Magnetic Bias Research Articles

Analysis of harmonic characteristics of UHV AC / DC power network considering the influence of geomagnetic storm

The resultant harmonic effect of geomagnetically induced current (GIC) caused by geomagnetic storm has an impact on the safe and reliable operation of ultra-high voltage (UHV) AC / DC power grid in China, which in severe cases may lead to power grid breakdown or even large-scale blackout. Firstly, the harmonic effect model of converter transformer under the action of GIC is established based on the GIC calculation results of China's UHV power grid during the super-geomagnetic storm in November 2004. The harmonic voltage of transformer equivalent model port is described in phasor form. Secondly, the characteristic harmonic model of the converter is established based on the switching function modulation theory. By analyzing the working principle of the equivalent circuit of 12-pulse rectifier, the harmonic coupling model between DC magnetic bias harmonic voltage of transformer and converter is established, and the harmonic coupling characteristics of AC / DC power network under the action of GIC are analyzed. Lastly, the simulation model of ±800 kV UHV AC / DC transmission system in China is built by using PSCAD/EMTDC software as the simulation platform. The non-characteristic harmonic current and voltage on the AC side and DC side of the AC / DC power grid are calculated. The results show that the harm caused by GIC invading converter transformer is more serious than that caused by GIC invading UHV transformer. When both of them are invaded by GIC, the non-characteristic harmonics generated by mutual superposition and coupling have a greater impact on the power grid. The research results of this paper can provide a theoretical basis for space disaster prevention of UHV AC / DC power grid in China.

Magnetic properties of an Fe-based amorphous soft magnetic composite prepared via in-situ steam oxidation

Reducing the core losses of motors and transformers is an important way to address the global environment and energy resource problems. Soft magnetic composites (SMCs) comprising of electrically insulated ferromagnetic powder has shown vast potential for applications at low to high applied frequencies. In this work, in-situ steam oxidation was used to coat the surface of amorphous Fe75.5Si4B6P8C6Mo0.5 powder with an insulating layer. The thickness and composition of the Fe-oxide layer were controlled by tuning the in-situ steam oxidation time, and more cracks and Fe2+ were formed on the powder surface with the extension of oxidation time. The magnetic Fe-oxide layer of amorphous powder not only plays an insulating role to reduce the Pe, but also affects the magnetic coupling of particles that leads to the change of Ph. And the amorphous SMCs with 10 h in-situ steam oxidation treatment exhibits good comprehensive soft magnetic properties, i.e., the magnetic loss (Pcv), effective permeability (μe) and DC bias are 61.44 W/kg (50 mT, 100 kHz), 30.4 (to 10 MHz) and 71.5% (100 Oe), respectively.

Commercial Chip-based Tunneling Magnetoresistance Sensor for Green-synthesized Magnetic Nanoparticles Assay

Owing to their significant roles in multiple sectors, the demand for high-performance, rapid, user-friendly, and low-cost sensors is crucial for biosensing. This paper reports the performance of a commercial chip-based tunneling magnetoresistance (TMR) sensor for detecting green-synthesized magnetic nanoparticles (MNP) as potential magnetic labels. A Simple and low-cost design consisting of a TMR chip ALT-025 integrated with an Arduino microcontroller and a basic differential amplifier was developed to provide real-time and measurable digital readouts. Three kinds of ferrite MNPs (Fe3O4, CoFe2O4 and MnFe2O4) was synthesized by the coprecipitation method on the green synthesis approach utilizing Moringa Oleifera extracts. All sample have a face-centered cubic inverse spinel structure with average grain size of 10.3 nm, 9.2 nm and 6.1 nm for Fe3O4, CoFe2O4 and MnFe2O4, respectively. Furthermore, soft ferromagnetic behavior is identified for all sample with magnetization saturation of 55.3 emu/g, 37.6 emu/g, 19.3 emu/g for Fe3O4, CoFe2O4 and MnFe2O4, respectively. The sensor showed a promising performance in the detection of MNPs. For the three particles, the sensitivity exhibited a linear function of the MNPs concentration. The sensitivity is related not only to the particle size but also to the magnetization of the nanoparticles in the bias field. The change in the output voltage was proportional to the bias magnetization (MBias), indicating that particles with a higher bias magnetization can produce a stronger magnetic stray field on the TMR sensor surface. The sensor system successfully detected MNPs at different stray field intensities. Furthermore, a low limit of detection was achieved using these methods. Moreover, the remarkable stability and repeatability of the sensor is further validated by the steady signal acquired for 30s with an RSD of 0.5-28.5%. Therefore, the integration of commercial chip-based TMR sensors and green-synthesized MNPs has great potential for advancing the detection of various biomolecules.

Exhibition of magnetic memory effects and exchange bias in nanocrystalline CoSb2O4

Herein, super-spin-glass behaviour has been demonstrated through comprehensive memory studies in nanocrystalline CoSb2O4 prepared by a hydrothermal method.

Ion irradiation modulated magnetic properties of materials and its applications

Ion irradiation, a technology in which ion beams are used to irradiate materials, has high manipulation precision, short processing time, and many applications in the fields of material modification, chip manufacturing, biomedicine, energy and chemicals. Especially in magnetic material modification, customized modifications of magnetic materials can be achieved by precisely controlling the energy, dose, and direction of the ion beam. To further enhance the performances of magnetic materials and explore new magnetic devices, this study focuses on how ion irradiation precisely modulates various magnetic interactions and the analysis of its influence on the spin Hall effect and magnetic structural dynamics. Firstly, the latest research achievements are emphasized of ion irradiation regulated magnetic characteristics such as perpendicular magnetic anisotropy, exchange bias, and RKKY interaction. These regulation methods are crucial for understanding and optimizing the microstructure and properties of magnetic materials. Secondly, the significant role played by ion irradiation in regulating spin-orbit torque devices is discussed in detail. These applications demonstrate the potential of ion irradiation technology in designing high-performance magnetic storage and processing devices. Finally, the future applications of ion irradiation technology in the preparation of multifunctional magnetic sensors and magnetic media for information storage are discussed, highlighting its great enormous innovation and application potential in the field of magnetic materials.

Giant exchange bias field above room temperature in perovskite YbCr1-xFexO3 (x = 0.6-0.9).

Temperature-induced exchange bias is observed in perovskite YbCr1-xFexO3 (x = 0.6-0.9) compounds. This is ascribed to the ferromagnetic (FM) coupling between Fe3+/Cr3+ ions and Yb3+ ions. During demagnetization, the Yb3+ ions will show a tendency to rotate with the direction of the applied magnetic field. However, the antiferromagnetic coupling of Fe3+/Cr3+ ions has a pinning effect, hindering their rotation, and thus producing an exchange bias effect. With increasing Fe3+ ion content, the magnetization and exchange bias field gradually become larger. When x = 0.8 and 0.9, the exchange bias field reaches up to -9.7 kOe and -13.6 kOe at 300 K, respectively. This giant room temperature exchange bias field will be more conducive for practical applications in magnetoelectronic devices.

Permanent magnet bias positioning magnetic needle for novel fluid magnetic mass transfer

Permanent magnet bias positioning magnetic needle for novel fluid magnetic mass transfer

Decoupled Topologies and Radial Levitation Control of a Novel Permanent Magnet Biased Bearingless Switched Reluctance Motor

Decoupled Topologies and Radial Levitation Control of a Novel Permanent Magnet Biased Bearingless Switched Reluctance Motor

Quantum Hall coherent perfect absorption in graphene

100%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$100\\%$$\\end{document} absorption in a two-dimensional electron gas (2DEG) with Dirac spectrum is demonstrated to be obtained by controlling the interference of multiple incident radiations, referred to as coherent perfect absorption (CPA). However, when a 2DEG such as graphene is exposed to a magnetostatic bias, it resonantly could absorb electromagnetic radiation by transitions of its Dirac electrons between non-equidistant and nonlinear Landau levels. Here, the magneto-optical terahertz (THz) CPA in graphene under the quantum Hall effect (QHE) regime at both strong and subtesla magnetostatic bias fields is addressed. Our findings show that an effective magneto-optical surface conductivity corresponding to right- and left-handed circular (RHC- and LHC) polarizations could model a magneto-tunable CPA in graphene in THz range. Significantly, graphene under QHE regime reveals different tunable CPA properties for each circularly polarized beams by the intensity of the applied magnetic bias. Moreover, it is observed that different phase modulations at CPA frequencies are achieved for RHC and LHC polarizations. Considering the maximum efficiency for a 2D absorber, our results demonstrate the magnetostatic tuning of CPA in 2D Dirac materials for long-wavelength sensing applications and signal processing.

Giant enhancement of nonreciprocity in gyrotropic heterostructures

Nonreciprocity is a highly desirable feature in photonic media since it allows for control over the traveling electromagnetic waves, in a way that goes far beyond ordinary filtering. One of the most conventional ways to achieve nonreciprocity is via employing gyrotropic materials; however, their time-reversal-symmetry-breaking effects are very weak and, hence, large, bulky setups combined with very strong magnetic biases are required for technologically useful devices. In this work, artificial heterostructures are introduced to enhance the effective nonreciprocal behavior by reducing the contribution of the diagonal susceptibilities in the collective response; in this way, the off-diagonal ones, that are responsible for nonreciprocity, seem bigger. In particular, alternating gyrotropic and metallic or plasmonic films make an epsilon-near-zero (ENZ) effective-medium by averaging the diagonal permittivities of opposite sign, representing the consecutive layers. The homogenization process leaves unaltered the nonzero off-diagonal permittivities of the original gyrotropic substance, which become dominant and ignite strong nonreciprocal response. Realistic material examples that could be implemented experimentally in the mid-infrared spectrum are provided while the robustness of the enhanced nonreciprocity in the presence of actual media losses is discussed and bandwidth limitations due to the unavoidable frequency dispersion are elaborated. The proposed concept can be extensively utilized in designing optical devices that serve a wide range of applications from signal isolation and wave circulation to unidirectional propagation and asymmetric power amplification.

The evolution of penta-graphene thermoelectrics: External fields as a key enabler for High-Performance devices

The tight-binding model is used in this study to examine the electronic and thermal properties of penta-Graphene (PG). To analyze these properties, approaches including the Kubo formula, linear response theory, and Green's function method are applied. The obtained numerical results show that by introducing bias voltage U, external magnetic field Π, and electron doping μ, significant modifications in electrical conductivity, magnetic susceptibility, and thermoelectric properties including power factor PF(T), Seebeck coefficients S(T), and figure of merit ZT(T) are observed. The band structure and density of states (DOS) of PG structure can be significantly altered by applying a bias voltage or an external magnetic field. The wide band gap of PG exhibited a marked reduction with the application of these external fields, displaying greater sensitivity to Π compared to U. Thermal properties of PG, including electrical conductivity σ(T), exhibit a peak as a function of temperature, which its peak position depends on the external fields and μ. Below the peak, σ(T) increases significantly with increasing external fields and μ, due to the reduced band gap and increased number of excited charge carriers, respectively. σ(T) demonstrates greater sensitivity to μ compared to Π, and both parameters have a more pronounced effect than bias voltage U. The magnetic field and bias voltage shift the peak position of the power factor PF(T) in opposite directions. Through tuning of the external fields, which alters the electronic structure of PG, the ZT(T) can be enhanced and its rate of increase is more dependent on bias voltage than on magnetic field.

Polarization-selective equilibration and quantum interference in graphene-based quantum Hall edge states

Quantum Hall (QH) edge states in two-dimensional electron gas systems have drawn attention as potential carriers of quantum information, owing to their chiral ballistic nature, arising from Landau level formation under a strong perpendicular magnetic field. They remain stable even in the presence of material disorder, making them ideal for phase-sensitive measurements like QH interferometry. We report the equilibration processes between electron waves in spin and valley polarized QH edge states, particularly in a four-terminal measurement configuration. We compare experimental results with theoretical estimations based on the Landauer formula, taking into account selective equilibration depending on spin polarization. Furthermore, quantum interference in co-propagating QH edge states is investigated, elucidating how factors such as magnetic fields, voltage bias, and gate configurations influence quantum phase. The research offers insights into the controllability of interactions between QH edge states, vital for harnessing the potential of graphene in QH interferometers and quantum information processing.

Microstructural and magnetic properties of BiFeO3/Bi2Fe4O9 binary nanoparticles with a bimodal size distribution

Binary nanoparticles composed of BiFeO3 and Bi2Fe4O9 have attracted considerable interest due to their magnetic properties and exchange bias effect. In this paper, two samples of well-crystallized BiFeO3/Bi2Fe4O9 binary nanoparticles with varying compositions, containing approximately 3 % and 32 % of the Bi2Fe4O9 crystalline phase, were investigated to understand the impact of microstructural variations on the magnetic behavior of the samples. Both BiFeO3 and Bi2Fe4O9 phases of binary nanoparticles were synthesized in the same synthesis process by calcining amorphous precursors obtained after solvent evaporation from aqueous solutions containing iron and bismuth nitrates and tartaric acid. The identification and quantification of crystalline phases of the produced samples were performed using the X-ray diffraction technique and multiphase Rietveld refinement, respectively. The structure and microstructures of the synthesized BiFeO3/Bi2Fe4O9 nanoparticles were carried out using transmission electron microscopy techniques. The atomic composition of the samples was investigated by energy-dispersive X-ray. The magnetic properties were studied by the DC magnetization technique. Transmission electron microscopy analyses showed that both samples exhibit a bimodal particle size distribution, with average particle sizes of ∼ 116 nm and ∼ 6.5 nm for the ∼ 3 % Bi2Fe4O9 sample. In contrast, for the ∼ 32 % Bi2Fe4O9 sample the average particle sizes were ∼ 108 nm and ∼ 7 nm, respectively. Energy-dispersive X-ray analyses revealed a Fe-rich impurity phase in both samples. Temperature-dependent zero-field-cooled and field-cooled magnetization curves present a large separation between them below 300 K and zero-field-cooled curves are consistent with a broad (bimodal) size distribution of the nanoparticles for both samples. Magnetic hysteresis loops measured at 5 K after the samples are cooled from 400 K in a zero magnetic field show weak ferromagnetism for the ∼ 3 % Bi2Fe4O9 sample, whereas the ∼ 32 % Bi2Fe4O9 sample presents an improved ferromagnetism. An exchange bias-type effect was observed in both samples. The results suggest that the increase of ultrasmall Bi2Fe4O9 nanoparticles leads to an improvement in the coercivity and remanence of BiFeO3/Bi2Fe4O9 binary nanoparticles.

Efficient loading of cesium atoms in a magnetic levitated dimple trap

We report a detailed study of magnetically levitated loading of ultracold 133Cs atoms in a dimple trap. The atomic sample was produced in a combined red-detuned optical dipole trap and dimple trap formed by two small waist beams crossing a horizontal plane. The magnetic levitation for the 133Cs atoms forms an effective potential for a large number of atoms in a high spatial density. Dependence of the number of atoms loaded and trapped in the dimple trap on the magnetic field gradient and bias field is in good agreement with the theoretical analysis. This method has been widely used to obtain the Bose–Einstein condensation atoms for many atomic species.

Enhancement of graphene absorption: a numerical formula to design a suitable Fabry-Perot cavity in a terahertz spectral window

In this article, we investigate the absorption characteristics of a graphene-embedded FP cavity in a terahertz spectral window. The optical attributes were determined by a 4 × 4 transfer matrix procedure. The findings demonstrate that perfect absorption is completely reliant on the structural characteristics of the FP cavity throughout a broad range of terahertz frequencies. From the obtained dataset, numerical formulae are generated for structural parameters (N FD , N BD ) using a linear regression machine learning algorithm to achieve higher than 90% absorption. The artificial neural network trained using our dataset provided a coefficient of determination (R2)=1, opening up new pathways to design perfect terahertz absorbers. Furthermore, we explored the influence of magnetic biasing on absorption traits, and our findings show that fine absorption improvement is conceivable. The formulated numerical relations have greater importance in the design of perfect terahertz absorbers.

Graphene-based terahertz antenna with polarization reconfiguration

This article presents the design of a terahertz antenna with the ability to reconfigure its polarization using graphene’s anisotropic properties. By applying a DC magnetic flux, the graphene conductivity tensor can be transformed into a non-diagonal tensor. Conversely, the magnetic bias can be switched off to revert the tensor to a diagonal state. This approach allows the antenna to switch between linear and circular polarizations, making it highly versatile for terahertz applications. Additionally, by altering the direction of the applied DC magnetic bias, one can select the rotation direction of circular polarization between left and right hand. The feeding line and ground plane of the antenna are made of silver metal, and its behavior in the terahertz band is described using the Modified Relaxation-Effect model, which is more accurate than the skin effect model for this band. The finite element numerical method is utilized to analyze the antenna, which yields results consistent with the design predictions. The antenna’s impedance bandwidth spans 2.8 to 3.22 terahertz for linear polarization and 2.9 to 3.18 THz for circular polarization. The antenna’s maximum gain is 6 dB for linear polarization and 5 dB for circular polarization. The axial ratio for circular polarization is 1.36 in the middle of the band, which is below the 3 dB threshold for circular polarization. The antenna produces directive patterns for both linear and circular polarizations. The key innovation of this antenna design lies in its unique utilization of a magnetostatic field to achieve reconfiguration in the terahertz graphene dielectric resonator antenna, a method that has not been previously documented in literature.

Completely switchable multi-mode narrowband terahertz absorber: Monolayer graphene, coupled topological interface states, and Rabi splitting

In the present study, we have explored the absorption properties of a monolayer graphene-based photonic heterostructure, which consists of topological photonic configurations, graphene, and a Bragg mirror. The optical attributes were computed by using the 4×4 transfer matrix method. The results indicate that the creation of topological interface states and strong coupling between these resonant modes result in mode splitting, leading to the formation of hybrid modes known as coupled topological interface states with unity transmittance. The number of absorption modes can be modulated with an appropriate selection of cascaded photonic structures. Our proposed design with 1, 3, 5, and 7 cascaded topological photonic structures provides 1, 5, 9, and 13 absorption modes with greater than 90% absorption. The findings also reveal that absorption peak strength is greatly influenced by Fermi-level and magnetic fields; switching of the modes from absorption to reflection and vice versa is achieved by suitable electrical and magnetic biasing. Our proposed design offers various applications, such as switchable filters, absorbers, and modulators.

Inverse exchange bias and training effect in Gd2Co0.5Mn1.5O6 with negative magnetization

The present experiments indicate that the phenomena of inverse exchange bias and negative magnetization are observed simultaneously in Gd2Co0.5Mn1.5O6 below the ferrimagnetic compensation point Tcomp. The training effect confirms the intrinsic nature of the inverse exchange bias and reveals that the frozen spins relax more slowly than the rotatable spins according to the fitting of exchange bias field (HEB) vs cycle loops (n) curve. The amplitude of HEB varies inversely with the cooling field Hcool and the inverse exchange bias is found to remain up to 50 kOe. The observed exchange bias is argued to be due to the fact that the inverse ferromagnetic moments below Tcomp can be flipped by adjusting the amplitude or direction of the magnetic field during the M(H) hysteresis loop measurement. The manipulability of the negative magnetization state is demonstrated by the remanent magnetization and magnetic switching measurements.

Effects of intrinsic and extrinsic multi-phased lattice structure and chemical heterogeneity on the control of ferrimagnetic properties in Co rich spinel ferrite

The present work reports the structural and magnetic properties in spinel ferrite composition around Co2.25Fe0.75O4 as the function of self-composite structure (ratio of two spinel phase components), coexisting impurity phase of hematite, and non-stoichiometric chemical composition. The neutron diffraction patterns have been used to determine lattice structure and magnetic moment. The neutron depolarization experiments have indicated spin reorientation at lower temperature for the single phased sample. The magnetization measurements confirmed the blocking of magnetic domains, possible magnetic spin compensation and magnetic exchange bias effect. The sample coexisting with small hematite phase has shown the maximum coercivity (∼14.682 kOe), remanent magnetization (∼1.4657 μB) and saturation magnetization (∼2.027 μB) with small exchange bias shift (- 71 Oe) at 5 K. The single phased sample exhibited the lowest magnetic moment (∼0.3775 μB) with maximum exchange bias shift (- 543 Oe) and the highest cooling-field induced enhancement of coercivity (3691 Oe). The experiments confirmed a large scope of tuning the ferrimagnetic parameters by controlling the phase fraction and interfacial phases in Co rich spinel ferrite.

Charge Oscillations in Bilayer Graphene Quantum Confinement Devices.

Quantum confinement structures are building blocks of quantum devices in fundamental physics exploration and technological applications. In this work, we fabricate dual-gated bilayer graphene Fabry-Pérot quantum Hall interferometers employing two different gating strategies and conduct finite element simulations to understand the electrostatics of the confinement structures and to guide device design and fabrication. We observe two types of resistance oscillations arising from the charging of quantum dots formed inside the interferometers. We obtain the size, location, and charging energy of the dots by measuring the dependence of the oscillations on the magnetic field, gate voltages, and dc bias. We analyze and discuss the origin of the quantum dots and their impact on quantum Hall edge state backscattering and interference. Insights gained in these studies shed light on the construction of van der Waals quantum confinement devices.