Small Magnetic Field Research Articles

Induction heating boosts water splitting on iron–coated nickel foam

Abstract Alkaline water electrolysis at high temperatures can rival acidic proton-exchange membranes.
However, they suffer from increased energy consumption, reduced lifespan of materials and heightened safety risks. Magnetic hyperthermia is a method of localizing intense heating in the presence of an external high-frequency alternating magnetic field. In this study, we developed a custom electromagnetic induction device capable of generating a small magnetic field of about 2 µT. High-permeability nickel foam is used as electrodes. Results show that the iron coated nickel foam decreases the overpotential of the hydrogen evolution reaction and oxygen evolution reaction by ~150 mV and 60 mV, respectively, at 20 mAcm−2 when subjected to magnetic heating in a high-frequency alternating magnetic field. The overall water splitting current of Ni foam/Fe increases 540% under intermittent induction. The enhanced stability of Ni foam/Fe is attributed to the high binding energy of metal-O on the surface. The density function theory calculations further indicates that the lattice expansion of the metal electrode under induction heating optimizes the adsorption and desorption of H*, thereby enhancing the HER performance.

Guanine-based spin valve with spin rectification effect for an artificial memory element.

Non-volatile electronic memory elements are very attractive for applications, not only for information storage but also in logic circuits, sensing devices and neuromorphic computing. Here, a ferroelectric film of guanine nucleobase is used in a resistive memory junction sandwiched between two different ferromagnetic films of Co and CoCr alloys. The magnetic films have an in-plane easy axis of magnetization and different coercive fields whereas the guanine film ensures a very long spin transport length, at 100K. The non-volatile resistance states of the multiferroic spintronic junction with two-terminals are manipulated by a combined action of small external magnetic and electric fields. Thus, the magnetic field controls the relative orientation of the magnetization of the metallic ferromagnetic electrodes, that leads to different magnetoresistance states. The orientation and the magnitude of the electric field controls the orientation of the polarization of the guanine ferroelectric barrier, that leads to different electroresistance states, respectively. Moreover, we have observed a strong interfacial coupling of the two parameters. Consequently, positive and negative magnetoresistance hysteresis loops corresponding to spin rectification effects and non-hysteretic (erased) resistive states are manipulated with the electric field by switching the orientation of the electrical polarization of the organic ferroelectric.

Zeeman Effect-Boosted Spin-Polarized Band Splitting in Diluted Magnetic Photocatalysis Semiconductors for Efficient CO2 Photoreduction.

Magnetic field regulation is an effective strategy to improve the photocatalytic activity of magnetic semiconductor photocatalysts, but it is not suitable for widely used nonmagnetic photocatalytic semiconductors. Here, we report a Zeeman effect-driven spin-polarized band splitting phenomenon in diluted magnetic semiconductors that show efficient photocatalytic CO2 reduction under visible-light irradiation. A flexible Ni2+-doped BaTiO3 nanofiber film is used as the diluted magnetic semiconductor model to prove this concept. The interstitial Ni2+ dopant induces the spin-polarized bands in Ni-BaTiO3 nanofibers to split under light excitation, generating spin-excited electrons and holes. This Zeeman effect induced by the magnetic field is more obvious since it intensifies the spin-polarized band splitting and generates more spin-excited electrons and holes, suppressing the carrier recombination and extending the carrier lifetime for CO2 photoreduction. As a result, the evolution rates of CO and CH4 are as high as 86.47 and 96.06 μmol/g/h under a small magnetic field of 50 mT. The proposed mechanism of Zeeman effect-driven spin-polarized band splitting is feasible to improve the CO2 photoreduction efficiency of broadly applied diluted magnetic semiconductors.

Aligning and Observing the Liquid Crystal Director in 3D Using Small Magnetic Fields and a Wedge‐Cell

AbstractThe mechanical and optical properties of liquid crystalline materials are largely dependent on the director profile. More complex soft robotic functions and programmed optical properties require spatially varying director profiles, ideally in 3D. However, it is challenging to achieve arbitrary director orientation with most established alignment techniques, as one needs to overcome surface interactions, use high electric or magnetic field strengths and temperatures. Another experimental difficulty is that there is a lack of suitable techniques that can be used to characterize the director in 3D. Here, this study first shows that the addition of 5CB to reactive mesogens permits cross‐linked liquid crystalline materials to be fabricated with a spatially varying 3D director profile using weak magnetic fields (0.13 T). This study also shows, how these can be characterized with an optical technique that uses a wedge cell to visualize the programmed 3D director profile. Interestingly, the method also permits the real‐time observation of the director. This work shows that it is possible to precisely control the director in 3D with low magnetic fields and that the dynamics can be directly observed, which facilitates potential applications of soft liquid crystalline (LC) gels and potentially also elastomers.

Electron beam-splitting effect with crossed zigzag graphene nanoribbons in high-spin metallic states

Here, we analyze the electron transport properties of a device formed of two crossed graphene nanoribbons with zigzag edges (ZGNRs) in a spin state with total magnetization different from zero. While the ground state of ZGNRs has been shown to display antiferromagnetic ordering between the electrons at the edges, for wide ZGNRs—where the localized spin states at the edges are decoupled and the exchange interaction is close to zero—in the presence of relatively small magnetic fields, the ferromagnetic (FM) spin configuration can become the state of lowest energy due to the Zeeman effect. In these terms, by comparing the total energy of a periodic ZGNR as a function of the magnetization per unit cell, we obtain the FM-like solution of the lowest energy for the perfect ribbon, the corresponding FM-like configuration of the lowest energy for the four-terminal device formed of crossed ZGNRs, and the critical magnetic field needed to excite the system to this spin configuration. By performing transport calculations, we analyze the role of the distance between layers and the crossing angle of this device in the electrical conductance, at small gate voltages. The problem is approached employing the mean-field Hubbard Hamiltonian in combination with non-equilibrium Green’s functions. We find that ZGNR devices subject to transverse magnetic fields may acquire a high-spin configuration that ensures a metallic response and tunable beam-splitting properties, making this setting promising for studying electron quantum optics with single-electron excitations.

Turbulent plasma flow, its energies, and structures: Velocity vortices, magnetic field cocoons, and plasmoids

Context. Turbulent flows are believed to be present in the solar corona, especially in connection with solar flares and coronal mass ejections. They are supposed to be very effective processes in energy transportation and can contribute to the heating of the solar corona. Aims. We study turbulence in reconnection outflows associated with flares and coronal mass ejections. We simulated the generation and evolution of the turbulent plasma flow and investigated its energies and formed plasma velocity and magnetic field structures. Methods. For the numerical simulations, we adopted a three-dimensional (3D) magnetohydrodynamic (MHD) model, in which we solved a full set of the 3D time-dependent resistive and compressible MHD equations using the LARE3D numerical code. Results. We numerically studied turbulence in the plasma flow in the model with the plasma parameters that could simulate processes in the magnetic reconnection outflows in solar flares. Starting from a non-turbulent plasma flow in the energetically closed system, we studied the evolution of the kinetic, internal, and magnetic energies during the turbulence generation. We found that most of the kinetic energy is transformed into the plasma heating (about 95%) and only a small part to the magnetic energy (about 5%). The turbulence in the system evolves to the saturation stage with the power-law index of the kinetic density spectrum, −5/3. Magnetic energy is also saturated due to its dissipation and reconnection in small and complex magnetic field structures. We show examples of the structures formed in studied turbulent flow: velocity vortices, magnetic field cocoons, and plasmoids.

Static Magnetic Field Meter Using Rotating Search Coil Method

Abstract Static magnetic fields always exist the environment around us; besides being useful there are also negative impacts on humans, therefore it is necessary to have a tool to measure static magnetic fields. The purpose of this research is to produce a static magnetic field meter that can measure weak magnetic field. In this research, the design and realization of a static magnetic field meter using rotating search coil method is carried out, including: search coil, instrumentation and calibration amplifier, and display. Based on the results of measurements and tests that have been carried out, a static magnetic field meter with a coil area A = m2 and the number of turns N = 14 at a certain angular frequency (ω), can detect and measure a fairly small static magnetic field density (B) from various sources, for stable conditions: in the laboratory without any source of magnetic field can detect B = 2.127 to 2.375 mT, and with magnetic source (circular can detect B = 7.422 to 8.194 mT, neodymium can detect B = 11.03 to 11.84 mT, and smartphone X can detect B = 10.37 to 11.78 mT). Overall the device can work to detect and measure weak static magnetic fields with good measurement stability as seen from the relationship curve between supply voltage and the DC motor rotation which is linear, and measurement sensitivity up to B = 2.127 mT.

Spin-valley locked excited states spectroscopy in a one-particle bilayer graphene quantum dot

Current semiconductor qubits rely either on the spin or on the charge degree of freedom to encode quantum information. By contrast, in bilayer graphene the valley degree of freedom, stemming from the crystal lattice symmetry, is a robust quantum number that can therefore be harnessed for this purpose. The simplest implementation of a valley qubit would rely on two states with opposite valleys as in the case of a single-carrier bilayer graphene quantum dot immersed in a small perpendicular magnetic field (B⊥ ≲ 100 mT). However, the single-carrier quantum dot excited states spectrum has not been resolved to date in the relevant magnetic field range. Here, we fill this gap, by measuring the parallel and perpendicular magnetic field dependence of this spectrum with an unprecedented resolution of 4 μeV. We use a time-resolved charge detection technique that gives us access to individual tunnel events. Our results come as a direct verification of the predicted spectrum and establish a new upper-bound on inter-valley mixing, equal to our energy resolution. Our charge detection technique opens the door to measuring the relaxation time of a valley qubit in a single-carrier bilayer graphene quantum dot.

Evidence of Zero-Field Wigner Solids in Ultrathin Films of Cadmium Arsenide

The quantum Wigner crystal is a many-body state where Coulombic repulsion quenches the kinetic energy of electrons, causing them to crystallize into a lattice. Experimental realization of a quantum Wigner crystal at zero magnetic field has been a long-sought goal. Here, we report on the experimental evidence of a Wigner solid in ultra-thin films of cadmium arsenide (Cd3As2) at zero magnetic field. We show that a finite bias depins the domains and produces an unusually sharp-threshold current-voltage behavior. Hysteresis and voltage fluctuations point to domain motion across the pinning potential and disappear at finite temperature as thermal fluctuations overcome the potential. The application of a small magnetic field destroys the Wigner solid, pointing to an unconventional origin. We use Landau-level spectroscopy to show that the formation of the Wigner solid is closely connected to a topological transition as the film thickness is reduced. Published by the American Physical Society 2024

Superparamagnetic Relaxation in Ensembles of Ultrasmall Ferrihydrite Nanoparticles

The paper examines the impact of interparticle interactions on the superparamagnetic relaxation of ultrasmall nanoparticle ensembles, using Fe2O3∙nH2O iron oxyhydroxide (ferrihydrite) nanoparticles as an example. Two samples were analyzed: ferrihydrite of biogenic origin (with an average particle size of d ≈ 2.7 nm) with a natural organic shell, and a sample (with d ≈ 3.5 nm) that underwent low-temperature annealing, during which the organic shell was partially removed. The DC and AC magnetic susceptibilities (χ′(T), χ′′(T)) in a small magnetic field in the superparamagnetic (SPM) blocking region of the nanoparticles were measured. The results show that an increase in interparticle interactions leads to an increase in the SPM blocking temperature from 28 to 52 K according to DC magnetization data. It is shown that below the SPM blocking temperature, magnetic interactions of nanoparticles lead to the formation of a collective state similar to spin glass in bulk materials. The scaling approach reveals that the dynamics of correlated magnetic moments on the particle surface slow down with increasing interparticle interactions. Simulation of χ′′(T) dependence has shown that the dissipation of magnetic energy occurs in two stages. The first stage is directly related to the blocking of the magnetic moment of nanoparticles, while the second stage reflects the spin-glass behavior of surface spins and strongly depends on the strength of interparticle interactions.

Tunable magnonic crystal in a hybrid superconductor–ferrimagnet nanostructure

One of the most intriguing properties of magnonic systems is their reconfigurability, where an external magnetic field alters the static magnetic configuration to influence magnetization dynamics. In this paper, we present an alternative approach to tunable magnonic systems. We studied theoretically and numerically a magnonic crystal induced within a uniform magnetic layer by a periodic magnetic field pattern created by the sequence of superconducting strips. We showed that the spin-wave spectrum can be tuned by the inhomogeneous stray field of the superconductor in response to a small uniform external magnetic field. Additionally, we demonstrated that modifying the width of superconducting strips and separation between them leads to the changes in the internal field which are unprecedented in conventional magnonic structures. The paper presents the results of semi-analytical calculations for realistic structures, which are verified by finite-element method computations.

On the trapped magnetic moment in type-II superconductors

Abstract Measurements of the trapped (remanent) magnetic moment, M trap ( H ) , when a small magnetic field H is turned off after cooling below the superconducting transition temperature, Tc , or ramping a magnetic field up and down after cooling in a zero magnetic field, offer substantial advantages in difficult cases of small samples and large field-dependent backgrounds, which is relevant for hydrogen-based ultra-high-T c superconductors (UHTS). Until recently, there was no need for a separate paper on the trapped magnetic flux for well-known critical-state models due to the simplicity of the physics involved. However, recent publications showed the need for such an analysis. This note summarizes the expectations for the Bean model with constant critical current density and the Kim model with field-dependent critical currents. It is shown that if the trapped moment is fitted to the power law, M trap ∝ H α , the fixed exponent α = 2 is exact for the Bean model, while Kim models show a wide interval of possible values, 2 ⩽ α ⩽ 4 . Furthermore, accounting for reversible magnetization expands the range of possible exponents to 1 ⩽ α ⩽ 4 . In addition, demagnetizing factors are essential and make the trapped moment orientation dependent even in isotropic materials. As a concrete application, it is shown that flux trapping experiments performed on H3S UHTS compounds can be well described using this generalized approach, lending further support to the type-II superconducting nature of H3S under ultra-high pressure.

Electronic 1/f noise as a probe of dimensional effects on spin-glass dynamics

Over the past decade, spin-glass simulations have improved to the point that they now access time- and length-scales comparable to experiments at the mesoscale. A recent series of thin-film field-cooled/zero-field-cooled magnetization (FC/ZFC) experiments demonstrated activated spin dynamics, with a temperature-independent activation energy proportional to the logarithm of the film thickness and with coefficients in remarkable agreement with the simulation. These measurements require the application of small magnetic fields, which has been shown to affect the spin-glass energy landscape. Measurements of the 1/f noise in metallic spin-glasses have been previously shown to be a sensitive probe of the spin dynamics, and the measurements can be made without applying a magnetic field. In this mini-review, we review these techniques and discuss how transport measurements can fit into the current landscape of spin-glass measurements. We compare previous measurements to more recent measurements on similar films, made with ostensibly different cooling protocols, and compare both the previous and recent measurements to the magnetometry. The transport measurements—taken over a wider range of temperature than magnetometry—suggest that the maximum spin-glass energy barrier height is temperature-dependent, not fixed, possibly due to two-dimensional dynamics. We discuss this possibility, along with future measurements, which may be able to resolve this mystery.

Control of the light polarization in ferromagnetic diode structures InGaAs/GaAs/δ-Mn

Electric-field influence on the polarization of the quantum well photoluminescence is studied in the diode structures InGaAs/GaAs/δ-Mn with narrow GaAs spacer dS = 2–5 nm at small magnetic field. Weakening of the circular polarization degree with increasing electric-field evidence about significant contribution of the stationary mechanism of the carriers’ polarization due to their exchange coupling with a nearby ferromagnetic δ-Mn-layer.

Ground state magnetic structure and magnetic field effects in the layered honeycomb antiferromagnet YbOCl

YbOCl is a representative member of the van der Waals layered honeycomb rare-earth chalcohalide RChX (R = rare earth; Ch = O, S, Se, and Te; and X = F, Cl, Br, and I) family reported recently. Its spin ground state remains to be explored experimentally. We grew high-quality single crystals of YbOCl and conducted comprehensive thermodynamic, elastic, and inelastic neutron scattering experiments down to 50 mK. The experiments reveal an antiferromagnetic phase below 1.3 K which is identified as a spin ground state with an intralayer ferromagnetic and interlayer antiferromagnetic ordering. By applying sophisticated numerical techniques to a honeycomb (nearest-neighbor)–triangle (next-nearest-neighbor) model Hamiltonian which accurately describes the highly anisotropic spin system, we are able to simulate the experiments well and determine the diagonal and off-diagonal spin-exchange interactions. The simulations give an antiferromagnetic Kitaev term comparable to the Heisenberg one. The experiments under magnetic fields allow us to establish a magnetic field–temperature phase diagram around the spin ground state. Most interestingly, a relatively small magnetic field (∼0.3 to 3 T) can significantly suppress the antiferromagnetic order, suggesting an intriguing interplay of the Kitaev interaction and magnetic fields in the spin system. The present study provides fundamental insights into the highly anisotropic spin systems and opens a window to look into Kitaev spin physics in a rare-earth-based system. Published by the American Physical Society 2024

Field‐Induced Butterfly‐Like Anisotropic Magnetoresistance in a Kagome Semimetal Co3In2S2

AbstractWith the interplay between magnetism and topological bands, magnetic kagome semimetals provide promising platforms for exploring exotic correlated electronic states and quantum phenomena such as anomalous Hall effect, quantum spin liquid, and unconventional magnetoresistance, as well as driving advances in electronic and spintronic applications. Here, a field‐induced butterfly‐like anomalous anisotropic magnetoresistance (AMR) effect in an intriguing kagome semimetal Co3In2S2 is reported. The kagome‐lattice Co3In2S2 single crystals are synthesized via a polycrystal‐source chemical vapor transport approach, possessing a high carrier mobility reaching 104 cm2V−1s−1. The Co3In2S2 single crystal exhibits a canted antiferromagnetic state below 5 K, but intriguingly, it is easily transformed into a ferromagnetic state under a small external magnetic field. Furthermore, the planar Hall effect (PHE) is detected, stemming from the complex contribution of field‐induced ferromagnetism and orbital magnetoresistance. Remarkably, as the magnetic field increases, the low‐temperature magnetoresistance behavior of the Co3In2S2 reveals a butterfly‐like AMR effect with a maximum value of 850%, exhibiting a superposition of the two‐, four‐, and six‐fold AMR terms. Band structure calculations suggest that such a field‐induced butterfly‐like AMR effect may originate from the modulations of the electronic structure near the Fermi level by the magnetic moment. The findings offer a valuable platform for understanding the anomalous AMR effect and for the development of advanced spintronic devices.

Modeling interactions between rubidium atom and magnetometer cell wall molecules

Magnetometer cell wall coat molecules play an important role in preserving the lifetime of pumped alkali metal atoms for use in magnetometers that are capable of measuring very small magnetic fields. The goal of this study is to help rationalize the design of the cell coat molecules. Rubidium-87 is studied in terms of its interaction with three template cell coat molecules: ethane, ethene, and methyltrichlorosilane (MeTS). Ab initio electronic structure methods are applied to investigate the effect that the coat molecules have on the 2S ground state and 2P excited state of 87Rb. We find that, from the ab initio results, the three template molecules have differing effects, with MeTS having the largest effect on the ground state and ethane or ethene having the largest effect on the non-degenerate excited states.

Weak magnetic vestibular stimulation decreases postural sway

BackgroundPerceptible galvanic vestibular stimulation (GVS) causes nystagmus and postural sway deterioration. Conversely, imperceptible GVS improves postural stability, suggesting the presence of stochastic resonance. Research questionSimilar to GVS, strong magnetic vestibular stimulation of 7 T induces nystagmus and increases body sway. Thus, a relatively small magnetic stimulation may improve postural stability. In this study, we measured the effect of a relatively small magnetic field on postural sway. MethodsPosturography was performed in eight healthy participants using a stabilometer with foam rubber on board. The center of pressure (COP) trajectories were recorded in both the anterior–posterior and medial–lateral directions for 60 s with the eye closed. Neodymium magnets (0.4 T) or aluminum disks of similar size (0 T) were placed bilaterally over the mastoid processes. ResultsBoth the trajectory length and envelopment area of the COP movement with 0.4 T were significantly smaller than those with 0 T. SignificanceThe relatively smaller magnetic vestibular stimulation decreased postural sway. This method may be useful for improving the vestibular function and related reflexes.

Searching for Evidence of Subchromospheric Magnetic Reconnection on the Sun

Within the coronae of stars, abundances of those elements with low first ionization potential (FIP) often differ from their photospheric values. The coronae of the Sun and solar-type stars mostly show enhancements of low-FIP elements (the FIP effect) while more active stars such as M dwarfs have coronae generally characterized by the inverse-FIP (I-FIP) effect. Highly localized regions of I-FIP effect solar plasma have been observed by Hinode's EUV Imaging Spectrometer in a number of highly complex active regions (ARs), usually around strong light bridges of the umbrae of coalescing/merging sunspots. These observations can be interpreted in the context of the ponderomotive force fractionation model, which predicts that plasma with I-FIP effect composition is created by the refraction of waves coming from below the plasma fractionation region in the chromosphere. A plausible source of these waves is thought to be reconnection in the (high-plasma-β) subchromospheric magnetic field. In this study, we use the 3D visualization technique of Chintzoglou & Zhang combined with observations of localized I-FIP effect in the corona of AR 11504 to identify potential sites of such reconnection and its possible consequences in the solar atmosphere. We found subtle signatures of episodic heating and reconnection outflows in the expected places, in between magnetic flux tubes forming a light bridge, within the photosphere of the AR. Furthermore, on either side of the light bridge, we observed small antiparallel horizontal magnetic field components, supporting the possibility of reconnection occurring where we observe I-FIP plasma. When taken together with the I-FIP effect observations, these subtle signatures provide a compelling case for indirect observational evidence of reconnection below the fractionation layer of the chromosphere, however direct evidence remains elusive.

High-precision reading of a weak magnetostatic field

Efforts have recently been put into developing proton precession magnetometers (PPM) in order to measure weak magnetic fields with high precision. Although high-cost commercial PPMs are available today, there are ongoing studies for low power consumption, low cost, high accuracy and resolution. Accordingly, in this study, a new PPM with a measurement range of 20,000 nT-120,000 nT, a production cost of $2000 and an accuracy of 0.13 nT was developed and its performance in determining the earth’s magnetic field was examined. It was determined that the developed PPM has an advantage over its commercial equivalents (G857, WCZ, etc.) in terms of price-performance ratio and can be used to determine small magnetic field values such as the earth’s magnetic field.