Low External Field Research Articles

Joining of single-core ex-situ MgB2/Fe wires by termination architecture

The development of reliable jointing technique is of great importance for the future of applied superconductivity, especially for superconducting magnets working in persistent mode. Superconducting joints with termination architecture between single-core not reacted ex-situ MgB2/Fe wires were manufactured and tested. I-V characteristics of joints were measured at 4.2 K and 20 K for variable external fields and compared with I-V of joined wire. The ratio of joints critical current to that of parent conductors (Ic-joint/Ic-wire) has been evaluated and discussed. It was found that Ic-joint with termination architecture may be even larger than Ic-wire at high external field, but decreasing for lower fields. Ic-joint/Ic-wire = 0.8 was measured for the best joint at 20 K and low external field 1.5 T, which is promising for functional persistent switch.

Phase-field simulation and machine learning of low-field magneto-elastocaloric effect in a multiferroic composite

Achieving appreciable elastocaloric effect under low external field is critical for solid-state cooling technology. Here, a non-isothermal Phase-Field Model (PFM) coupling martensitic transformation with mechanics, heat transfer and magnetostrictive behavior is proposed to simulate Magneto-elastoCaloric Effect (M-eCE) that is induced by magnetic field in a multiferroic composite (e.g., Magnetostrictive-Shape Memory Alloys (MEA-SMA) composite). In the PFM, a nonlinear constitutive hyperbolic tangent model is utilized to model the macroscopic magnetostrictive behavior of MEA, and the heat transfer coupled with phase transformation is employed to calculate the adiabatic temperature change (ΔTad) during M-eC cooling cycles. The influences of magnetic field, geometrical dimension, and ambient temperature on ΔTad are comprehensively investigated. Machine Learning (ML) is further conducted on the database from PFM simulations to accelerate the prediction and design of MEA-SMA composite with an improved ΔTad. It is found that a large ΔTad of 10–14 K and a wide working temperature window of 30 K can be achieved under ultra-low magnetic field of 0.15–0.38 T by optimizing the composite’s geometrical dimension. The present work combining PFM and ML for evaluating M-eCE provides a theoretical framework for the optimization of M-eC cooling devices, and is also potentially extended to other multicaloric effects (e.g., electro-elastocaloric effect).

Reflective, polarizing, and magnetically soft amorphous neutron optics with 11B-enriched B4C.

The utilization of polarized neutrons is of great importance in scientific disciplines spanning materials science, physics, biology, and chemistry. However, state-of-the-art multilayer polarizing neutron optics have limitations, particularly low specular reflectivity and polarization at higher scattering vectors/angles, and the requirement of high external magnetic fields to saturate the polarizer magnetization. Here, we show that, by incorporating 11B4C into Fe/Si multilayers, amorphization and smooth interfaces can be achieved, yielding higher neutron reflectivity, less diffuse scattering, and higher polarization. Magnetic coercivity is eliminated, and magnetic saturation can be reached at low external fields (>2 militesla). This approach offers prospects for substantial improvement in polarizing neutron optics with nonintrusive positioning of the polarizer, enhanced flux, increased data accuracy, and further polarizing/analyzing methods at neutron scattering facilities.

Stereo‐Active Lone Pairs Induced Giant Polarization in a 2D Ge‐Based Halide Perovskite Antiferroelectric

AbstractAntiferroelectrics, characterized by electrically controlled antipolar‐polar phase transformation, have attracted tremendous attention as a class of promising electroactive materials for assembling electronic devices. The emerging two‐dimensional (2D) halide perovskites with superior compositional diversity offer an ideal platform for exploring electroactive materials, whereas lead‐free antiferroelectric counterparts are still scarcely reported. Herein, for the first time, a new lead‐free 2D germanium iodide perovskite antiferroelectric (i‐BA)2CsGe2I7 (1, i‐BA is iso‐butylammonium) has been presented, which exhibits a high Curie temperature (Tc) up to 403 K. Remarkably, benefiting from the lone pair stereochemical activity in Ge2+ induced large structural distortion and Cs+ ion off‐center displacement, 1 shows well‐defined double P–E hysteresis loops in a wide temperature range with a giant maximum polarization up to 18.8 µC cm−2, which achieves a new high record among molecular antiferroelectrics. Moreover, under a low external electric field of 22.5 kV cm−1, the antipolar‐polar phase transformation in 1 affords a recoverable energy storage density Wrec of 0.27 J cm−3 and high storage efficiency up to 79.76%. Such lead‐free halide perovskite antiferroelectric with intriguing antiferroelectric behaviors, including high Tc, large polarization and remarkable energy storage properties, is exciting, which provides an alternative candidate for high‐performance antiferroelectrics for environmentally friendly electronic devices.

Stability of moving solitons in trans -polyacetylene in an electric field

In this work we study the dynamics and stability of charged solitons in trans-polyacetylene (tPA), and revisit the issue of the stability of these non-linear excitations under the effect of an external electric field applied parallel to the polymer. Using the formalism of the Su-Schrieffer-Heeger (SSH) model, we solve the coupled dynamical equations for electrons and classical nuclei at the mean-field level and in the regime of low external electric field $E$, where the dynamics of the moving soliton is adiabatic. Analyzing observable quantities in real space and frequency space, we identify the microscopic mechanisms triggering the dynamical instabilities of the soliton. In addition, we put forward the definition of a proper quantitative measure of its stability, an issue which to the best of our knowledge has remained an open question. Besides its intrinsic interest from the fundamental point of view, our work might be relevant for the design of novel organic electronic devices based on soliton-mediated transport.

Solid-state NMR signals at zero-to-ultra-low-field

Zero-to-ultra-low-field nuclear magnetic resonance (ZULF NMR) is fast emerging as a viable spectroscopic approach to study samples under conditions dominated by internal spin interactions. In the absence of the truncating effects of Zeeman interaction, the NMR signal is determined by J-coupling, dipole-dipole, and/or quadrupolar interactions. But, the low spin-precession frequencies and equilibrium spin polarisation in low external fields necessitate the use of special techniques for detecting the signals. In this article, spin evolution in ultra-low-field regime for various systems is studied and the expected NMR signals are evaluated for solid samples. The methodologies that can be used to make low-field detection feasible especially in case of solid samples are described.

Conductance quantization control and neuromorphic properties in Pt-nanoparticle incorporated HfAlOx alloy memristor

In this work, platinum nanoparticles have been utilized to achieve better control of conductance quantization for high-density memory storage nonvolatile memory applications. Here, atomic layer deposited Pt-nanoparticles are sandwiched between HfAlOx switching layers. An Au/Ti/HfAlOx/Pt-NP/HfAlOx/ITO resistive random-access memory (RRAM) device exhibits bipolar resistive switching SET/RESET properties at a very low external electric field with memory window >10 and an endurance of >103 cycles. With a very slow voltage sweep rate (0.002 V/step) during current–voltage characteristics under both SET and RESET conditions, a controlled stepwise increase/decrease in distinct conductance quantization behavior with integer and half-integer multiples was observed. This phenomenon predicts atomic contact formation and rupture of oxygen vacancies between conductive filaments and Pt-NPs. Control of post-synaptic conductance properties with modulation of pre-spike width, number, and frequency showed the robustness of the RRAM device studied here. Gradual, controlled change in conductance obtained under dc and pulse conditions in the experiments is very promising for next-generation multi-level high-density storage RRAM devices to develop artificial electric synapses for neuromorphic applications.

Remote Modular Electronics for Wireless Magnetic Devices.

Small‐scale wireless magnetic robots and devices offer an effective solution to operations in hard‐to‐reach and high‐risk enclosed places, such as inside the human body, nuclear plants, and vehicle infrastructure. In order to obtain functionalities beyond the capability of magnetic forces and torques exerted on magnetic materials used in these robotic devices, electronics need to be also integrated into them. However, their capabilities and power sources are still very limited compared to their larger‐scale counterparts due to their much smaller sizes. Here, groups of milli/centimeter‐scale wireless magnetic modules are shown to enable on‐site electronic circuit construction and operation of highly demanding wireless electrical devices with no batteries, that is, with wireless power. Moreover, the mobility of the modular components brings remote modification and reconfiguration capabilities. When these small‐scale robotic modules are remotely assembled into specific geometries, they can achieve, if not impossible, challenging electrical tasks for individual modules. Using such a method, several wireless and battery‐free robotic devices are demonstrated using milli/centimeter‐scale robotic modules, such as a wireless circuit to power light‐emitting diodes with lower external fields, a device to actuate relatively high force‐output shape memory alloy actuators, and a wireless force sensor, all of which can be modified on‐site.

Giant low-field actuated caloric effects in a textured Ni43Mn47Sn10 alloy

In this work, based on composition adjusting and texture control, we demonstrate giant elastocaloric and magnetocaloric effects in a directionally solidified Ni43Mn47Sn10 polycrystalline with strong <001>A texture. On removing a low compressive stress of 210 MPa, giant adiabatic temperature change up to −17.3 K is achieved. Such value is much higher than those in some elastocaloric materials reported previously. Besides, giant magnetic field induced entropy changes of 23.5 Jkg−1K−1 and 37.1 Jkg−1K−1 are also obtained under the magnetic field changes of 2 T and 5 T, respectively. Simultaneously achieving giant elastocaloric and magnetocaloric effects actuated by relatively low external field is of great importance to explore multi-caloric or coupled caloric effects in developing efficient and eco-friendly solid-state refrigeration technology.

Local and global geometry of the 2D Ising interface in critical prewetting

Consider the Ising model at low temperatures and positive external field λ on an N×N box with Dobrushin boundary conditions that are plus on the north, east and west boundaries and minus on the south boundary. If λ=0, the interface separating the plus and minus phases is diffusive, having O( N) height fluctuations, and the model is fully wetted. Under an order one field, the interface fluctuations are O(1), and the interface is only partially wetted, being pinned to its southern boundary. We study the critical prewetting regime of λN↓0, where the height fluctuations are expected to scale as λ−1/3 and the rescaled interface is predicted to converge to the Ferrari–Spohn diffusion. Velenik (Probab. Theory Related Fields 129 (2004) 83–112) identified the order of the area under the interface up to logarithmic corrections. Since then, more refined features of such interfaces have only been identified in simpler models of random walks under area tilts. In this paper we resolve several conjectures of Velenik regarding the refined features of the Ising interface in the critical prewetting regime. Our main result is a sharp bound on the one-point height fluctuation, proving e−Θ(x3/2) upper tails reminiscent of the Tracy–Widom distribution, capturing a tradeoff between the locally Brownian oscillations and the global field effect. We further prove a concentration estimate for the number of points above which the interface attains a large height. These are used to deduce various geometric properties of the interface, including the order and tails of the area it confines and the polylogarithmic prefactor governing its maximum height fluctuation. Our arguments combine classical inputs from the random-line representation of the Ising interface with novel local resampling and coupling schemes.

Tin‐rich stannides CePd2Sn3, REPd2Sn4, Pr2Pd4Sn7 and RE3Pd6Sn10 – Crystal chemistry and 119Sn Mössbauer spectroscopy

AbstractThe stannides CePd2Sn3, REPd2Sn4 (RE=Ce, Pr, Nd, Sm), Pr2Pd4Sn7 and RE3Pd6Sn10 (RE=Ce, Pr) were synthesized from the elements by arc‐melting and subsequent annealing in muffle or induction furnaces. The praseodymium, neodymium and samarium compounds are reported for the first time. The structures of PrPd2Sn4 and Pr2Pd4Sn7 were refined from single crystal X‐ray diffractometer data. The common structural building units of these stannides are condensed Pd@Sn5 square pyramids and Pd@Sn6 octahedra. The RE2Pd4Sn7 and RE3Pd6Sn10 structures are intergrowth variants of CePd2Sn3 and CePd2Sn4 related slabs. PrPd2Sn4, NdPd2Sn4 and Pr3Pd6Sn10 show Curie‐Weiss behavior with experimental magnetic moments close to the free‐ion values. Antiferromagetric ordering is detected at Néel temperatures of 8.2(1) K (PrPd2Sn4), 10.3(1) K (NdPd2Sn4) and 6.7(1) K (Pr3Pd6Sn10). The three stannides show metamagnetic transitions at low external field strengths. The stannides CePd2Sn3, REPd2Sn4 (RE=Ce, Pr, Nd) and RE3Pd6Sn10 (RE=Ce, Pr) were further characterized via their 119Sn Mössbauer spectra which reflect the different tin substructures: (i) cis‐Sn4 units, (ii) Sn2 pairs and (iii) square grids.

Electric-field induced fluctuations in laser generated plasma plume

The effect of an external electric field on laser-generated plasma has been studied. It is observed that the laser-generated plasma can be used for the ignition of a spark in the presence of a low voltage external electric field. An eight-fold emission intensity enhancement in Cu I spectral lines are measured as compared to the signal intensity in the absence of an external electric field. The plasma parameters remain the same initially, up to a few microseconds after the generation of plasma, and this feature makes it more interesting for the quantitative analysis of any sample using laser induced breakdown spectroscopy (LIBS). In the presence of an external electric field, fluctuations (contraction and expansion) in the laser-generated plasma are observed which increase the plasma decay time and consequently result in enhanced signal intensity.

Frequency- and magnetic-field-dependent properties of ordered magnetic nanoparticle arrangements

We present investigations of the frequency and magnetic field dependent properties of ordered magnetic nanoparticles (MNPs) arrangements consisting of magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) nanoparticles with an average diameter of 20 nm by employing micro Brillouin light scattering microscopy. We utilized electron beam lithography to prepare hexagonally arranged, circularly shaped MNP assemblies consisting of a single layer of MNPs using a variant of the Langmuir-Blodgett technique. By comparing the results with nonstructured, layered superlattices of MNPs, further insight into the influence of size and geometry of the arrangement on the collective magnetic properties is obtained. We show that at low static external field strengths, two signals occur in frequency dependent measurements for both nonstructured and structured assemblies. Increasing the static external field strength results in a sharpening of the main signal, while the satellite signal decreases in its intensity and increases in its linewidth. The occurrence of multiple signals at low external field strengths is also confirmed by sweeping the static external field and keeping the excitation frequency constant. Furthermore, two-dimensional spatial mapping of the resonances reveals that the main and the satellite signal originate from the center and the edge, respectively, of a single circular MNP assembly. Micromagnetic simulations confirm these assignments and the dependence of the two signals on the static external field strength, justifying an interpretation of the observed characteristics in terms of different local environments of MNPs forming the MNP assembly.

Controlling reaction rate of phase transfer hydrogenation of acetophenone by application of low external electric field

AbstractThe controllable mass transfer and reaction rate for phase transfer hydrogenation of acetophenone across a well‐defined boundary were investigated. The effect of solvent was found important and 1‐butanol exhibited the best performance among the five investigated homologous alcohol solvents, consistent with its higher solubility in water and greater dielectric constant. Initial reaction rates increased with increasing electric potential, consistent with enhanced mass transfer across the aqueous/organic boundary. At longer reaction times, deactivation was apparent. It correlated with increasing voltage and is ascribed to lower equilibrium concentration of reactive species at the interface. External control over reaction rate was demonstrated by switching the applied electric potential over the course of the reaction. Effects of external electric field on enantioselectivity were also explored with reversal field direction. The changes correlate with catalyst decomposition.

Field-induced Néel vector bi-reorientation of a ferrimagnetic insulator in the vicinity of compensation temperature* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303202 and 2016YFA0300803), the National Natural Science Foundation of China (Grant Nos. 11904194, 11727808, and 11674159), and the Fundamental Research Funds for the Central Universities, China (Grant No. 020414380121).

The spin Hall magnetoresistance (SMR) effect in Pt/Gd3Fe5O12 (GdIG) bilayers was systematically investigated. The sign of SMR changes twice with increasing magnetic field in the vicinity of the magnetization compensation point (T M) of GdIG. However, conventional SMR theory predicts the invariant SMR sign in the heterostructure composed of a heavy metal film in contact with a ferromagnetic or antiferromagnetic film. We conclude that this is because of the significant enhancement of the magnetic moment of the Gd sub-lattice and the unchanged moment of the Fe sub-lattice with a relatively large field, meaning that a small net magnetic moment is induced at T M. As a result, the Néel vector aligns with the field after the spin-flop transition, meaning that a bi-reorientation of the Néel vector is produced. Theoretical calculations based on the Néel’s theory and SMR theory also support our conclusions. Our findings indicate that the Néel-vector direction of a ferrimagnet can be tuned across a wide range by a relatively low external field around T M.

Laboratory study of non-ideal effects in magnetically collimated astrophysical outflows

Central outflow’s collimation by magnetic field is an important theoretical mechanism for explaining the astrophysical objects’ morphology formation, and its credibility has been tested in many laser plasma experiments in a dimensionless manner. This article introduces integrated simulation and experiment work based on the present laboratory magnetically collimated jet framework, to explore how non-ideal terms’ strength including radiative cooling and magnetic diffusion from different targets can affect the outflow shape. The interaction between outflow from a target with low atomic number and external field satisfies the ideal magneto-hydrodynamic conditions, and the outflow shape results in diamagnetic cavity and jet; on the other hand, a heavy element target brings strong magnetic diffusion that destroys the collimation structure, together with the stagnation of outflow introduced by radiative cooling, and outflow shape results in weakly collimated hemisphere near the target and a detached magnetized density clump. The detailed dimensionless analysis shows that the large-scale dissipation of jets in young stellar objects can possibly be an analog of the laboratory jet’s magnetic diffusion breakup, also similar structures like the loosely collimated lobes and bright ansaes in planetary nebula can be observed in highly diffusive laboratory outflows. This article shows for the first time that a series of non-relativistic astronomical outflows’ dynamic behaviors can be explained by the non-ideal magneto-hydrodynamic evolution of laboratory plasmas.

Electrical and magnetic properties of amorphous Fe22.5Co22.5Ni22.5Cr22.5Zr10 alloy

This report is focused on the investigation of electrical and magnetic properties of distorted bcc system. The inspiration for the selection of amorphous Fe22.5Co22.5Ni22.5Cr22.5Zr10 alloy with a distorted bcc structure is based on the theoretical band calculation. It is confirmed that the free electron density of state of disordered bcc FeNiCr alloy increases at Fermi surface.Amorphous Fe22.5Co22.5Ni22.5Cr22.5Zr10 alloy, hereafter denoted as a-FeCoNiCrZr, shows among all the metallic materials the weakest temperature dependence of the electrical resistivity, ρ0(T). The weak ρ0(T) has been interpreted in the context of different physical processes such as tunnelling of free electrons and electron phonon interactions.Magnetically, a-FeCoNiCrZr consists of independent soft ferromagnetic clusters with average cluster size of about 1840 atoms. The Curie temperature, TC, was verified using Landau’s theory of second-order phase transitions in low non-zero external fields. The reduced magnetization, M(T)/M(0), as a function of reduced temperature, T/TC, is explained in the framework of the exchange integral fluctuation theory. The exchange integral fluctuation is dominant only at low external fields. At Bex = 9 T, the exchange integral fluctuation has no noticeable effect on reduced magnetization. At Bex = 9 T, the precision of magnetic clusters around the Bex controls the behaviour of reduced magnetization as function of temperature.

Phase field simulation of grain size effects on the phase coexistence and magnetostrictive behavior near the ferromagnetic morphotropic phase boundary

The grain size effects on the phase coexistence and magnetostrictive response of Tb1−xDyxFe2 polycrystals near the ferromagnetic morphotropic phase boundary (MPB) are revealed through phase-field modeling. It shows that phase coexistence is a universal phenomenon in polycrystals for both ferromagnetic and ferroelectric MPB and that the range of compositions for phase coexistence increases with decreasing grain sizes. A large, reversible, and anhysteretic magnetostrictive response at low external fields is also found in the fine-grained polycrystals around the ferromagnetic MPB, which offers us a route to developing nanocrystalline magnetostrictive materials.

Tunable filter and modulator with controlled bandwidth and wide dynamic range based on planar thin films structure.

Tunable narrowband spectral filters with high throughput and wide dynamic range are in high demand for many applications, however, a cost is usually associated with the filter narrowing either in the dynamic range, in the throughput or the manufacturability. Here it is shown for the first time that using the coupling between waveguide and lossy modes (LMs) in lossy ultrathin films through thin film coupling layer it is possible to obtain a reflection peak with controllable width (sub-Angstroms till tens of nm) and tunability over wide spectral range (>500nm in the visible and near infrared). The excitation of broadband LM is enabled using an ultrathin absorptive layer with high imaginary to real part ratio of the dielectric constant (i.e 6nm of Cr). The wider dynamic range and higher contrast are observed more with TE polarization than TM. The tuning is achieved by incidence angle scan of few degrees or by modulating the waveguide layer from the visible till the near infrared and in principle it can be designed to operate in any spectral range. Such a thin waveguide layer can allow tuning at ultrahigh speed using conventional electrooptic, magnetooptic, piezoelectric or thermooptic materials using relatively low external fields. The tuning sensitivity and range depend strongly on the waveguide layer thickness and the refractive index mismatch between the waveguide and the coupling layer. Under small index mismatch new peaks are seen via Rabi type splitting with gaps as high as 700nm or more, thus exhibiting ultrahigh tuning with negative sensitivity.

Electrostatically enhanced catalytic phase transfer hydrogenation of acetophenone under low external electric field

The oriented external electric field (OEEF) promoted phase transfer catalysis (PTC) reaction is reported for the first time. It was demonstrated experimentally that an OEEF can act as promoter or inhibitor for the phase transfer hydrogenation of acetophenone depending on its direction. Organic solvents showed differing influences on the reaction, attributed to different electrical conductivities. A non-linear dependence of product yield on OEEF potential was also discovered. This work presents a methodology for promoting PTC reactions in the absence of a chemical phase transfer agent and shows promise for significantly improving the performance of organic reactions in biphasic liquid systems.