External Dc Field Research Articles

Negative capacitance switching in size-modulated Fe3O4 nanoparticles with spontaneous non-stoichiometry: confronting its generalized origin in non-ferroelectric materials.

Persistent low-frequency negative capacitance (NC) dispersion has been detected in half-metallic polycrystalline magnetite (Fe3O4) nanoparticles with varying sizes from 13 to 236 nm under the application of moderate dc bias. Using the Havriliak-Negami model, 3D Cole-Cole plots were employed to recapitulate the relaxation times (τ) of the associated oscillating dipoles, related shape parameters (α, β) and resistivity for the nanoparticles with different sizes. The universal Debye relaxation (UDR) theory requires a modification to address the shifted quasi-static NC-dispersion plane in materials showing both +ve and -ve capacitances about a transition/switching frequency (f0). A consistent blue-shift in 'f0' is observed with increasing external dc field and decreasing particle size. Based on this experimental data, a generalized dispersion scheme is proposed to fit the entire positive and negative capacitance regime, including the diverging transition point. In addition, a comprehensive model is discussed using phasor diagrams to differentiate the underlying mechanisms of the continuous transition from -ve to +ve capacitance involving localized charge recombination or time-dependent injection/displacement currents, which has been adequately explored in the scientific literature, and the newly proposed 'capacitive switching' phenomenon. An inherent non-stoichiometry due to iron vacancies [Fe3(1-δ)O4], duly validated from first principles calculations, builds up p-type nature, which consequently promotes more covalent and heavier dipoles and slows the dipolar relaxations; this is incommensurate with Maxwell-Wagner interfacial polarization (MWIP) dynamics. This combinatorial effect is likely responsible for the sluggish response of the associated dipoles and the stabilization of NC.

Light amplification in tunable multicolor liquid crystalline system

Multicolor emission exhibited by complex organic systems is interesting and desired phenomenon for many optical applications. Laser devices capable to emit the light in the spectral range of three fundamental colors (red, green and blue) can revolutionize the modern display and lighting technologies. In this paper we demonstrate a multicolor stimulated emission (STE) phenomenon obtained in dye doped, liquid crystalline (LC) material. The polychromatic STE was obtained for LC cell, filled with nematic mesophase doped with three luminescent dyes. The CIE XYZ chromaticity diagram (1931) demonstrates effective color modulation with respect to relative concentration of utilized dyes and applied voltage. The precise color adjustment can be provided by applying the external electric DC fields, with voltages ranging from 0 to 20 V.

Anisotropic Magnetoelectric Effect in a Magnetostrictive Fiber Composite—Piezoelectric Plate Structure

The magnetoelectric (ME) effect in a planar structure containing a mechanically coupled layer of a magnetostrictive fiber composite (MFC) and a layer of piezoelectric lead zirconate titanate (PZT) is investigated. MFC was a single layer of Ni wires 25 μm in diameter located at a distance of ~150 µm from each other and fixed in a polymer matrix. The structure was placed in an external dc field H and excited by ac magnetic field. It was shown that MFC magnetostriction anisotropy leads to a strong ME effect anisotropy. The magnetoelectric coefficient for parallel orientation of the magnetic field relative to the fiber was ~1.15 V/(Oe cm), and for perpendicular orientation ~0.2 V/(Oe cm). Nonlinear frequency doubling effect was observed in the structure for parallel orientation of H.

Slow Magnetic Relaxation in a Mononuclear Five‐Coordinate Cu(II) Complex

A mononuclear five‐coordinate Cu(II) complex [Cu(12‐TMC)Cl][B(C6H5)4] (1) (12‐TMC = 1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane) with distorted square pyramidal geometry has been synthesized and characterized by single‐crystal X‐ray diffraction, direct‐current and alternating‐current magnetic measurements and electron paramagnetic resonance (EPR) spectroscopy. The complex displays slow magnetic relaxation under an external dc field. The analysis of the field‐ and temperature‐dependence of the relaxation rates suggests that the slow dynamics of the magnetization is dominated by the Raman‐like process in the temperature range of 1.8 K to 11 K under an applied dc field of 0.15 T. In addition, the results of magnetic dynamics for the diluted sample with diamagnetic Zn(II) ions confirms the molecular origin of slow magnetic relaxation.

Cholesteric Flakes in Motion Driven by the Elastic Force from Nematic Liquid Crystals.

The paper presents a methodology to control the motion and orientation of suspended reflective cholesteric flakes in a nematic liquid crystal (LC) matrix. The flakes exhibit a dielectric anisotropy which controls their alignment with their in-plane axes parallel to an external electrical dc field. The elastic forces imposed by the LC host affect the switching behavior of the flakes and take care of the realignment to the planar state as soon as the dc field is switched off. When the LC host has a positive dielectric anisotropy, the switching voltage of the flakes is reduced by a factor of 2 in comparison with a LC host with negative dielectric anisotropy or in comparison with an isotropic host. We discovered that the LC host further regulates the back relaxation of cholesteric to return to the planar state upon retrieving the electric field. Whereas, in the isotropic fluid, flakes do not exhibit a preferred orientation when relaxed. Based on this newly proposed principle, we demonstrated its application as an optical switch for smart windows. Depending on the pitch of the cholesteric helix of the flakes, the light of a preset wavelength is reflected. Upon application of an electric field, the embedded flakes rotate their planes perpendicular to the substrate and consequently the incident light becomes fully transmitted without reflection or scattering of light.

Low-Noise Orthogonal Fluxgate Using Flipped Current Joule Annealing

It has been shown that annealing of amorphous magnetic wire used as a core for orthogonal fluxgates in the fundamental mode can significantly decrease the noise of the sensors in the $1/f$ region. This is due to an increase in the circumferential anisotropy due to the dc current flowing through the wire during annealing. This method, however, presents some drawbacks: first, it requires an infrared furnace and precise compensation of the magnetic field inside it. Second, it is very slow, because it requires the cooling of the whole furnace before removing the wire. Most importantly, while the $1/f$ noise decreases, the noise floor increases. In this paper, we present a method that allows the simultaneous reduction of $1/f$ noise and noise floor. This method is based on Joule annealing by means of a very large current in the wire. The current is periodically flipped with 0.25 Hz frequency in order to avoid an excessive increase in the offset. The annealing is performed in a four-layer shielding to avoid the presence of an external dc field. Annealing for 1 min, 1 wire-based sensor returned a considerable noise reduction both in the $1/f$ regions from 2.5 to 1.5 $\text {pT}/\sqrt {\mathrm{ Hz}}$ ) while the noise floor was unchanged at $650~\text {fT}/\sqrt {\mathrm{ Hz}}$ . For larger annealing time, however, the noise floor rose. We tried to compensate this problem by increasing the number of wires to four, but also in this case, we achieved the best noise reduction (from 1.7 to $0.75~\text {pT}/\sqrt {\mathrm{ Hz}}$ at 1 Hz and from 470 to $350~\text {fT}/\sqrt {\mathrm{ Hz}}$ noise floor) with 1 min annealing. By the use of thermocamera, we discovered that the problem of long-time annealing was that the sensor head support was warming up too much. Therefore, we repeated the experiment by annealing 21 min as a series of 1 min annealing followed by 3 min of no current for 21 times to let the sensor head totally cool to room temperature after every annealing period. In this way, we achieved the lowest noise of 630 $\text {fT}/\sqrt {\mathrm{ Hz}}$ at 1 Hz and $400~\text {fT}/\sqrt {\mathrm{ Hz}}$ noise floor.

Control of terahertz pulse polarization by two crossing DC fields during femtosecond laser filamentation in air

Terahertz (THz) radiation induced by the femtosecond laser filament in air can be strongly enhanced through applying an external DC field. In the present work, this technique has been extended to two-dimensional biasing; namely, two pairs of electrodes, which are placed perpendicular to each other, are applied to provide two crossing DC fields. The experimental results have demonstrated that the amplitudes of two orthogonal components of the polarized THz pulse could be modulated by the electric field parallel to it, in an in situ and real-time manner, while the phase delay between them remains constant. Hence, the polarization of the output THz pulse could be freely controlled by adjusting the voltages of the two crossing DC fields, achieving arbitrary ellipticity. It might be the simplest method to manipulate the THz polarization during femtosecond laser filamentation in air.

Trinuclear and Mononuclear Lanthanoid Complexes Containing 2‐Methyl‐8‐quinolinolate: Synthesis, Structures, and Magnetic Properties

Two series of new lanthanoid complexes containing 2‐methyl‐8‐quinolinolate (MQ–) have been prepared at elevated temperatures. Lighter elements yield trinuclear complexes [Ln3(MQ)9(HMQ)]·5H2O·3MeOH (1Ln, where Ln = Pr, Eu, Gd, Tb, Dy and Ho) by solvothermal reactions, and heavier lanthanoids yield the mononuclear complexes [Ln(MQ)3(HMQ)]·HMQ·MeOH (2Ln, where Ln = Dy, Ho, Er and Yb), with overlap between the two series found in the cases of Dy and Ho. Magnetic studies reveal that 1Tb, 1Dy, and 2Er show distinct slow magnetic relaxation under external DC fields. The magnetic data for 1Gd yielded the J value for weak antiferromagnetic coupling and the magnetocaloric –ΔSm value of 20 J kg–1 K–1 at 2 K and ΔH = 5 T.

Single-ion magnet and luminescent properties in a Dy(III) triangular dodecahedral complex

In 2016, our group first reported a single-ion magnet (SIM) [Dy(bbpen)Br] (H2bbpen = N,N′‑bis(2‑hydroxybenzyl)‑N,N′‑bis(2‑methylpyridyl)ethylenediamine) with the highest effective energy barrier (Ueff) over 1000 K. To explore further for the high performance single-ion magnets (SIMs), we used functional ligand to replace the Br− anion. The H2bbpen and 4′‑(1,2,2‑triphenylvinyl)benzoic acid (tpe-COOH) ligand with aggregation-induced-emission-active (AIE) property, formed a triangular dodecahedral SIM, [Dy(bbpen)(tpe-COOH)]⋅2MeOH. The structural, magnetic and luminescent properties of the SIM have been investigated. Related to its lower local symmetry around Dy(III) ion, the Ueff was sharply reduced to 77 K under an external dc field of 0.15 T. Therefore, it proved the importance of local symmetry to get the high performance SIMs from the opposite side.

Building Block and Directional Bonding Approaches for the Synthesis of {DyMn4}n (n = 2, 3) Metallacrown Assemblies

Four 3d-4f heterometallic complexes based on salicylhydroxamic acid (H3sha) have been synthesized. Single-crystal X-ray diffraction analysis reveal that all complexes are comprised of {Dy[12-MCsha,Mn(III)-4]}n (n = 2 or 3, MC = metallacrown) supramolecular architectures, where the MC building blocks are further ligated by the ditopic subunit ligands with different lengths and bite angles. Along with the angular orientations ranging from 120° to 180°, the resulting architectures vary from dimeric double-layer to trimeric triangular MC assemblies based on the building block and directional bonding approaches. Magnetic measurements reveal the presence of dominant antiferromagnetic interactions for all complexes, thereinto, one of which displays slow relaxation of magnetization under external dc field.

Modification of transfer-matrix method for electromagnetic waves in layered superconductor in presence of dc magnetic field

In the present paper, we modify the transfer-matrix method to study the dissipation-free transition of electromagnetic waves of terahertz range through a plate of layered superconductor embedded in the dielectric environment in the presence of external direct current (dc) magnetic field. In this work, we сonsider TM-polarized electromagnetic waves. The setup is arranged in such a way that the dielectric and superconducting layers in the plate are perpendicular to its interface, and the external magnetic field is directed along the plate and parallel to the layers. We consider the case of a weak external dc field at which magnetic vortices do not penetrate the plate. Due to the nonlinearity of the Josephson plasma formed in the layered superconductor, the dc magnetic field penetrates non-uniformly into the plate and affects the electromagnetic wave. Hence, the magnitude of the external dc magnetic field can be used as a variable parameter to tune various phenomena associated with the propagation of an electromagnetic waves in layered superconductors. In the presence of the external homogeneous dc magnetic field, linear electromagnetic waves in the layered superconductor turn out to be non-exponential. Therefore we cannot directly apply the transfer matrix method, in which the amplitudes of the corresponding exponents are compared. However, in the present paper, it is shown that for a sufficiently thick plate, the matrices describing the wave transfer through the plate can be introduced. The analytical expressions for these matrices are derived explicitly in terms of special Legendre functions. The obtained transfer-matrices can be used for the further study of the wave transfer through the layered superconductor in the presence of an external dc magnetic field.

A decanuclear [DyZn] cluster: a {Zn} rectangle surrounding an octahedral {Dy} single molecule magnet.

An octahedral {DyIII6} cage within a diamagnetic {ZnII4} rectangle is reported, with magnetic relaxation studies revealing single-molecule magnet behaviour for the complex under zero external dc field with Ueff = 43 K and τo = 1 × 10-5 s.

Oximato-Based Ligands in 3 d/4 f-Metal Cluster Chemistry: A Family of {Cu3Ln} Complexes with a "Propeller"-like Topology and Single-Molecule Magnetic Behavior.

The organic chelating and bridging ligands 9,10-phenanthrenedione-9-oxime (phenoxH) and 9,10-phenanthrenedione-9,10-dioxime (phendoxH2) were synthesized and subsequently employed for the first time in heterometallic 3 d/4 f-metal cluster chemistry. The general reaction between CuCl2·2H2O, LnCl3·6H2O, phenoxH, and NEt3 in a 1:2:2:4 molar ratio, in a solvent mixture comprising MeCN and MeOH, afforded brown crystals of a new family of [Cu3LnCl3(phenox)6(MeOH)3] clusters (Ln = Gd (1), Tb (2), Dy (3)) that possess an unprecedented [Cu3Ln(μ-NO)6]3+ "propeller"-like core. Complexes 1-3 are the first {Cu3Ln} clusters in which the outer CuII and the central LnIII atoms are solely bridged by diatomic oximato bridges. The {Cu-N-O-Ln} bridging units are very distorted with torsion angles spanning the range 35.5-48.9° and 25.2-55.6° in 1 and 2, respectively. As a result, complexes 1-3 are antiferromagnetically coupled, in agreement with previously reported magnetostructural criteria for oximato-bridged Cu/Ln complexes. The magnetic susceptibility data for all complexes were nicely fit to an isotropic spin Hamiltonian (for 1) or a Hamiltonian that accounts for the spin of the CuII atoms, the spin component of the LnIII, the spin-orbit coupling (λ), an axial ligand-field component around the LnIII atoms (Δ), and the Zeeman effect (for the anisotropic 2 and 3). The resulting fit parameters were J = -1.34 cm-1 and g = 2.10 (1), J = -1.42 cm-1, gCu = 2.10, and Δ = -26.3 cm-1 (2), and J = -1.70 cm-1, gCu = 2.05, and Δ = -38.1 cm-1 (3). The reported fitting procedure, implemented in the PHI program, is here used for the first time. Even if this method is only valid in high-symmetry Ln environments, when it is properly used allows a very simple and efficient method to obtain the exchange parameters. In light of the negative anisotropy, compounds 2 and 3 were found to exhibit frequency-dependent tails of out-of-phase signals in the presence of a small external dc field, characteristic of the slow magnetization relaxation of a single-molecule magnet. By using the Kramers-Kronig equations, the effective energy barriers ( Ueff) were derived and reported as Ueff = 10.1 and 5.4 cm-1 for 2 and 3, respectively.

Electrical Control of Unpolarized Reflectivity in Polymer‐Stabilized Cholesteric Liquid Crystals at Oblique Incidence

AbstractThe cholesteric liquid crystal phase at or near normal incidence selectively reflects circularly polarized light matching the handedness of the helicoidal superstructure. At large angles of incidence, both senses of circularly polarized light are reflected over a certain wavelength range. Stabilizing the cholesteric liquid crystal phase with low concentrations of polymer network that maintains structural chirality allows the pitch (and accordingly, the selective reflection) to be adjusted with an electric field. Here, the behavior of these polymer stabilized cholesteric liquid crystals is investigated at large angles of incidence, to determine how the total reflection evolves when the polymer network is deformed by an external DC field. Strong, unpolarized reflection is observed in polymer stabilized cholesteric liquid crystals at oblique angles over a wide range of field values. As the application of a DC electric field can adjust the position or bandwidth of the total reflection in polymer stabilized cholesteric liquid crystals, these reconfigurable devices could find potential end use in optical applications where polarization independent properties are needed, such as spectroscopy and polarimetry.

Synthesis, crystal structure, and magnetic properties of a series of binuclear lanthanide compounds derived from the 4-Bromo-2-((quinolin-8-ylimino)methyl)phenol ligand

A series of binuclear lanthanide compounds, derived from the Schiff-base ligand 4-bromo-2-((quinolin-8-ylimino)methyl)phenol (HL), namely [Ln2(L)2(CH3CH2CH2 COO)2(NO3)2(MeOH)2] (Ln = Gd (1), Tb (2), Dy (3), Ho (4)), has been synthesized. Compounds 1–4 have a similar dinuclear lanthanide core bridged by a butyrate ion. Magnetic studies reveal that compound 1 has a weak ferromagnetic interaction. On the other hand, compound 3 displays slow magnetic relaxation under an external dc field with the anisotropic barrier of 27.08 K and pre-exponential factor τ0 = 4.12 × 10−6 s. Therefore the Dy(III) compound is a field-induced SMM.

Robust lanthanide metal–organic frameworks with highly sensitive sensing of aniline and slow magnetization relaxation behaviors

Two new three-dimensional lanthanide frameworks [Ln(BTB)(H2O)]n (Ln = Eu (1) and Dy (2)) based on 1,3,5-benzenetribenzoic acid (H3BTB) were synthesized and characterized. Structural analysis reveals that they were isostructural, in which two lanthanide ions are bridged by four carboxylic groups from different BTB3− in the modes of μ2-η1η1 and μ2-η1η2 to form a binuclear unit. The binuclear units are built into one-dimensional chain via two μ2-η1η1 carboxylic groups, which are further linked by BTB3− to generate three-dimensional frameworks. Compound 1 displays excellent solvent stability. The luminescent investigations suggest that 1 exhibits high sensitive sensing to aniline. Importantly, 1 as a promising luminescent detector for aniline can be recycled at least for four times. Additionally, magnetic studies reveal that 2 displays slow magnetic relaxation behaviors with the energy barrier (ΔE/kB) of 9.36 K under an external dc field (dc = 2500 Oe).

Heterometallic lanthanide-centred [NiLnIII] rings.

A [NiII6DyIII] heptanuclear complex featuring a rare six-membered {NiII6} metal ring surrounding the central Dy(iii) ion is reported. Magnetic studies reveal single-molecule magnet behaviour for the complex under zero external dc field, while replacing the DyIII ion with [capital Upsilon]III or GdIII ions allows for a comprehensive understanding of the magnetic behaviour.

Investigation of dipolar interaction in FINEMET ribbons through longitudinally driven magneto-impedance effect

The magnetic dipolar interactions among multiple FINEMET ribbons have been studied by longitudinally driven magneto-impedance effect (LDMI) and hysteresis loops in this paper. The effect of dipolar fields on LDMI apparently expands the “bell” magneto-impedance profiles and raises its characteristic frequency. This is essentially correlated with the domain nucleation process under the combined effect of ac driving field and dc external field. A theoretical model was utilized to explicate the LDMI variation with the number of ribbons N. Basically, the nucleation field varied linearly with N. The influence of the frequency of ac current causes the increase of the nucleation field by adding a term He∼f0.38 before 4 MHz, but the dipolar field barely decreases with ac current. At frequency of 10 kHz, the dipolar field is fitted to be about 0.69 Oe, and the geometric factor can be estimated to be 5.60 × 10−5. Additionally the nucleation field reduces slightly due to the compensation of the alternating field, while the LDMI ratio changes obviously. The results indicate that LDMI can be employed as a sensitive tool to reveal the dipolar interaction in FINEMET ribbons and facilitate the design of the materials for magnetic devices.

Slow magnetic relaxation in a carboxylate-bridged CoII complex

A new Co(II) complex, {[Co(MPA)(4,4′-bpy)(H2O)]·2H2O}n (1) (H2MPA = 4-methylphthalic acid, 4,4′-bpy = 4,4′-bipyridine), has been prepared under solvothermal condition and fully characterized. The X-ray crystallography test demonstrates that 1 exhibits two-dimensional (2D) layer structure in which the helical Co(II) chains linked by syn-anti carboxylate bridges are connected by 4,4′-bpy ligands. Weak ferromagnetism was observed on direct-current (dc) susceptibility due to the collective effects of the helical chain structure and antiferromagnetic couplings between Co(II) ions transmitted by μ2-1,3-carboxylate groups. Interestingly, slow magnetic relaxation was observed in 1 at liquid helium temperatures without an external dc field.

Pure quadratic or higher-order optical effects in anisotropic crystals induced by external dc fields and probed by a single low-intensity plane electromagnetic wave

The determination of a clear theoretical demarcation between a true or a false quadratic or higher-order low-intensity optical effect induced by an externally applied static or quasistatic (dc) vector field in anisotropic crystals is the scope of the present work. A complete set of necessary and sufficient conditions required for the practical possibility of direct detection, measurement, and tabulation of only those pure optical contributions is finally obtained. The dc electro-optic effect stands out as the most representative of all of these low-power dc optical effects. However, although the dc Kerr effect remains the main topic of application of the analytical treatment developed in this work, the current theoretical formalism is extended to include other dc conventional crystal optics effects, such as electrogyration, electroabsorption, and externally induced ray or energy propagation. Even more, the theoretical conditions are further generalized to apply to any pure higher-order crystal optics effect induced by external dc fields. These can be electric, magnetic, force, and even temperature or concentration gradient fields. The current treatment does not extend to multiple-beam high-intensity nonlinear optics effects induced by optical (ac) fields. Compared to previously published expressions, a more general Fresnel equation is also provided here together with the most general Jones vectors describing the eigenpolarizations of the single probing beam of light. All the generalizations and extensions mentioned in this article are valid as long as the field-dependent coefficients of the particular optical effect under consideration satisfy the equation of a positive-definite complex Hermitian form.