Dc Field Research Articles

Boosting the proton conduction in a magnetic dysprosium-organic framework by introducing conjugate NH4+-NH3 pairs

Metal-organic frameworks (MOFs) with inherent porosity and suspended acidic groups are promising proton conducting materials in water or aqua-ammonia media. Herein we report a new lanthanide phosphonate, namely, Dy2(amp2H2)2(mal)(H2O)2·5H2O (MDAF-6). It possesses a 3D open-framework structure, and shows a high NH3 adsorption capacity of 142.4 cm3/g at P/P0 = 0.98 at 298 K due to acid-base interaction. Interestingly, the proton conductivity of MDAF-6-NH3 is enhanced by five orders of magnitude compared to MDAF-6 after 8.5 h exposure in saturated NH3-H2O vapor, indicating the importance of coexistent conjugate acid-base pairs of H3O+-H2O and NH4+-NH3 in promoting proton conduction. Magnetic studies of MDAF-6 revealed slow magnetization relaxation under zero dc field, characteristic of single-molecule magnet behavior. This work provides not only a new multifunctional MOF material, but also a new strategy to improve proton conduction in aqua-ammonia medium.

Recent Advances in Electro-Optic Response of Polymer-Stabilized Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLC) are molecules that can self-assemble into helicoidal superstructures exhibiting circularly polarized reflection. The facile self-assembly and resulting optical properties makes CLCs a promising technology for an array of industrial applications, including reflective displays, tunable mirror-less lasers, optical storage, tunable color filters, and smart windows. The helicoidal structure of CLC can be stabilized via in situ photopolymerization of liquid crystal monomers in a CLC mixture, resulting in polymer-stabilized CLCs (PSCLCs). PSCLCs exhibit a dynamic optical response that can be induced by external stimuli, including electric fields, heat, and light. In this review, we discuss the electro-optic response and potential mechanism of PSCLCs reported over the past decade. Multiple electro-optic responses in PSCLCs with negative or positive dielectric anisotropy have been identified, including bandwidth broadening, red and blue tuning, and switching the reflection notch when an electric field is applied. The reconfigurable optical response of PSCLCs with positive dielectric anisotropy is also discussed. That is, red tuning (or broadening) by applying a DC field and switching by applying an AC field were both observed for the first time in a PSCLC sample. Finally, we discuss the potential mechanism for the dynamic response in PSCLCs.

Micromagnetics of Microwave-Assisted Switching in Co-Pt-Based Nanostructures: Switching Time Minimization

Microwave-assisted switching (MAS) is simulated for different CoPt and CoPt/Co3Pt nanosrtuctures as a function of applied DC field and microwave frequency. In all the cases, the existence of microwave excitation can lower the switching field by more than 50%. However, this coercivity reduction comes at a cost in the required switching time. The optimal frequencies follow the trends of the ferromagnetic resonances predicted by the Kittel relations. This implies that: (a) when the DC field is applied along the easy axis, the coercivity reduction is proportional to the microwave frequency, whereas (b) when the coercivity is lowered by applying the DC field at an angle of 45° to the easy axis, extra MAS reduction requires the use of high frequencies.

Ground states and magnonics in orthogonally coupled symmetric all-antiferromagnetic junctions

In this paper, the rich ground-state structure of orthogonally coupled symmetric all-antiferromagnetic junctions with easy-plane anisotropy is reported. A spin reorientation process rather than the traditional spin flop (SF) occurs, resulting in a phase in which N\'eel vectors preserve the mirror-reflection symmetry (termed MRS phase). The phase transitions in the ground state between SF and MRS phases can be either first or second order. After disturbed by external stimuli, magnons with different parities emerge. For in-plane dc fields, no couplings between magnons occur. When dc fields become oblique, coherent couplings between magnons with opposite parity emerge, leading to anticrossings in resonance frequencies. However, self-hybridization among magnons with the same parity never happens. More interestingly, spin waves based on MRS phases are linearly polarized and their polarization directions can be fine controlled.

Zero Sequence Voltage Modulation Strategy of Variable Flux Reluctance Machine Based on Single Leg Excitation Control

Conventionally, the variable flux reluctance machines (VFRMs) are controlled by armature current and field current, which can be generated through a separated H-bridge. However, high switching losses in the H-bridge are not avoidable. To reduce the switching losses and improve efficiency, a VFRM control system based on single-leg excitation control is proposed. The field winding of VFRM is connected to the neutral point of armature winding by using single-leg to utilize the zero-sequence current as the DC excitation. A novel zero sequence voltage modulation technique is developed to generate the equivalent DC field excitation, i.e., the DC bias in the current of the armature winding. Compared with the separated DC field excitation, lower copper losses and higher efficiency can be obtained. Finally, experimental results on a prototype VFRM are presented for verification.

Crystal structure, fluorescence, magnetic properties and DNA interaction of four novel binuclear LnⅢ2 compounds with Schiff ligand

Four new Ln2 compounds, [Ln2(H2L−)2(acac)2(CH3O)2] (Ln = Sm(1), Tb(2), Ho(3) and Er(4), H3L = 2‑hydroxy-N’-[(1E)-(8-hydroxyquinolin-2-yl)methylidene]benzohydrazide), were synthesized by using a multidentate Schiff ligand and lanthanide β-diketone salts via solvent evaporation methods. The X-ray structural analysis reveal that the four Ln2 compounds are dinuclear compounds and each Ln(Ⅲ) ion is eight-coordinated. The two central Ln(Ⅲ) ions are bridged by two μ2O atoms from 8-hydroxyquinoline Schiff base ligands, leading to a parallelogram Ln2O2 core. The magnetism investigation shows that compound 2 display slow magnetic relaxation behavior under zero dc field (ΔE/kB = 19.77 K and τ0 = 1.35 × 10−6 s). Subsequent studies strongly suggested that the four Ln2 compounds binded to Calf thymus DNA (CT-DNA) through intercalative mode and have pBR322 plasmid DNA cleavage ability. This work will provide more insights for the design and investigation of lanthanide-based SMMs, and has certain reference value to study the interaction between compound and DNA.

Complex Berry phase and imperfect non-Hermitian phase transitions

In many classical and quantum systems described by an effective non-Hermitian Hamiltonian, spectral phase transitions, from an entirely real-energy spectrum to a complex spectrum, can be observed as a non-Hermitian parameter in the system is increased above a critical value. A paradigmatic example is provided by systems possessing parity-time ($\mathcal{PT}$) symmetry, where the energy spectrum remains entirely real in the unbroken $\mathcal{PT}$ phase while a transition to complex energies is observed in the broken $\mathcal{PT}$ phase. Such spectral phase transitions are universally sharp. However, when the system is slowly and periodically cycled, the phase transition can become smooth, i.e., imperfect, owing to the complex Berry phase associated to the cyclic adiabatic evolution of the system. This remarkable phenomenon is illustrated by considering the spectral phase transition of the Wannier-Stark ladders in a $\mathcal{PT}$-symmetric class of two-band non-Hermitian lattices subjected to an external dc field, revealing that a nonvanishing imaginary part of the Zak phase---the Berry phase picked up by a Bloch eigenstate evolving across the entire Brillouin zone---is responsible for imperfect spectral phase transitions.

High-Resolution Magnetoelectric Sensor and Low-Frequency Measurement Using Frequency Up-Conversion Technique.

The magnetoelectric (ME) sensor is a new type of magnetic sensor with ultrahigh sensitivity that suitable for the measurement of low-frequency weak magnetic fields. In this study, a metglas/PZT-5B ME sensor with mechanical resonance frequency fres of 60.041 kHz was prepared. It is interesting to note that its magnetic field resolution reached 0.20 nT at fres and 0.34 nT under a DC field, respectively. In order to measure ultralow-frequency AC magnetic fields, a frequency up-conversion technique was employed. Using this technique, a limit of detection (LOD) under an AC magnetic field lower than 1 nT at 8 Hz was obtained, and the minimum LOD of 0.51 nT was achieved at 20 Hz. The high-resolution ME sensor at the sub-nT level is promising in the field of low-frequency weak magnetic field measurement technology.

The 0-3 Lead Zirconate-Titanate (PZT)/Polyvinyl-Butyral (PVB) Composite for Tactile Sensing.

In this study, a 0-3 piezoelectric composite based on lead zirconate-titanate (PZT)/polyvinyl-butyral (PVB) was fabricated and characterized for its potential application in tactile sensing. The 0-3 composite was developed to incorporate the advantages of both ceramic and polymer. The paste of 0-3 PZT-PVB composite was printed using a conventional screen-printing technique on alumina and mylar substrates. The thickness of the prepared composite was approximately 80 μm. After printing the top electrode of the silver paste, 10 kV/mm of DC field was applied at 25 °C, 120 °C, and 150 °C for 10 min to align the electric dipoles in the composite. The piezoelectric charge coefficient of d33 and the piezoelectric voltage coefficient of g33 were improved by increasing the temperature of the poling process. The maximum values of d33 and g33 were 14.3 pC/N and 44.2 mV·m/N, respectively, at 150 °C. The sensor's sensitivity to the impact force was measured by a ball drop test. The sensors showed a linear behavior in the output voltage with increasing impact force. The sensitivity of the sensor on the alumina and mylar substrates was 1.368 V/N and 0.815 V/N, respectively. The rising time of the sensor to the finger touch was 43 ms on the alumina substrate and 35 ms on the mylar substrate. Consequently, the high sensitivity and fast response time of the sensor make the 0-3 PZT-PVB composite a good candidate for tactile sensors.

Chemical bonding and crystal structure in flash sintered Y2O3 under DC or AC field

Undoped Y2O3 polycrystals were densified by flash sintering under DC or AC electric field with varying holding times up to 60 min, and their chemical bonding state and crystal structure were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. Only DC flash sintering with longer holding times (≥ 30 min) resulted in blackening in cathodic regions, indicating the electrode-dependent introduction of neutral oxygen vacancies. The Y 3d3/2 and Y 3d5/2 binding energies decreased with increasing holding time in both DC- and AC-flash-sintered Y2O3 specimens, regardless of polarity, indicating that positively charged oxygen vacancies were introduced by the flash sintering in an electrode-independent manner, even under DC electric field. In addition, the XRD analysis revealed that flash-sintered specimens with longer holding times exhibited short-range structural fluctuations. These electrode-dependent and independent point defect generations under electric fields may be related to the enhanced atomic diffusion during flash sintering.

Magnetic Shielding for Electric Car Power Cables

In this article, the effect of different magnetic shielding materials for electric car power cables has been studied using finite element analysis (FEA), verified by experimental results. Bearing in mind the complex spectrum modulation, typically using pulsewidth modulation (PWM), a magnetic field shielding up to 2 kHz is required. Therefore, magnetic shielding was studied for frequencies of 2 kHz, 50 Hz, and dc field. The employed magnetic shielding materials were high permeability layers, such as permalloy, as well as composites based on resin matrix including flakes or powders from nanocrystalline ribbons, such as nanocrystalline FINAMET ribbons. Magnetic shielding was studied for cable illustrating good shielding ability using a combination of Cu and permalloy sheets for frequencies up to 2 kHz. Furthermore, magnetic shielding was studied for the case of floor shielding, where permalloy appeared to be an acceptable shielding solution.

Experimental and Theoretical Investigation on the Dynamic Response of Ferrofluid Liquid Marbles to Steady and Pulsating Magnetic Fields.

Liquid marbles are droplets enwrapped by a layer of hydrophobic micro/nanoparticles. Due to the isolation of fluid from its environment, reduction in evaporation rate, low friction with the surfaces, and capability of manipulation even on hydrophilic surfaces, liquid marbles have attracted the attention of researchers in digital microfluidics. This study investigates the manipulation of ferrofluid liquid marbles (FLMs) under DC and pulse width-modulated (PWM) magnetic fields generated by an electromagnet for the first time. At first, the threshold of the magnetic field for manipulating these FLMs is studied. Afterward, the dynamic response of the FLMs to the DC magnetic field for different FLM volumes, coil currents, and initial distances of FLM from the coil is studied, and a theoretical model is proposed. By applying the PWM magnetic field, it is possible to gain more control over the manipulation of the FLMs on the surface and adjust their position more accurately. Results indicate that with a decrease in FLM volume, coil current, and duty cycle, the FLM step length decreases; hence, FLM manipulation is more precise. Under the PWM magnetic field, it is observed that FLM movement is not synchronous with the generated pulse, and even after the coil is turned off, FLMs keep their motion. In the end, with proper adjustment of the electromagnet pulse width, launching of FLMs at a distance farther than the coil is observed.

Method for measuring magnetic susceptibility of magnetic nanoparticles up to millimeter-wave frequency range

A novel method for measuring the complex magnetic susceptibility of magnetic nanoparticles (MNPs) has been developed by a broadband microstrip line-type probe with a frequency range of up to 50 GHz. Two external orthogonal DC fields, Hhard and Heasy, were applied to change the susceptibility of MNPs (Streptavidin coated, 200 nmϕ, number of MNPs about 1010) based on magnetization switching theory. The susceptibility of MNPs in the liquid, solid state, and after biotin-avidin reaction with polymer beads (2 μmϕ, number of polymer beads about 108) was evaluated. A clear FMR frequency of MNPs and the same FMR frequency shift trend as Hhard changed was observed around 2–10 GHz in both liquid and solid states. The susceptibility of MNPs reached maximum when Hhard was 0.1 T which is approximately equal to Heasy (0.135 T). The maximum values of real and imaginary parts increased by a factor of 3.39 and 1.66 compared to when Hhard was 0. The obtained results indicate that the magnetic anisotropy was successfully controlled in the liquid and solid states of MNPs aggregate. Furthermore, after the biotin-avidin reaction of MNPs with polymer beads, the imaginary part of susceptibility showed a decreasing trend regardless of the increase in Hhard, and its half-width expanded by 35.1% at maximum when Hhard was 0.15 T.

Dynamics of Discharge Development in a Homogeneous DC Field Initiated by a Plasma Filament Created by a Femtosecond Laser Pulse

Dynamics of Discharge Development in a Homogeneous DC Field Initiated by a Plasma Filament Created by a Femtosecond Laser Pulse

Realization and Tests of Prototype Fluxgate Magnetic Sensors for the ITER Neutral Beam Injectors.

In the ITER neutral beam injectors (NBI), the presence of an external variable magnetic field generated by the ITER tokamak itself, could deflect the ion beam during acceleration and cause a loss of beam focusing. For this reason, the ion source, the accelerator and the neutralizer will be shielded from external magnetic field by means of a passive magnetic shield and a system of active correction and compensation coils (ACCC). The ACCC will operate in a feedback control loop and thus require the measurement of magnetic field inside the NBI vessel. Magnetic sensors for this application must be capable of measuring DC and slow variable magnetic fields, and be vacuum-compatible, radiation-hard and robust, since they will be subjected to neutron flux produced by fusion reactions in the tokamak and inaccessible for maintenance. This paper describes the realization and tests of fluxgate magnetic sensors prototypes specifically designed for this purpose before the installation in MITICA and ITER.

3d Ion-Driven Hexanuclear Heterometallic Clusters with Amazing Structures and Magnetic Properties

By using a heptadentate Schiff base ligand with three NO2 pockets to react with 3d and 4f acetate precursors, three series of M4Ln2 complexes (M, Ln = Ni, Gd for 1NiGd; Ni, Dy for 2NiDy; Co, Gd for 3CoGd; Co, Dy for 4CoDy; Cu, Gd for 5CuGd; Cu, Dy for 6CuDy) have been obtained and characterized. X-ray single-crystal diffraction analyses indicated that Ni4Ln2 series exhibit butterfly structures with extended wings, the Co4Ln2 series are made up of four fused linear defective open cubes, and the Cu4Ln2 series display H-typed topology structures. The dc magnetic susceptibility data revealed that all of the Gd derivatives possess significant magnetic entropy change, whereas the alternating current (ac) magnetic susceptibility measurements identified that only the NiDy derivative displays single-molecule magnet behavior with energy barrier of 15.69 K under zero dc field. These results reveal that 3d ions are important in adjusting the structure and magnetic properties for 3d–4f complexes.

Electrorheological effects of waxy crude oils under high-voltage AC electric field

Electrical treatment is an emerging method for improving the cold flowability of waxy crude oil by taking advantage of the electrorheological effect of the oil. Previous investigations have been focused on the effect of DC electric fields, with the effect of AC electric fields unknown. In this paper, experiments were made by exposing a waxy crude oil to AC electric fields with different frequencies, field amplitudes, and waveforms (square wave, sine wave, and triangle wave), and the viscosity and yield stress of the crude oil before and after the treatment were compared. It was found that the AC electric field could also improve crude oil cold flowability, but its electrorheological effect is somewhat weaker than that of a DC field under the same test conditions of treatment time and temperature. The lower the frequency, the stronger the effect. The essential reason could be the reduced force of charged particle migration under an AC field with a higher frequency. At the same frequency, the viscosity reduction is significantly improved with increased field strength amplitude. At the same field strength amplitude, the electrorheological effect of the three studied waveforms is ranked, from strong to weak, as square one, sinusoidal one, and triangle one, which is essentially attributed to the difference in the Root Mean Square (RMS) values of the field strength of each waveform. The higher the RMS value, the better the electrorheological effect. The main conclusion was confirmed by additional experiments using another crude oil.

Design and analysis of a hybrid magnetic shielding system: application for the magnetic non-destructive testing of circuits

A magnetic shielding system combining three-axis active magnetic shielding and a high-Tc superconducting ring (TASR) was developed. The active shield mainly responds to DC field while the ring operates for shielding AC fields below 1 kHz. In this study, the system’s shielding effects were evaluated experimentally, and the results suggest that this system can effectively shield the interference of static and low-frequency magnetic fields to ensure the operation of high-precision magnetic sensors. The shielding factor reached 99.9% for geomagnetic field (DC, 38 500 nT), which exceeded 98% for about 10 000 nT AC magnetic field at 30–1000 Hz. Then, magnetic non-destructive testing of the circuit was performed in this TASR shielding system. The images clearly showed the magnetic field distribution in different kinds of circuits that could be inverted to current density information, indicating that the shielding system can detect circuit defects and has great application potential in integrated circuit diagnosis.

The Versatility of Ethylene Glycol to Tune the Dimensionality and Magnetic Properties in DyIII-Anilato-Based Single-Ion Magnets.

We exploit the high versatility of the solvent ethylene glycol (eg = CH2OH-CH2OH) acting as a ligand with three different coordination modes: terminal (κO), chelate (κ2O,O'), and bridge (1κO,2κO') to prepare a novel family of six different coordination polymers with DyIII and three different anilato ligands (3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone dianion = C6O4X22-, with X = H, Cl, and Br). With the X = H derivative (dhbq2-), we have prepared [Dy2(dhbq)3(eg)2(μ-eg)]·4eg·2H2O (1), a 3D diamond-like network with a chelate and bridging eg molecules. With the X = Cl derivative (chloranilato), we have prepared [Dy2(C6O4Cl2)3(eg)4]·2eg·H2O (2) and [Dy2(C6O4Cl2)3(μ-eg)(H2O)4]·2eg·7H2O (3). Compound 2 has a 2D (6,3)-gon brick-wall lattice and contains a chelate and a terminal eg molecule. Compound 3 has a 3D diamond-like topology as 1, although now the chelate eg has been replaced by two water molecules. Finally, with the X = Br derivative (bromanilato), we have obtained [Dy2(C6O4Br2)3(eg)2(CH3OH)2]·2eg·4CH3OH (4), [Dy2(C6O4Br2)3(eg)4]·4eg (5), and [Dy2(C6O4Br2)3(eg)3(H2O)]·2eg·H2O (6). Compound 4 has a 2D (6,3)-gon herringbone topology and contains a chelate eg and a MeOH molecule. Compounds 5 and 6 have a 2D (6,3)-gon brick-wall topology with a chelate and a terminal eg molecules (in 5 and in one of the two independent Dy centers of 6). The other Dy center in 6 has a chelate eg and a water molecule. All the compounds show slow relaxation of the magnetization at low temperatures (in compounds 1, 2, and 5 with no applied DC field). The magnetization of compounds 1-6 relaxes through Orbach and direct mechanisms when a DC field is applied and through an Orbach and/or quantum tunneling mechanism when no DC field is applied.

Synthesis, luminescence and magnetic properties of dinuclear complexes based on a “pincer” Schiff base and different β-diketonate ligands

Twelve new dinuclear rare earth complexes in three series were successfully synthesized with a “pincer” Schiff base H2L and different β-diketonate ligands: Ln2(acac)2(L)2·2C2H5OH (Ln = Eu (1), Tb (2), Dy (3), Er (4)), Ln2(dbm)2(L)2·2C3H7NO (Ln = Eu (5), Tb (6), Dy (7), Er (8), Yb (9)), Ln2(TTA)2(L)2·2H2O (Ln = Eu (10), Tb (11), Dy(12)). (H2L = 3-aminopyrazine-2-carboxylic acid [(2-hydroxybenzylidene)] hydrazide; Hacac = acetylacetone; Hdbm = dibenzoylmethane; HTTA = 2-thiophene trifluoroacetone). These complexes are centrosymmetric dinuclear structures and RE3+ ions are bridged by oxygen atoms in carbonyl of Schiff base to form the core unit of Ln2O2. The characteristic emission peaks of Er3+ and Yb3+ in 4, 8 and 9 can be observed in Near-infrared luminescence spectra, that is confirmed the central ions were sensitized effectively via ligands. Moreover, 1, 5, 6 and 10 were sensitized in various degrees. The dynamic magnetization curves indicate that 3, 7 and 12 are typical SMMs with slow magnetic relaxation behaviors. Their effective energy barriers are 48.20 K (3) and 42.30 K (12) under 3000 Oe dc field and 81.89 K (7) under zero dc field. The pre-exponential factors (τ0) are 1.35 × 10−6 s (3), 1.38 × 10−10 s (7) and 4.93 × 10−6 s (12), respectively. It is confirmed that the magnetic properties of SMMs can be regulated by different β-diketonate ligands.