Dynamics in Ni80Fe20 Nanostripe Arrays

AbstractFerromagnetic nanostripes have gained massive attention due to their intriguing magnetic properties associated with dimensional confinements and shape anisotropy leading toward potential applications in magnetic storage, memory, and spin‐wave‐based devices. Consequently, reconfiguration of their static and dynamic magnetic properties by the geometric parameters and external field is imperative. Here, a combined experimental and numerical study of the reconfigurable spin‐wave dynamics in arrays of ferromagnetic nanostripes by the stripe thickness and external magnetic field strength and orientation is presented. Different uniform, localized, and standing spin waves in the nanostripes and their monotonic and nonmonotonic variation, including mode merging with these parameters, are observed. The observed variations are interpreted with the aid of simulated spin configurations, magnetostatic field maps, and spin‐wave mode profiles. Further numerical study reveals anisotropic spin‐wave propagation in nanostripes for different thicknesses and in different bias‐field geometry opening potential applications in magnonic circuit components such as reconfigurable magnonic waveguides and omnidirectional spin‐wave emitters.

A mechanically strong and ductile soft magnet with extremely low coercivity

Soft magnetic materials (SMMs) serve in electrical applications and sustainable energy supply, allowing magnetic flux variation in response to changes in applied magnetic field, at low energy loss1. The electrification of transport, households and manufacturing leads to an increase in energy consumption owing to hysteresis losses2. Therefore, minimizing coercivity, which scales these losses, is crucial3. Yet meeting this target alone is not enough: SMMs in electrical engines must withstand severe mechanical loads; that is, the alloys need high strength and ductility4. This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that can pin magnetic domains, thus increasing coercivity and hysteresis losses5. Here we introduce an approach to overcome this dilemma. We have designed a Fe–Co–Ni–Ta–Al multicomponent alloy (MCA) with ferromagnetic matrix and paramagnetic coherent nanoparticles (about 91 nm in size and around 55% volume fraction). They impede dislocation motion, enhancing strength and ductility. Their small size, low coherency stress and small magnetostatic energy create an interaction volume below the magnetic domain wall width, leading to minimal domain wall pinning, thus maintaining the soft magnetic properties. The alloy has a tensile strength of 1,336 MPa at 54% tensile elongation, extremely low coercivity of 78 A m−1 (less than 1 Oe), moderate saturation magnetization of 100 A m2 kg−1 and high electrical resistivity of 103 μΩ cm.

Functionalized Magnetic Nanoparticles for Alternating Magnetic Field- or Near Infrared Light-Induced Cancer Therapies.

The multi-faceted nature of functionalized magnetic nanoparticles (fMNPs) is well-suited for cancer therapy. These nanocomposites can also provide a multimodal platform for targeted cancer therapy due to their unique magnetic guidance characteristics. When induced by an alternating magnetic field (AMF), fMNPs can convert the magnetostatic energy to heat for magnetic hyperthermia (MHT), as well as for controlled drug release. Furthermore, with the ability to convert near-infrared (NIR) light energy to heat energy, fMNPs have attracted interest for photothermal therapy (PTT). Other than MHT and PTT, fMNPs also have a place in combination cancer therapies, such as chemo-MHT, chemo-PTT, and chemo-PTT–photodynamic therapy, among others, due to their versatile properties. Thus, this review presents multifunctional nanocomposites based on fMNPs for cancer therapies, induced by an AMF or NIR light. We will first discuss the different fMNPs induced with an AMF for cancer MHT and chemo-MHT. Secondly, we will discuss fMNPs irradiated with NIR lasers for cancer PTT and chemo-PTT. Finally, fMNPs used for dual-mode AMF + NIR-laser-induced magneto-photo-hyperthermia (MPHT) will be discussed.

MuMag2022: a software tool for analyzing magnetic field dependent unpolarized small-angle neutron scattering data of bulk ferromagnets.

The MATLAB-based software tool MuMag2022 is presented for the analysis of magnetic-field-dependent unpolarized small-angle neutron scattering (SANS) data of bulk ferromagnets such as elemental nanocrystalline ferromagnets, magnetic nanocomposites or magnetic steels. On the basis of the micromagnetic theory for the magnetic SANS cross section, the program analyzes unpolarized total (nuclear and magnetic) SANS data within the approach-to-saturation regime. The main features of MuMag2022 are the estimation of the exchange-stiffness constant, and of the strength and spatial structure of the magnetic anisotropy field and the magnetostatic field due to longitudinal magnetization fluctuations. MuMag2022 is open source and available as a standalone executable for Windows at https://mumag.uni.lu.

Transition from antiferromagnetic to quadrupole order in a modified square artificial spin ice

The results of a numerical study of an ordered substitution of a double-segment into the unit cell of square artificial spin ice are presented. Each pattern vertex has three magnetic moment configurations that compete to form a magnetic ground state in zero applied magnetic field, including nearest-neighbor and next-nearest-neighbor interaction contributions to the total magnetostatic energy. The ground state depends on the number of lattice sites considered and the spacing between the two film segments that comprise the double segment. Monte Carlo simulations reveal that the double-segment sublattice undergoes long-range antiferromagnetic order that can be supplanted by magnetic quadrupole order by applying an in-plane magnetic field perpendicular to the double-segment easy axis. The quadrupole-ordered state consists of a sublattice of weakly correlated chains of double-segments with antiparallel polarizations, which admits many different ground states that reflect a very high degree of frustration.