Large External Magnetic Field Research Articles

The delicate coupling between magnetism and magneto-transport in Fermi-energy-adjusted MnBi2Te4 crystals

We explored the coupling between magnetic and magneto-transport properties in MnBi2Te4 crystals with Fermi energy EF ranging from 10 to 100 meV in the conduction band. Electrical, magnetic, and magneto-transport measurements reveal distinct behaviors depending on EF. At lower EF values (10 meV), MnBi2Te4 exhibits degenerate-semiconductor-like electrical transport and ferrimagnetism, with weak coupling between magneto-resistance and ferrimagnetism. In contrast, MnBi2Te4 displays metallic transport and antiferromagnetism (AFM) at higher Fermi energies, with magneto-resistance strongly coupled to antiferromagnetism and canted antiferromagnetism under a large external magnetic field. Remarkably, Hall measurements demonstrate a pronounced anomalous Hall resistivity (AHR) when the EF of MnBi2Te4 is 10 meV, larger than that reported for other bulk MnBi2Te4 crystals in the literature. Significant AHR is attributed to the Berry-phase effect in electronic-band structure based on first-principles calculation. The evolution of magnetic and magneto-transport properties in EF shifted MnBi2Te4 can be semi-quantitatively explained by the Ruderman–Kittel–Kasuya–Yosida interaction between neighboring MnTe layers. Our work suggests that the strongly Fermi-energy-sensitive magneto-transport properties observed in MnBi2Te4 may be useful in developing magnetic sensors/detectors.

A walking soft robot driven by electromagnetism inside the body

Existing magnetically driven soft robots mainly rely on external electromagnetic, leading to a substantial energy consumption due to the requirement of a large external magnetic field. Moreover, the precise control of these soft robots relies on electric current, making them highly susceptible to disturbances and deviations induced by minute variations in the current. To overcome these challenges, we propose and evaluate a novel approach employing a miniature walking soft robot empowered by its internal electromagnets. The overall robot size is 18 mm × 6 mm × 12 mm (length × height× width). This design enables the robot to achieve precise and stable motion using a 240-mA current with a 6 V low voltage. In addition, the incorporation of specially designed sheet-leg mechanism with varying degrees of friction facilitates the transformation of linear motion into an effective forward gait. This paper outlines the principles and control strategies of the robot, illustrates the robot fabrication process, at the same time verifies the structural integrity through experimental validation. Further evaluations include comprehensive analysis of the robot’s gait and speed. The results show that the robot attains a speed of 2.86 mm s–1. This study marks a stride towards the realization of a fully autonomous, unrestrained, cost-effective, and energy-conserving magnetic soft robot.

Nitrogen-vacancy magnetometry of CrSBr by diamond membrane transfer

Magnetic imaging using nitrogen-vacancy (NV) spins in diamonds is a powerful technique for acquiring quantitative information about sub-micron scale magnetic order. A major challenge for its application in the research on two-dimensional (2D) magnets is the positioning of the NV centers at a well-defined, nanoscale distance to the target material required for detecting the small magnetic fields generated by magnetic monolayers. Here, we develop a diamond “dry-transfer” technique akin to the state-of-the-art 2D-materials assembly methods and use it to place a diamond micro-membrane in direct contact with the 2D interlayer antiferromagnet CrSBr. We harness the resulting NV-sample proximity to spatially resolve the magnetic stray fields generated by the CrSBr, present only where the CrSBr thickness changes by an odd number of layers. From the magnetic stray field of a single uncompensated ferromagnetic layer in the CrSBr, we extract a monolayer magnetization of MCSB = 0.46(2) T, without the need for exfoliation of monolayer crystals or applying large external magnetic fields. The ability to deterministically place NV-ensemble sensors into contact with target materials and detect ferromagnetic monolayer magnetizations paves the way for quantitative analysis of a wide range of 2D magnets assembled on arbitrary target substrates.

Pressure-driven evolution of upper critical field and Fermi surface reconstruction in the strong-coupling superconductor Ti4Ir2O

We report pressure-driven evolution of the superconducting transition temperature (${T}_{\mathrm{c}}$) and upper critical field ${B}_{\mathrm{c}2}$ $(0)$ of strong-coupling superconductor ${\mathrm{Ti}}_{4}{\mathrm{Ir}}_{2}\mathrm{O}$ that possesses an unusually large ${B}_{\mathrm{c}2} (0)$ at ambient pressure (AP). Our results reveal an extremely low-pressure coefficients of ${T}_{\mathrm{c}}$, i.e., $d{T}_{c}/dP\ensuremath{\approx}\ensuremath{-}0.047$ K/GPa for $P<15$ GPa and \ensuremath{-}0.017 K/GPa for $15<P<50$ GPa, presumably associated with an inherent large bulk modulus of $\ensuremath{\sim}252$ GPa. Interestingly, we find that its ${B}_{\mathrm{c}2}$ $(0)$ undergoes a smooth crossover at 35.6 GPa from well beyond to less than the Pauli paramagnetic limit ${B}_{\mathrm{p}}^{\mathrm{BCS}}(0)=1.84\phantom{\rule{0.16em}{0ex}}{T}_{c}$; i.e., the ${B}_{\mathrm{c}2}(0)\ensuremath{\approx}18.2 \mathrm{T}=1.7\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at AP decreases to $\ensuremath{\sim}5.8 \mathrm{T}=0.88\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at 50 GPa. The density-functional calculations predict the possible occurrence of pressure-induced Fermi surface reconstruction for the energy bands near the $K$ point with strong spin-orbit coupling in 31--41 GPa. The present work sheds more light on this intriguing superconductor capable of resisting large external compression and strong magnetic fields.

Magnetic field effects in an octupolar quantum spin liquid candidate

Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to an external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here we use neutron scattering to show that the application of a large external magnetic field to ${\mathrm{Ce}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$, an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wave vectors in ${\mathrm{Ce}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ pyrochlore, and its absence in ${\mathrm{Ce}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$. Therefore, our results identify ${\mathrm{Ce}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ as a strong candidate for an octupolar $U(1)$ QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.

INFLUENCE OF TEMPERATURE ON THE BEARING CAPACITY OF PASSIVE MAGNETIC BEARINGS

When magnetic-passive bearings are heated, the magnetic properties of permanent magnets change and, therefore, the force of interaction between them changes. Depending on the temperature and exposure time, irreversible changes in the strength of the interaction of magnets occur, caused by changes in the magnetic structure, phase composition, as well as reversible changes due to the temperature dependence of the magnetization and coercive force of the magnet material. A distinctive feature of the operation of magnets in bearings is the effect of large external magnetic fields on them. For this reason, it is impossible to predict in advance the properties of bearings at elevated temperatures using only the data given in the literature for magnetically rigid materials or individual magnets. The influence of temperature on the interaction of two coaxially arranged magnets modeling a part of a bearing is theoretically and experimentally considered. Numerical values of temperature coefficients of changes in bearing capacity and stiffness of bearings are determined, as well as the theoretical conclusion about the constancy of these coefficients for bearings with optimal magnet sizes is confirmed.

Acoustically Induced Giant Synthetic Hall Voltages in Graphene.

Any departure from graphene's flatness leads to the emergence of artificial gauge fields that act on the motion of the Dirac fermions through an associated pseudomagnetic field. Here, we demonstrate the tunability of strong gauge fields in nonlocal experiments using a large planar graphene sheet that conforms to the deformation of a piezoelectric layer by a surface acoustic wave. The acoustic wave induces a longitudinal and a giant synthetic Hall voltage in the absence of external magnetic fields. The superposition of a synthetic Hall potential and a conventional Hall voltage can annihilate the sample's transverse potential at large external magnetic fields. Surface acoustic waves thus provide a promising and facile avenue for the exploitation of gauge fields in large planar graphene systems.

Enhanced second harmonic Hall resistance in in-plane synthetic antiferromagnets

Synthetic antiferromagnet (SyAF) has been demonstrated to be an ideal candidate for spin–orbit torque (SOT) based spintronic devices. However, the detailed mechanism needs to be clarified due to the coexistence of multiple effects. This paper studies SOT and the thermoelectric effect in SyAF of Pt/Co/Ru/Co/Pt by harmonic Hall resistance measurements. Different from the traditional Co/Pt bilayers, the second harmonic Hall resistance signals of the SyAF-based devices are obviously enhanced under a large external magnetic field (Bext), which is caused by the antiferromagnetic exchange coupling fields weakening the influence of Bext. By fitting the Hall resistance curves, the field-like torque is demonstrated to be the main contribution to the Hall resistance. Interestingly, both the SOT effective fields are greatly enhanced for antiparallel alignment. This study separates the contributions of SOT and the thermoelectric effect in the SyAF structures and enables the design of the spintronic devices with stability under a large magnetic field.

Strong Zeeman splitting in orbital-hybridized valleytronic interfaces

Stacked heterointerfaces of two-dimensional (2D) materials are well-suited to obtain novel electronic functionality at small length scales, because the intrinsic properties of both materials can be blended by interlayer coupling with remarkable effects on the electronic band structure. In particular, band degeneracy at the K and \(K'\) Brillouin zone valleys of 2D materials can be broken by symmetry reductions by applying an external magnetic field, with possible application in valleytronic devices. Nevertheless, this usually requires impractically large external magnetic fields > 100 T. An alternative and more practical route is provided via stacking a non-centrosymmetric layered material on a magnetic substrate to induce valley splitting through the magnetic proximity effect; however, the splitting magnitude can vary sharply between similar interfaces. In this work, first-principle calculations are used to calculate the valley splitting of eight low-strain interfaces with transition metal dichalcogenides stacked on 2D magnetic substrates. It is shown that the interlayer band hybridization plays a major role when the two layers’ bands are closely aligned in energy, with the resulting band repulsion and the total valley splitting being strongly dependent on the Hubbard U correction , showing up to an order of magnitude change. The WSe\(_2\)/CrGeTe\(_3\) interface in particular shows strong interlayer interactions, which can potentially lead to a significantly high valley splitting of 26 meV; however further experimental investigations will be needed to resolve uncertainties in the predicted valley splitting.

Stark broadening of low-[formula omitted] hydrogen lines in strongly magnetized hydrogen plasmas: Influence of [formula omitted]-degeneracy removal due to the quadratic Zeeman effect

The Stark broadening of hydrogen lines with a low upper principal quantum number n is reexamined in the case of plasmas subject to a large (≳1 kT) external magnetic field. This investigation has been motivated by the observation of strong Zeeman splitting in a significant number of white dwarf spectra. The diamagnetic Zeeman effect term present in the Hamiltonian removes the degeneracy of the energy levels in orbital quantum number l, which results in an alteration of the Stark perturbation due to the microfield. In this work, we investigate this effect by applying a computer simulation method designed for Stark broadening modeling. A focus is put on the Lyman α and H-α lines. Based on line shape calculations, we show that the width of the Zeeman components is strongly dependent on the magnetic field.

Reducing the efficiency roll-off in organic light-emitting diodes at high currents under external magnetic fields

Singlet-polaron and triplet-polaron annihilation mechanisms are the most detrimental exciton quenching processes that lower the efficiency of organic light-emitting diodes (OLEDs) at high current densities, causing so-called efficiency roll-off in these devices. These exciton loss mechanisms are also the critical obstacles towards the realization of electrically pumped organic semiconductor lasers, which require very high current densities to reach threshold. Herein, under a relatively large external magnetic field, we demonstrate that the efficiency roll-off at high current densities in europium (Eu 3 + )-based solution-processed OLEDs can be suppressed to some extent while the luminance is enhanced. We achieve this by reducing the Förster-type exciton–polaron annihilation processes. Under the applied magnetic field, we show that manipulation of the polaron-spin and exciton dynamics lead to a quantitative roll-off suppression. • Modified exciton dynamics under an external magnetic field in lanthanide-based OLEDs. • Efficiency roll-off suppression due to reduced exciton–polaron quenching. • Brightness enhancement under an external magnetic field.

Phonon Hall effect with first-principles calculations

Phonon Hall effect (PHE) has attracted a lot of attention in recent years with many theoretical and experimental explorations published. While experiments work on complicated materials, theoretical studies are still hovering around the phenomenon-based models. Moreover, previous microscopic theory was found unable to explain large thermal Hall conductivity obtained by experiments in strontium titanate (STO). Therefore, as a first attempt to bridge this gap, we implement first-principles calculations to explore the PHE in real materials. Our work provides a new benchmark of the PHE in sodium chloride (NaCl) under a large external magnetic field. Moreover, we demonstrate our results in barium titanate (BTO), and discuss the results in STO in detail about their deviation from experiments. As a possible future direction, we further propose that the inner electronic Berry curvature plays an important role in the PHE in STO.

Reducing Dzyaloshinskii-Moriya interaction and field-free spin-orbit torque switching in synthetic antiferromagnets

Perpendicularly magnetized synthetic antiferromagnets (SAF), possessing low net magnetization and high thermal stability as well as easy reading and writing characteristics, have been intensively explored to replace the ferromagnetic free layers of magnetic tunnel junctions as the kernel of spintronic devices. So far, utilizing spin-orbit torque (SOT) to realize deterministic switching of perpendicular SAF have been reported while a large external magnetic field is typically needed to break the symmetry, making it impractical for applications. Here, combining theoretic analysis and experimental results, we report that the effective modulation of Dzyaloshinskii-Moriya interaction by the interfacial crystallinity between ferromagnets and adjacent heavy metals plays an important role in domain wall configurations. By adjusting the domain wall configuration between Bloch type and Néel type, we successfully demonstrate the field-free SOT-induced magnetization switching in [Co/Pd]/Ru/[Co/Pd] SAF devices constructed with a simple wedged structure. Our work provides a practical route for utilization of perpendicularly SAF in SOT devices and paves the way for magnetic memory devices with high density, low stray field, and low power consumption.

On the interplay between magnetic field and anisotropy in holographic QCD

We investigate the combined effects of anisotropy and a magnetic field in strongly interacting gauge theories by the gauge/gravity correspondence. Our main motivation is the quark-gluon plasma produced in off-central heavy-ion collisions which exhibits large anisotropy in pressure gradients as well as large external magnetic fields. We explore two different configurations, with the anisotropy either parallel or perpendicular to the magnetic field, focusing on the competition and interplay between the two. A detailed study of the RG flow in the ground state reveals a rich structure where depending on which of the two, anisotropy or magnetic field, is stronger, intermediate geometries with approximate AdS4 × ℝ and AdS3 × ℝ2 factors arise. This competition is also manifest in the phase structure at finite temperature, specifically in the dependence of the chiral transition temperature on anisotropy and magnetic field, from which we infer the presence of inverse magnetic and anisotropic catalyses of the chiral condensate. Finally, we consider other salient observables in the theory, including the quark-antiquark potential, shear viscosity, entanglement entropy and the butterfly velocity. We demonstrate that they serve as good probes of the theory, in particular, distinguishing between the effects of the magnetic field and anisotropy in the ground and plasma states. We also find that the butterfly velocity, which codifies how fast information propagates in the plasma, exhibits a rich structure as a function of temperature, anisotropy and magnetic field, exceeding the conformal value in certain regimes.

Route to tunable room temperature electric polarization in SrTiO3-CoFe2O4 heterostructures.

Utilizing the magnetostrictive properties of CoFe2O4, we demonstrate reversible room temperature control of the Ti electronic structure in SrTiO3–CoFe2O4 heterostructures, by inducing local and reversible strain in the SrTiO3. By means of X-ray absorption spectroscopy, we have ascertained the changes that take place in the energy levels of the Ti 3d orbitals under the influence of an external magnetic field. The observed Ti electronic state when the sample is subjected to moderately large external magnetic fields and the disappearance of the induced phase upon their removal indicates lattice distortions that are suggestive of the development of a net electric polarization.

Effect of nuclear quadrupole interaction on spin beats in photoluminescence polarization dynamics of charged excitons in InP/(In,Ga)P quantum dots

The spin dynamics of positively (X$^{+}$) and negatively (X$^{-}$) charged excitons in InP/In$_{0.48}$Ga$_{0.52}$P quantum dots subject to a magnetic field is studied. We find that a characteristic feature of the system under study is the presence of nuclear quadrupole interaction, which leads to stabilization of the nuclear and electron spins in a quantum dot in zero external magnetic field. In detail, the nuclear quadrupole interaction leads to pinning of the Overhauser field along the quadrupole axis, which is close to the growth axis of the heterostructure. The nuclear effects are observed only when resident electrons are confined in the quantum dots, i.e. for X$^{-}$ trion photoexcitation. The presence of X$^{-}$ and X$^{+}$ trion contributions to the photoluminescence together with the quadrupole interaction significantly affects the dynamics of optical orientation in Voigt magnetic field. In absence of dynamic nuclear spin polarization the time evolution of the photoluminescence polarization was fitted by a form which describes the electron spin relaxation in "frozen" nuclear field fluctuations. In relatively large external magnetic fields exceeding 60 mT good agreement between theory and experiment is achieved.

Magnetars and Magnetic Separation of Chiral Radicals in Interstellar Space: Homochirality.

Pasteur was the first to realize Earth's homochirality. Consequently, he attempted to design experiments revealing a mechanism that would expose life's chiral preference. Some of these experiments involved the application of magnetic fields to chemical reactions. His experiments failed, in part, because B-fields are pseudo-vectors and cannot couple preferentially to one handedness. However, extremely large magnetic fields cause the Maxwell equations to break down. This allows the motions of spin and charge densities in paramagnetic anion radicals to produce polarized axial B-fields that can undergo preferential coupling to one handedness. Hence, when a racemic mixture of paramagnetic organic molecules passes by an extremely large external gradated magnetic field, the enantiomers experience different torque forces and acquire different translational directions. B-fields of the required magnitude are unknown on this planet. In fact, they would be lethal, thereby eliminating any chance of Pasteur's success. On the other hand, Duncan and co-workers have recently discovered and garnered physical understanding of magnetars in interstellar space. Some of these neutron star systems produce B-fields greater than the quantum electrodynamic field strength, which is more than enough to generate the required torque for the interstellar enantiomeric separation. In space, chiralitically enriched materials can be deposited on planetesimals and result in homochiral "islands" on the planets. The formation of magnetars is a consequence of weak force events. We assert that, in interstellar space, a plethora of enantiomerically enriched dust clouds resulted from inter-magnetar-paramagnetic molecule force fields.

Magnetic field distribution in a magnetic liquid spike

Abstract The Rosensweig, or normal field instability, is a well known phenomenon where formation of a spike like shape appears on a magnetic liquid surface when it is exposed to a sufficiently large external magnetic field. While the surface shape of magnetic liquid has an impact on the magnetic field distribution and vice versa, it is somehow essential to know the magnetic field distribution within the magnetic liquid. The aim of the magnetic field calculation is to show that the magnetic field density is distributed similarly as the shape of the free surface. The calculation is placed in the 3D space and performed at the equilibrium state of magnetic liquid using the Finite Element Method based software Opera Field. Only a single spike of the magnetic liquid is included in the computational model, while the pattern of the surface deformation is assumed to be hexagonal and distributed periodically over the surface.

The gyrokinetic limit for the Vlasov–Poisson system with a point charge

We consider the asymptotics of large external magnetic field for a 2D Vlasov–Poisson system governing the evolution of a bounded density interacting with a point charge. We show that the solution converges to a measure-valued solution of the Euler equation with a defect measure.

Electrochemical Performance of Iron Oxide Nanoflakes on Carbon Cloth under an External Magnetic Field

In this work, the iron oxide (Fe2O3) nanoflakes on carbon cloth (Fe2O3@CC) were triumphantly prepared and served as the electrode of supercapacitors. By applying an external magnetic field, we first find that the magnetic field could suppress the polarization phenomenon of electrochemical performance. Then, the influences of the mono-/bi-valent cations on the electrochemical properties of the Fe2O3@CC were investigated under a large external magnetic field (1 T) in this work. The chemical valences of the cations in the aqueous electrolytes (LiNO3 and Ca(NO3)2) have almost no influences on the specific capacitance at different scan rates. As one of important parameters to describe the electrochemical properties, the working potential window of the Fe2O3@CC electrode was also investigated in this work. The broad potential window in room-temperature molten salt (LiTFSI + LiBETI (LiN(SO2CF3)2 + LiN(SO2C2F5)2)) has been obtained and reached 1.2 V, which is higher than that of the traditional aqueous electrolyte (~0.9 V).