Field Dependence Research Articles

Effects of Cr doping and size reduction on the structural and magnetic properties of α–CoV2O6

We report the effect of Cr-doping on the structural and magnetic properties of powder α–CoV2O6 samples. Samples were synthesized using a wet chemical synthesis method and calcined at temperatures between 350 ℃ and 500 ℃ to promote phase formation. For α–CoV2O6 and Cr-doped α–CoV2O6 samples, a single phase only occurs when the samples are calcined at 450 ℃ and 500 ℃, respectively, as confirmed using powder x-ray diffraction (XRD). Cr-doping results in a compression distortion of the lattice along the c-axis. Transmission electron microscopy (TEM) measurements indicate that Cr-doping causes a reduction in particle size and particle agglomeration. Metamagnetism was observed in field dependence of magnetization, M(μ0H) for α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05. The hysteresis in M(μ0H) curves for μ0H < 5T at 2K indicate that the magnetization is metastable and suggests magnetic irreversibility. Zero-field-cooled (ZFC) and field-cooled (FC) temperature dependence of magnetization, M(T), show antiferromagnetic (AFM) ordering of the α–Co(V1−xCrx)2O6 samples at fields, μ0H ≤ 1T. Increasing field strength to 2.5T causes a transition from the AFM state to a ferrimagnetic (FI) state, and an introduction of spin-glass (SG)-like behavior. At 5T, a transition from the FI state to a ferromagnetic (FM) state and a suppression of SG-like behavior is observed.

Fe(Se,Te) Thin Films Deposited through Pulsed Laser Ablation from Spark Plasma Sintered Targets

Iron-based superconductors are under study for their potential for high-field applications due to their excellent superconducting properties such as low structural anisotropy, large upper critical fields and low field dependence of the critical current density. Between them, Fe(Se,Te) is simple to be synthesized and can be fabricated as a coated conductor through laser ablation on simple metallic templates. In order to make all the steps simple and fast, we have applied the spark plasma sintering technique to synthesize bulk Fe(Se,Te) to obtain quite dense polycrystals in a very short time. The resulting polycrystals are very well connected and show excellent superconducting properties, with a critical temperature onset of about 16 K. In addition, when used as targets for pulsed laser ablation, good thin films are obtained with a critical current density above 105 A cm−2 up to 16 T.

Observation of Linear Magnetoresistance in MoO2

Magnetoresistance, the change in resistance with applied magnetic fields, is crucial to the magnetic sensor technology. Linear magnetoresistance has been intensively studied in semimetals and semiconductors. However, the air-stable oxides with a large linear magnetoresistance are highly desirable but remain to be fully explored. In this paper, we report the direct observation of linear magnetoresistance in polycrystalline MoO2 without any sign of saturation up to 7 T under 50 K. Interestingly, the linear magnetoresistance reaches as large as 1500% under 7 T at 2 K. The linear field dependence is in great contrast to the parabolic behavior observed in single-crystal MoO2, probably due to phonon scattering near the grain boundaries. Our results pave the way to comprehending magneto-transport behavior in oxides and their potential applications in magnetic sensors.

Does the ionic distribution in the electrical double layer modify second harmonic scattering?

Surface-specific nonlinear optical techniques are ideally suited to investigate the complex structure of aqueous interfaces. For colloidal particles dispersed in aqueous solutions, interfacial properties can be retrieved with angle-resolved second harmonic scattering (AR-SHS). The mathematical framework of AR-SHS does not require a priori knowledge on the electrostatic distribution in the first few nanometers close to the interface, therefore allowing us to formulate a molecular-level description of the electrical double layer (EDL) based on the experimental data. However, farther away from the interface, an analytical form of the electrostatic potential decay is necessary to account for the distance dependence of the surface electrostatic field propagating into the solution. This requirement is especially important at low ionic strengths, where the electrostatic field is not efficiently screened by counterions. Here, we examine to what extent the analytical form of the electrostatic potential decay impacts the AR-SHS data analysis. We analyze the effect of different functions on the scattering form factors, on the integrated AR-SHS signal intensity, and on the surface parameters extracted from fitting the AR-SHS data. We find that the trends of the surface parameters remain similar regardless of the chosen function, demonstrating the robustness of our approach to establish a molecular-level picture of the EDL. At ionic strengths <10-4M for 100-nm diameter particles, a functional form that physically represents counterions packed more densely in the vicinity of the surface than in the case of the Poisson-Boltzmann distribution has the largest impact, resulting in an overestimation of the obtained surface potential.

Effect of the global electroneutrality condition on electromigration Taylor-Aris dispersion in a microcapillary with finite Debye layer thickness.

In this work, the electromigration dispersion (EMD) due to the charged electrolytes in microchannels is considered without using the thin electrical double layer assumption. The electrokinetic flow and transport of ions are actuated within a rectangular micro-/nanochannel having a negative surface charge density under the influence of an external electric field. Due to the local variation of the conductivity and the nonvalidity of the electroneutrality condition, the local electric field varies as a function of the solute concentration, wall surface charge density, valency, and Debye layer length. The resulting electrokinetic flow due to the external electric field drives the fluid along with the charged species, where the Taylor-Aris dispersion separates the solutes into their different constituents. The local concentration dependence of the electric field leads to the formation of concentration profiles that are slightly asymmetric with respect to the standard Gaussian distribution. Including a finite Debye layer thickness has an effect on the advection of the species as well as the diffusion of the species. It is found that in cases where Debye layers are thicker, the species advects faster within the microchannel. This might give valuable insights into the nature of the EMD. Our model aims to predict the evolution of ionic concentration at all positions within the channel. A study of the higher-order statistics in skewness and kurtosis has also been conducted to obtain a better understanding of the idealized model consisting of a buffer solution.

The relative effects of direct and indirect written corrective feedback on L2 learning: The moderating role of field dependence/independence

The current study investigated the relative effects of direct and indirect written corrective feedback (WCF) on the learning of regular past tense, and the extent to which learning outcomes were related to individual variation in field dependence/independence (FDI). Fifty-nine Chinese intermediate second language (L2) learners of English were randomly assigned to two treatment groups and a control group. The two treatment groups carried out three written dictogloss tasks and received direct WCF and indirect WCF, respectively, on their erroneous use of regular past tense. The control group performed the same writing tasks but received feedback on the content of their writing. To evaluate the effect of treatment, a grammaticality judgement task and a picture description task were administered as the pretest, immediate posttest, and delayed posttest. The Group Embedded Figures Test was used to measure learners’ FDI orientation. The results showed that both treatment groups outperformed the control group on the posttests, but no significant differences were observed between the two treatment groups. In addition, the results suggested that FDI was related to learning outcomes as measured by the picture description task. Field independent learners had an overall advantage over field dependent learners, especially where indirect WCF was provided.

Unplugged Activity and Cognitive Style on The Speaking Abilities of Early Children

The ability to speak well and purposefully must be instilled in children from an early age. This is possible through the learning process at school using interesting learning methods and media according to the child's cognitive style. Because spoken language conveyed through speaking is a tool for expressing thoughts and feelings and a means of communication between humans. This will help improve children's speaking ability. This research aims to determine the differences in speaking abilities between students using the unplugged activity method and those using the beyond center and circle times (BCCT). Determine the differences in speaking ability between students with a field-dependence cognitive style and those with a field-independence cognitive style. The researcher tests the interaction between learning methods and cognitive style regarding speaking ability. This research uses an experimental design. The subjects were PGRI Jambon Kindergarten students (TK B1) and (TK B2) who studied the theme of Animals, totaling 44 children. The cognitive style was measured using a questionnaire based on the Children Embedded Figure Test (CEFT). Data analysis of relationships and interactions between variables used the two-way ANOVA test. The research results show that differences influence speaking ability between students using unplugged activity learning and BCCT. Some differences influence the student’s speaking abilities with a field-independence cognitive style and those with a field-dependence cognitive style. There is an interaction between learning methods and cognitive style regarding speaking ability.

Numerical solutions to hyperbolic Maxwell quasi-variational inequalities in Bean-Kim model for type-II superconductivity

This paper is devoted to the finite element analysis for the Bean-Kim model governed by the full 3D Maxwell equations. Describing type-II superconductivity at the macroscopic level, this model leads to a challenging coupled system consisting of the Faraday equation and a hyperbolic quasi-variational inequality (QVI) of the second kind with $L^1$-type nonlinearity, that arises explicitly from the magnetic field dependency in the critical current. With the involved Maxwell coupling in the 3D $\H(\curl)$-setting, the hyperbolic QVI character poses the primary challenge in the numerical investigation. Two mixed finite element methods based on implicit Euler and leapfrog time-stepping are proposed. On the one hand, the implicit Euler method results in a nonstandard system of curl-curl elliptic QVI with a first-order curl-type nonlinearity. Though the well-posedness of this system is guaranteed, its numerical realization is not straightforward and requires the use of a two-stage iteration process of high computational complexity. On the other hand, by approximating the electric and magnetic fields at two different time step levels, the leapfrog method turns out to be more suitable as it naturally eliminates the notorious QVI structure. More importantly, utilizing suited subdifferential and optimization techniques, we are able to prove an efficiently computable explicit formula for its exact solution in terms of the electric field, which makes its numerical computation substantially more favorable than the Euler method. As further advantages, the leapfrog method applies to broad scenarios involving low regular data of bounded variation (BV) in time for both the applied current source and the temperature distribution. Through nonstandard technical arguments tailored to the BV data, our analysis proves the conditional stability and, eventually, the uniform convergence of the proposed leapfrog method. This paper is closed by 3D numerical tests showcasing the reasonable and efficient performance of the proposed numerical solution.

Muon spin relaxation and emergence of disorder induced unconventional dynamic magnetic fluctuations in Dy2Zr2O7.

The disordered pyrochlore oxide Dy2Zr2O7shows the signatures of field-induced spin freezing with remnant zero-point spin-ice entropy at 5 kOe magnetic field. We have performed zero-field and longitudinal field Muon spin relaxation (μSR) studies on Dy2Zr2O7. Our zero field studies reveal the absence of both long-range ordering and spin freezing down to 62 mK. The μSR relaxation rate exhibits a temperature-independent plateau below 4 K, indicating a dynamic ground state of fluctuating spins similar to the well-known spin ice system Dy2Ti2O7. The low-temperature spin fluctuations persist in the longitudinal field of 20 kOe as well and show unusual field dependence of the relaxation rate, which is uncommon for a spin-liquid system. Our results, combined with the previous studies do not show any evidence of spin ice or spin glass ground state, rather point to a disorder-induced dynamic magnetic ground state in the Dy2Zr2O7material.

Interplay of lattice-spin-orbital coupling and Jahn-Teller effect in -noncollinear spinel TixMn1-x(FeyCo1-y)2O4: A neutron diffraction study.

Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel Mn1-xTix(Co1-yFey)2O4has been reported by means of neutron diffraction and magnetization studies. Two distinct set of compositions (i) x = 0.8 and y = 0.18 and (ii) x = 0.6 and y = 0.435 have been considered for the analysis. The first set of equivalent stoichiometry (Mn0.80Ti0.20Fe0.36Co1.64O4) exhibits enhanced tetragonal distortion across the ferrimagnetic Néel temperatures (TFNas high as 298 K) in comparison to the end compound MnCo2O4(TFN~180 K) with a characteristic ratio ct/√tof 0.99795(8). However, in the second case with higher B-site compositions, the presence of Jahn-Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility, χac(T) reveals a cluster spin-glass state with Gaby-Toulouse (GT) like mixed phases behavior below TFN. Such dispersive behavior appears to be sensitive to the level of octahedral substitution. Further, the field dependence of χac(T) follows the weak anisotropic GT-line behavior with crossover exponent Φ lies in the range 1.38 - 1.52 on the H-T plane which is in contrast to the B-site Ti substituted MnCo2O4 spinel that appears to follow irreversible non-mean-field AT-line behavior (Φ ~ 3 + δ). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition (T < 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime. &#xD.

A phase-field model for the brittle fracture of Euler–Bernoulli beams coupling stretching and bending

Damage gradient models seek to simulate fracture mechanics by modulating the material stiffness. Within this framework, a singular scalar field representing damage is commonly utilized to globally decrease elastic energy. However, when considering structural models like beams and plates, this approach often fails to adequately capture important aspects due to the interaction between stretching and bending contributions. In this work, we propose a model for planar Euler–Bernoulli beams that incorporates the following features: firstly, we utilize two phase-field damage parameters to describe material damaging, specifically addressing the ‘erosion’ occurring above and below the original, undamaged beam; secondly, we assume a simple linear dependence of the axial stress field on the thickness coordinate, along with linear dependence on the axial force and bending moment. By appropriately identifying the constitutive response, our model effectively considers the coupling between stretching and bending induced during through-the-thickness damage, demonstrating good agreement with 2D observations.

Origin of the Apparent Electric‐Field Dependence of Electrostrictive Coefficients

AbstractElectrostrictive materials exhibit a strain that is proportional to the square of the induced polarization. In linear dielectrics where the permittivity is constant, this electromechanical strain is also proportional to the square of the electric field. However, under increasing amplitudes of the driving field, the electromechanical strain sometimes saturates; the electrostrictive coefficients therefore appear to depend on the amplitude of the electric field used to measure them. Here, a methodology showing that this apparent field dependence is a consequence of neglecting higher‐order electromechanical phenomena is presented. When these are taken into account, not only do the electrostrictive coefficients remain constant but the signs of the high‐order coefficients enable the prediction of the saturation behavior from a single measurement. This approach is illustrated on both classical and non‐classical (so‐called “giant”) electrostrictors.

Exploring magnetocaloric effect of coordination polymer based on Mn(II) and Nb(IV) by relaxation calorimetry

Magnetocaloric properties of {MnII2(imH)2(H2O)4 [NbIV(CN)8]·4H2O}n molecular magnet are reported. The coordination polymer crystallizes in the monoclinic P21/c space group and displays a magnetic second-order phase transition at Tc ≈ 24.1 K. The relaxation calorimetry measurements are analyzed in order to determine the temperature dependence of the magnetic entropy change ΔSM as well as the adiabatic temperature change ΔTad for an array of applied field changes ranging from 0 to 9 T. For μ0ΔH = 5 T the entropy change ΔSM attains the maximum value of 6.55 J K−1 mol−1 (9.5 J K−1 kg−1) at 25.5 K, while the corresponding maximum value of ΔTad amounts to 2.02 K at 25.1 K. The critical behavior of the isothermal entropy change is discussed. In particular, the analysis of the field dependence of ΔSM in terms of the power law implies that the studied compound belongs to the mean-field universality class.

Magnetotransport properties of BiSe whiskers

Doped Bi2Se3 superconductors, such as CuxBi2Se3, SrxBi2Se3, NbxBi2Se3 are shown to be nematic superconductors in addition to the observed topological superconductivity. Recently it was found superconductivity in PdxBi2Se3 whiskers. The superconductivity has a residual character and probably exists on the whisker surface. The present paper deals with checking a possible nematicity existing in PdxBi2Se3 whiskers. The suppression of superconductivity was measured in the magnetic fields with induction up to 1.5 T. It was found the dependency of upper critical fields on the angle of magnetic field application. The dependency is explained in the framework of Ginzburg–Landau model and described by rather small anisotropy parameter 1.2. Besides blurring of temperature Curie near 5.3 K was observed of about 0.5 K that additionally confirmed the nematicity in PdxBi2Se3 whiskers. The rather high Curie temperature as well as nematicity existing is probably connected with strong spin–orbit interaction (SOI) of the charge carriers in the whiskers which stabilizing the superconductive state. When the temperature exceeds T c weak antilocalization of charge carriers is observed in the whisker magnetoresistance.

Unraveling the Molecular Weight Dependence of High Magnetic Field to Manipulate the Semiconducting Polymer Molecular Orientation.

The magnetic alignment of molecules, which exploits the anisotropy of diamagnetic susceptibility, provides a clean and versatile approach to the structural manipulation of semiconducting polymers. Here, the magnetic-alignment dynamics of two molecular-weight (MW) batches of a diketopyrrolopyrrole (DPP)-based copolymer (PDVT-8) were investigated. Microstructural characterizations revealed that the magnetically aligned, high-MW (Mn = 53.7 kDa) PDVT-8 film exhibited a higher degree of backbone chain alignment and film crystallinity compared with the low-MW (Mn = 17.6 kDa) PDVT-8 film grown via the same magnetic alignment method. We found that as the MW increases, the degree of preaggregation of the polymer molecules in solution significantly increases and the aggregation mode changes from H-aggregation to J-aggregation through a cooperative assembly mechanism. These events improved the responsiveness of high-MW polymer molecules to magnetic fields. Field-effect transistors based on the magnetic aligned high-MW PDVT-8 films exhibited a 6.8-fold increase in hole mobility compared to the spin-coated films, along with a mobility anisotropy ratio of 12.6. This work establishes a significant correlation among chain aggregation behavior in solution, polymer film microstructures, magnetic responsiveness, and carrier transport performance in donor-acceptor polymer systems.

Giant magneto-impedance effect in amorphous CoP@Cu microstrip based device in a wide frequency range

The giant magneto-impedance effect (GMI) consists in a large variation in the electrical impedance that occurs in some magnetic materials when subjected to a variable external magnetic field. Here we present an experimental study of the GMI in a wide frequency range observed in a structure of the metallic amorphous magnetic alloy CoP electrodeposited on copper microwires by using a microstrip configuration. In the low frequency range, the magnetic field dependence of the impedance is due to the behavior of the circumferential ac susceptibility in the magnetic coating. In the microwave range the behavior of the GMI is dominated by the main dynamical magnetic phenomenon at high frequencies, namely ferromagnetic resonance (FMR). The experimental data are well fit by Kittel equation for the FMR frequency for a ferromagnetic layer with a saturation magnetization similar to the ones reported in previous works. The structural stability of the CoP alloy against aging effects was also investigated. The overall results indicate that the CoP alloy is a good candidate for applications in sensors and in microwave devices.

Polarity in Liquid Crystals Formed by Self-Assembled Umbrella-Shaped Subphthalocyanine Mesogens.

We investigated the properties of p-type semiconducting columnar phases in self-assembled umbrella-shaped mesogens that have subphthalocyanine cores and oligo-thienyl arms. These compounds have nonswitchable phases that exhibit remanent electric polarization and nonlinear optical activity. Additionally, these compounds can generate photocurrents in the visible spectral range due to their wide absorption band. The photocurrent can be significantly increased by doping materials with fullerene. The charge mobility shows an anomalous field dependence, which decreases with the temperature.

Antiferromagnetic ground state and evidence for metamagnetic transition in itinerant-electron compounds YCo12−xFexB6 (x = 1.5, 2, 2.5)

In contrast to the parent compound YCo12B6, the Fe-doped YCo12−xFexB6 (1.5 ≤ x ≤ 2.5) alloys exhibit an antiferromagnetic ground state. It is further revealed that the antiferromagnetic state gets transformed into a forced ferromagnetic state by way of a magnetic-field-induced metamagnetic transition. The results demonstrate a progressive reinforcement of the antiferromagnetic interactions upon increasing Fe content along the YCo12−xFexB6 series of compounds. The magnetic phase diagram of each (x = 1.5, 2, and 2.5) composition is determined by combining isothermal and isofield magnetization measurements. The composition and temperature dependencies of the critical field are derived and discussed. Whereas for x = 2 and 2.5, the magnetic state changes from antiferromagnetic (AFM) to ferromagnetic (FM) and then to the paramagnetic (PM) state upon heating, exhibiting three distinct temperature regions, a multicritical point of magnetic origin is proposed for the x = 1.5 compound at a temperature of about 117 K. The ordering temperature is found to decrease upon increasing Fe concentration, while the critical field of the AFM-FM metamagnetic phase transition shows the opposite trend. The critical transition field diminishes upon increasing temperature.

An analytic expression for the field dependence of three-order Zernike aberrations in decentered and/or tilted optical systems

The double Zernike polynomials can describe the aperture dependence and field dependence of non-coaxial optical system aberrations simultaneously, and characterize the complex optical aberrations, which are of great value in the optical system design and alignment. In this paper, the specific analytic forms of all third-order double Zernike aberrations in decentered and/or tilted optical systems are derived based on the nodal aberration theory. This paper realizes the orthogonalization of the third-order aberrations with different aperture dependence and field dependence. The calculation process of the third-order double Zernike aberrations is presented. The ray tracing of the Cassegrain telescope verifies the analytic derivation of the double Zernike aberrations.

A broad-band investigation of ferromagnetic resonance in exchange coupled permalloy/phthalocyanine bilayer heterostructures at room temperature

The phenomenon of ferromagnet/molecule interfacial electronic hybridization enables the possibility of modifying magnetic characteristics of downscaled systems for spintronic applications. Ferromagnetic resonance (FMR) at the nanoscale level through a coplanar waveguide (CPW) over a wide frequency range is a remarkably reliable technique for investigation of the magnetic heterostructure’s interfacial properties and their feasibility in applications as spintronic devices. Here, magnetron sputtering and organic molecular beam epitaxy methods are used to fabricate the following nanoscale heterostructures: permalloy (Py)/Fe phthalocyanine (Pc), Py/ZnPc, and Py/Cr. Magnetization dynamics of the bilayer heterostructures is investigated in a CPW by studying frequency dependence (5–50GHz) of the resonance field and linewidth with parallel and perpendicular geometries of the static applied magnetic field, thereby providing insights into possible intrinsic and extrinsic sources of FMR damping. Planar Pc molecules at the Py layer interface are found to have the potential to contribute to magnetic stabilization based on calculated magnetic parameters. This is evidenced by the larger results of surface anisotropy of the Py/MPc bilayer heterostructures induced by exchange coupling interaction at room temperature as well as the lower values of damping constant.