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Ferromagnetism in quantum dot plaquettes
摘要： Following recent experimental progress concerning Nagaoka ferromagnetism in finite-size quantum dot plaquettes, a general theoretical analysis is warranted in order to ascertain in rather generic terms which arrangements of a small number of quantum dots can produce saturated ferromagnetic ground states and under which constraints on interaction and interdot tunneling in the plaquette. This is particularly necessary since Nagaoka ferromagnetism is fragile and arises only under rather special conditions. We test the robustness of ground state ferromagnetism in the presence of a long-range Coulomb interaction and long-range as well as short-range interdot hopping by modeling a wide range of different plaquette geometries accessible by arranging a few (～4) quantum dots in a controlled manner. We find that ferromagnetism is robust to the presence of long-range Coulomb interactions, and we develop conditions constraining the tunneling strength such that the ground state is ferromagnetic. Additionally, we predict the presence of a partially spin-polarized ferromagnetic state for 4 electrons in a Y-shaped 4-quantum-dot plaquette. Finally, we consider 4 electrons in a ring of 5 dots. This does not satisfy the Nagaoka condition; however, we show that the ground state is spin 1 for strong, but not infinite, on-site interaction. Thus, even though Nagaoka’s theorem does not apply, the ground state for the finite system with one hole in a ring of 5 dots is partially ferromagnetic. We provide detailed fully analytical results for the existence or not of ferromagnetic ground states in several quantum dot geometries which can be studied in currently available coupled quantum dot systems.
关键词： long-range Coulomb interactions,Nagaoka ferromagnetism,interdot tunneling,ferromagnetic ground states,quantum dot plaquettes
更新于2025-09-12 10:27:22
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<i>Ab initio</i> exact diagonalization simulation of the Nagaoka transition in quantum dots
摘要： Recent progress of quantum simulators provides insight into the fundamental problems of strongly correlated systems. To adequately assess the accuracy of these simulators, the precise modeling of the many-body physics, with accurate model parameters, is crucially important. In this paper, we employed an ab initio exact diagonalization framework to compute the correlated physics of a few electrons in artificial potentials. We apply this approach to a quantum-dot system and study the magnetism of the correlated electrons, obtaining good agreement with recent experimental measurements in a plaquette. Through control of dot potentials and separation, including geometric manipulation of tunneling, we examine the Nagaoka transition and determine the robustness of the ferromagnetic state. While the Nagaoka theorem considers only a single-band Hubbard model, in this work we perform extensive ab initio calculations that include realistic multiorbital conditions in which the level splitting is smaller than the interactions. This simulation complements the experiments and provides insight into the formation of ferromagnetism in correlated systems. More generally, our calculation sets the stage for further theoretical analysis of analog quantum simulators at a quantitative level.
关键词： Nagaoka transition,ab initio exact diagonalization,ferromagnetic state,quantum simulators,strongly correlated systems,quantum-dot system
更新于2025-09-11 14:15:04
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Inverse Spin Hall Effect in Electron Beam Evaporated Topological Insulator Bi <sub/>2</sub> Se <sub/>3</sub> Thin Film
摘要： Spintronics exploiting pure spin current in ferromagnetic (FM)/heavy metals (HM) is a subject of intense research. Topological insulators having spin momentum locked surface states exhibit high spin–orbit coupling and thus possess a huge potential to replace the HM like Pt, Ta, W, etc. In this context, the spin pumping phenomenon in Bi2Se3/CoFeB bilayers has been investigated. Bi2Se3 thin films are fabricated by electron beam evaporation method on Si (100) substrate. In order to confirm the topological nature of Bi2Se3, low temperature magnetotransport measurement on a 30 nm thick Bi2Se3 film which shows 10% magnetoresistance (MR) at 1.5 K has been performed. A linear increase in MR with applied magnetic field indicates the presence of spin momentum-locked surface states. A voltage has been measured at room temperature to quantify the spin pumping which is generated via inverse spin Hall effect (ISHE). For the separation of spin rectification effects mainly produced by the FM CoFeB layer, in plane angular dependence of the dc voltage with respect to applied magnetic field has been measured. Our analysis reveals that spin pumping induced ISHE is the dominant contribution in the measured voltage.
关键词： spin pumping,inverse spin Hall effect,topological insulator/ferromagnetic interface,magnetoresistance,ferromagnetic resonance
更新于2025-09-10 09:29:36
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Handbook of Advanced Non-Destructive Evaluation || Induction Thermography of Surface Defects
摘要： A survey on theory, characteristic quantities, and the experimental technique of induction thermography is given. Induction thermography is used for surface defect detection in forged parts of ferromagnetic steel at typical frequencies of 100–300 kHz. Values for the detection limits for various types of cracks and approaches to determine crack depths are given. The sensitivity for crack detection is comparable to magnetic particle inspection. A hidden defect in ferritic steel with a coverage of 140 μm was detected by lowering the induction frequency down to 1500 Hz. Cracks in silicon solar cells were detected. Defects of ?bers were detected in carbon ?ber reinforced polymer (CFRP). Inductive excitation is complementary to ?ash excitation. Crack detection in railway components like rails and wheels was shown. In rails, a larger defect could be detected from a test car moving at a speed of up to 15 km/h. A fully automated demonstrator for wheel testing was built up, which can detect surface defects in railway wheels with sensitivity comparable to magnetic particle testing. Standardization of thermography has gained progress in the last years and led to ?rst standards on active thermography and induction thermography.
关键词： surface defect detection,induction thermography,crack detection,carbon fiber reinforced polymer,railway component testing,ferromagnetic steel
更新于2025-09-10 09:29:36
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Nonlinear converse magnetoelectric effects in a ferromagnetic-piezoelectric bilayer
摘要： The strain mediated nonlinear converse magnetoelectric effect (CME) is investigated in a bilayer of an amorphous ferromagnet FeBSiC and piezoelectric lead zirconate titanate (PZT). The magnetic response of the sample to an AC electric ?eld (e) applied to PZT at an acoustic resonance frequency of 76 kHz was measured with a coil wound around the bilayer. With an increase in the amplitude of e over the range 0–250 V/cm, the variation in amplitude of the ?rst and the second harmonics of the induced voltage due to the variation in the magnetic induction B was measured for DC bias magnetic ?eld H ? 0–80 Oe. The coef?cients of the linear and nonlinear converse ME effects were 5.5 G cm/V and 1.9 (cid:2) 10(cid:3)2 G cm2/V2, respectively. The nonlinearity of the CME arises due to the nonlinear dependence of the magnetic induction on the stress. A theoretical model for the nonlinear CME is discussed.
关键词： acoustic resonance,nonlinear converse magnetoelectric effect,stress,magnetic induction,ferromagnetic-piezoelectric bilayer
更新于2025-09-10 09:29:36
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[IEEE 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018) - Paris, France (2018.7.8-2018.7.13)] 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018) - Spin Hall Effect Device for Magnetic Sensor Application
摘要： In this paper, the spin Hall device was fabricated to magnetic sensor applications. This device based on the spin Hall effect by observing in ferromagnetic and heavy metal multi-layer structures. We measured shift of switching current with changing a z-component of small magnetic field through anomalous Hall effect measurement. The shift of switching current can be applied the magnetic sensor. Also, we controlled the sensitivity of this device changing efficiency of spin Hall effect. This device is a good candidate for magnetic sensor to measure a wide dynamic range of magnetic field.
关键词： ferromagnetic layer,magnetic sensor,spin Hall effect
更新于2025-09-10 09:29:36
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Reference Module in Materials Science and Materials Engineering || III-V and Group-IV-Based Ferromagnetic Semiconductors for Spintronics
摘要： From the first development of the transistor, the rapid growth of solid-state electronic circuits has been quite impressive. Innovations of semiconductor (SC) processing technologies have helped keep the pace of developing smaller electronic devices with higher performance. Operations of most electronic devices rely on the ability to control the flow of charges. However, when electronic devices reach the scale of 10 nm or even smaller, two big problems appear. First, many kinds of fluctuation make it difficult to control the flow of electron charges. For example, when the channel length of field effect transistors (FETs) reaches several nanometers, the transport of electrons cannot be described by the conventional diffusion equation. In nanoscale devices, even fluctuation of positions of doped atoms strongly affects the movement of electrons, making it difficult to predict and control the output current. This results in large fluctuation of device parameters. The second problem is the leak off-current that results in the large idling energy dissipation. When the channel length reaches sub-10 nm, the leak off-current appears due to the quantum tunneling effect. As the device size becomes smaller, a larger number of devices working at higher speed are integrated. Thus, larger idling energy is consumed. To overcome these problems, many emerging technologies are being explored and developed. One of the prospective technologies, called 'spintronics,' may help us to find solutions or a totally new framework of electronics.
关键词： Group-IV semiconductors,ferromagnetic semiconductors,Mn-doped,Fe-doped,electrical control of ferromagnetism,III-V semiconductors,tunneling magnetoresistance,spintronics
更新于2025-09-10 09:29:36
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The metal-free magnetism and ferromagnetic narrow gap semiconductor properties in graphene-like carbon nitride
摘要： In spintronics, if a two-dimensional (2D) organic metal-free material has stable magnetism and narrow gap semiconductor properties, it will have a very bright application prospect. A graphene-like carbon nitride (g-C13N13) that we design just meets these requirements. As a new structure, firstly the stability of the g-C13N13 has been verified. It has stable electron spin polarization and the magnetic moment of each primitive cell is 1μB. It exhibits ferromagnetic narrow gap semiconductor properties through our analysis of energy band structure and charge density. Ferromagnetic ordering between two adjacent primitive cells is stable. The Monte Carlo simulation using the Ising model shows the Curie temperature material is 204K. Our research is an inspiration for the applications of this kind of materials in spintronics devices.
关键词： Metal-free magnetism,Electron spin polarization,Ferromagnetic ordering,Narrow gap semiconductor properties,Monte Carlo simulations
更新于2025-09-10 09:29:36
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[IEEE 2018 IEEE 38th International Conference on Electronics and Nanotechnology (ELNANO) - Kiev (2018.4.24-2018.4.26)] 2018 IEEE 38th International Conference on Electronics and Nanotechnology (ELNANO) - Harmonic Balance Method and Effective Magnetization Curves in Finite Element Analysis of Eddy-Current Losses
摘要： A nonlinear model of the magnetic field in a conductive ferromagnetic medium is presented. The model based on the simultaneous use of the harmonic balance method and the finite element method. The case of a magnetic field excitation by an alternating current source is considered. The values of eddy current losses are compared, determined using the presented method as well as by applying the monoharmonic approach using the normal magnetization curve and the curves obtained from it, by means of two energy methods.
关键词： finite element method,ferromagnetic,conductive medium,eddy current losses,harmonic balance method
更新于2025-09-10 09:29:36
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Penta-AlN2 monolayer: A ferromagnetic insulator
摘要： A ferromagnetic insulating behavior is necessary to realize quantum anomalous Hall effect. Here we study, using first-principles calculations and a Monte Carlo simulation, the electronic structure and magnetism of AlN2 monolayer. Our results show that the AlN2 monolayer has a ferromagnetic insulating behavior. By using the crystal field analysis, we find that the axes of the neighboring N2 dimers are perpendicular, and the superexchange between them accounts for the ferromagnetic (FM) order in the AlN2 monolayer. We also carried out a Monte Carlo simulation, which shows that the AlN2 monolayer remains FM with Curie temperature (Tc) ~ 22 K. Moreover, the tensile strain would make a stronger overlap between the neighboring N2 dimers, and steadily enhances the FM stability. As a result, the Tc can be increased with the tensile strain.
关键词： tensile strain,ferromagnetic insulator,AlN2 monolayer,superexchange,quantum anomalous Hall effect
更新于2025-09-10 09:29:36