Function Of Bias Field Research Articles

X-ray detected ferromagnetic resonance techniques for the study of magnetization dynamics

Element-specific spectroscopies using synchrotron-radiation can provide unique insights into materials properties. The recently developed technique of X-ray detected ferromagnetic resonance (XFMR) allows studying the magnetization dynamics of magnetic spin structures. Magnetic sensitivity in XFMR is obtained from the X-ray magnetic circular dichroism (XMCD) effect, where the phase of the magnetization precession of each magnetic layer with respect to the exciting radio frequency is obtained using stroboscopic probing of the spin precession. Measurement of both amplitude and phase response in the magnetic layers as a function of bias field can give a clear signature of spin-transfer torque (STT) coupling between ferromagnetic layers due to spin pumping. In the last few years, there have been new developments utilizing X-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz frequency range. The techniques of diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) provide novel ways for the probing of the dynamics of chiral and multilayered magnetic materials, thereby accessing key information relevant to the engineering and development of high-density and low-energy consumption data processing solutions.

An analytical model for nonlinear magnetoelectric effect in laminated composites

This paper presents an analytical and explicit theoretical model for the nonlinear magnetoelectric (ME) effect in laminated composites, in which the multi-field coupled properties of magnetostrictive materials are taken into account. The adopted explicit nonlinear constitutive equation is equivalent to the one in series form through expanding the magnetostriction as a Taylor series in the vicinity of the applied bias field. Then the field-dependent materials’ parameters are derived as functions of bias field and stress. Analytical solutions of nonlinear ME voltages in various modes are obtained simultaneously using mechanical differential equations, boundary conditions, and electric equations. It is predicted that the frequency-multiplying behavior observed in the composites can be tuned by bias magnetic field. Only even harmonics occur in the absence of bias field, while odd harmonics appear once a bias field is applied. An optimal bias field corresponding to the maximum linear voltage and zero nonlinear voltage is obtained, where the highest piezomagetic effect generates. It is proved that the ME distortions under a fixed ac field is actually caused by the competition results between the linear piezomagnetic effect and high-order magnetostrictive effect. Moreover, pre-stress may enhance ME voltages and reduce the optimal field due to the multi-field coupled character, and control the resonance frequency via ΔE effect. Eventually, the origin of the observed dual-peak phenomenon around resonance frequencies is revealed and explained theoretically, indicating that strong ME effect may be achieved in a wider bias field range. The analytical solutions from this study can be applied to design tunable ME sensors, energy harvesters, and some other nonlinear devices via manipulating the external bias field and pre-stress.

MR image segmentation based on level set method

In this paper, a level set model combining probabilistic statistics for image segmentation is proposed. Through adding a single-point pixel distribution into the energy function, the step size of each iteration is increased and the efficiency of the algorithm is improved. By adding the membership function of fuzzy clustering and bias field function, this method can effectively segment the image with intensity inhomogeneities. In addition, a new rule item is added to improve the edge segmentation effect of the image. Experiments on MR images of the brain show that the proposed model can provide ideal segmentation results compared with several level set segmentation models.

Mechanisms of electromechanical response in (1 − x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

Contributions to the piezoelectric response of (1 − x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics are quantified by small signal measurements made as functions of bias field and temperature. The highest fraction of intrinsic contributions is observed far from phase boundaries, of extrinsic contributions around phase boundaries, and of irreversible switching in the orthorhombic phase. The largest piezoelectric response, d33 = 475 ± 85 pC/N, is found near the orthorhombic to tetragonal phase boundary due to both reversible and irreversible switching. A peak in reversible switching above the Curie temperature for all compositions suggests a line of critical points associated with first order phase transitions, indicating that concurrence of triple and tricritical points in the zero-field phase diagram is not the responsible mechanism of enhanced piezoelectricity.

Lightness biased cartoon-and-texture decomposition for textile image segmentation

With the development of robust image processing tools in the textile industry, fabric designers are beginning to use feature extraction methods for both analysis and pattern design of fabric materials. In the design evaluation process, one of the basic problems is the efficient segmentation for textile fabric images. This is equivalent to partitioning the images into several meaningful regions that often correspond to units of design patterns, repeats, woven yarn or fibres. The main challenge in this problem is identifying robustly the boundaries of various components of fabric materials. In this paper, we propose a novel model to solve the problem. The model is established on the analysis of the characteristic of textile/fabric images. The main contributions of the model are: (1) a cartoon-and-texture decomposition process is incorporated into the model, which can reduce the influence of the random texture noise on the segmentation process; (2) to overcome the drawback of the lightness inconsistency for the segmentation process, a bias field function is introduced to measure the deviation degree between the cartoon image and the piecewise constant approximation of the cartoon image. Then, the regions of textile images can be more accurately estimated; (3) following the advantages of the fuzzy region competition based image segmentation models, we also use the fuzzy membership functions (FMFs) to indicate the regions of images. However, to restrain the FMFs from degeneration, a new penalty term on the FMFs is introduced in our model. In addition, by using the augmented Lagrange multiplier method and the Chambolle׳s dual projection method, we derive an efficient algorithm to solve the model. Experimental results demonstrate that the proposed model can generate better segmentation results for textile images than classical FMF based models.

Brain MR image segmentation based on local Gaussian mixture model and nonlocal spatial regularization

Brain Magnetic Resonance (MR) images often suffer from the inhomogeneous intensities caused by the bias field and heavy noise. The most widely used image segmentation algorithms, which typically rely on the homogeneity of image intensities in different regions, often fail to provide accurate segmentation results due to the existence of bias field and heavy noise. This paper proposes a novel variational approach for brain image segmentation with simultaneous bias correction. We define an energy functional with a local data fitting term and a nonlocal spatial regularization term. The local data fitting term is based on the idea of local Gaussian mixture model (LGMM), which locally models the distribution of each tissue by a linear combination of Gaussian function. By the LGMM, the bias field function in an additive form is embedded to the energy functional, which is helpful for eliminating the influence of the intensity inhomogeneity. For reducing the influence of noise and getting a smooth segmentation, the nonlocal spatial regularization is drawn upon, which is good at preserving fine structures in brain images. Experiments performed on simulated as well as real MR brain data and comparisons with other related methods are given to demonstrate the effectiveness of the proposed method.

Fuzzy region competition-based auto-color-theme design for textile images

We propose a model to recolor textile images by different color themes. The model contains three phases. The first phase is to partition an input textile image into several homogeneous regions. The CIELab color mean of each region and a bias-field function are obtained from the segmentation results. The combination of the color mean values of all regions is considered as the color theme of the input image. The second phase is to retrieve the relevant color themes from a given dataset. The retrieved color themes preserve the color mood of the input image in the sense of the similarity measurement defined in the color mood space. In the third phase, we reconstruct new images with different appearances from the input image by using the retrieved color themes. The proposed method provides a powerful tool for designers to generate and search for all relevant color combinations related to a given theme. Numerical results indicate that our recolorization model performs well on complex textile design patterns.

Optical modulation effects on nonlinear electron transport in graphene in terahertz frequency range

We describe very fast electron dynamics for a graphene nanoribbon driven by a control electromagnetic field in the terahertz frequency regime. The mobility as a function of bias field has been found to possess a large threshold value when entering a nonlinear transport regime. This value depends on the lattice temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum governed by the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependences of the mobility are seen in the linear and nonlinear transport regimes, which is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through elastic scattering and moving electrons from low-energy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating. Additionally, a self-consistent device modeling has been proposed for graphene transistors under an optical modulation on its gate, which employs Boltzmann moment equations up to the third-order for describing fast carrier dynamics and full wave electromagnetics coupled to the Boltzmann equation for describing spatial-temporal dependence of the total field. Finally, a detailed comparison of the derived Maxwell–Boltzmann moment equations in this paper with the well known Vlasov–Maxwell equations is also included.

Polarization reversal and backswitching dynamics in epitaxial BaTiO3 thin films

Polarization switching dynamics in polydomain epitaxial barium titanate (BaTiO3) thin films were investigated over the temperature range of 293–363K. To determine domain dynamics, the transient response of the linear electro-optic effect was measured as a function of bias field under pulsed excitation and temperature. Upon removal of the bias pulse, domain backswitching is observed, which is described by the Kohlrausch–Williams–Watts (KWW) extended exponential function in time. From the KWW kinetic parameter ⟨τ⟩ and its temperature and field dependences, the activation field and nucleation energy for polarization reversal are determined. The measured thermal energy barrier for nucleation is in the range of 0.1eV–0.3eV and decreases with the electric field. These values are in good agreement with those recently obtained in molecular dynamics simulation studies of domain nucleation and growth in BaTiO3.

Dynamic response of polydomain ferroelectric barium titanate epitaxial thin films and its field dependence

The nature of ferroelectric domain dynamics in polydomain epitaxial barium titanate thin film was investigated using the linear electro-optic effect. The dynamic response was studied as a function of bias field under pulsed excitation. Upon removal of the bias pulse a millisecond long transient response was observed that is attributed to 90° domain reversal. The dynamic response and its field dependence are described by the Kohlrausch–Williams–Watts stretched exponential function in time with an average relaxation time ⟨τ⟩. This average time for polarization reversal is a function of applied electric field E and is given by ⟨τ⟩=τ0 exp(α/E), where τ0 is the prefactor and α is the activation field. The measured activation fields depended on bias. Activation fields of 2.0 and 0.7 V/μm measured under high and low bias conditions are comparable to those observed for bulk barium titanate.

Non-equilibrium Dynamics of Interacting Tunneling States in Glasses

After the temporary application of a strong electric bias field to a glass sample at temperatures in the milli-Kelvin range its dielectric constant is increased and then decays slowly back to its equilibrium value. We studied these dielectric non-equilibrium properties of the polyester glass Mylar and the borosilicate glass BK7 as a function of bias field and temperature. We find that the decay to equilibrium depends on the duration of the applied bias field substantially only if the sample has been biased for several thousand seconds or longer. The decay curves after shorter bias field applications are influenced rather by the rate by which the field has been changed. Following the ”dipole gap” theory1 we assume that the observed excess dielectric response originates in the non-equilibrium dynamics of tunneling states (TSs) that are strongly coupled. Our analysis of the data within the framework of strongly coupled pairs of TSs indicates three competing equilibrium destroying processes, leading to the observed dynamics. Obviously the energy relaxation rate of TSs depends on the bias field since it changes the energy splitting of TSs by coupling to their dipole moments. In addition, a quick enough field sweep can drive TSs non-adiabatically between their energy eigen-states yielding sweep rate dependent decay times. The third process results in decay times independent of temperature and the bias field sweep rate and duration. We propose a picture where the field lifts the tunneling particle in potential wells beyond its original double well as the cause of the third contribution. The decay towards equilibrium is by quantum mechanical tunneling. Moreover, our observations indicate that below a material dependent temperature the relaxation of TSs is caused primarily by interactions between them. PACS numbers: 61.43.Fs, 05.70.Ln, 66.35.+a, 77.22.-d

A dual image approach for bias field correction in magnetic resonance imaging

In this paper, we propose a dual image approach to correcting intensity inhomogeneities for MR images acquired using surface coils. Previous methods are usually not satisfactory due to restricted application domains, considerable human interactions, or some undesirable artifacts. The proposed algorithm provides nice correction results for a variety of surface-coil MR images. It is accomplished by using an additional body-coil MR image of a smaller size captured at the same position as that of the surface-coil image to facilitate the estimation of the bias field function. The correction algorithm consists of aligning the surface-coil image with the body-coil image and fitting a spline surface from a sparse set of data points for the associated bias field function. Experiments on some real images show satisfactory correction results by using the proposed algorithm.

Dynamic magnetic and magnetoelastic properties of epoxy-TbFe 2 composites

The static and dynamic magnetic and magnetoelastic properties of epoxy-bonded TbFe 2 were investigated as functions of bias field, frequency and AC drive field. The peak in d 33 vs. bias field was shifted to higher bias field. A small negative Δ E effect was observed in field <80 kA/m. Both d 33 and k 33 were much lower, compared with bulk Terfenol-D or its composites.

Effective magnetostriction and magnetomechanical coupling of terfenol-D composites

Epoxy and glass matrix Terfenol-D composites have been produced by using a cold and hot compression-molding technique, respectively. The static and dynamic magnetic and magnetomechanical properties of samples with a volume fraction Vf in the range of 10% to 82% have been investigated as functions of bias field, frequency, and ac drive field. The epoxy composites have a greater saturation magnetostrain than the glass composite with the same Vf. The elastic modulus of the epoxy composites is dominated by that of the matrix, Em. Increasing the value of Em reduces the measured magnetostriction λc of the composites. A model, based on the strain related energy equilibrium, has been developed to describe the experimental results for Vf and Em dependences of λc and the magnetomechanical coupling coefficient k33. It is concluded that an optimum k33 value can be obtained by choosing a matrix with an appropriate elastic modulus.

Low temperature synthesis and electrical properties of epitaxial Sr0.8Bi2.2Ta2O9 thin films

High quality epitaxial thin films of Sr0.8Bi2.2Ta2O9 (SBT) were grown on LaNiO3 (LNO) bottom electrodes. The SBT/LNO heterostructure was fabricated on (001) oriented LaAlO3 substrates by pulsed laser ablation. X-ray diffraction and high resolution transmission electron microscopy revealed epitaxial growth of SBT and LNO layers along the (001) direction and sharp interfaces between the epilayers. The SBT films exhibited a dielectric constant of ∼270 and the loss tangent varied from 0.02 to 0.04. The dielectric constant measured as a function of bias field revealed that the films were not ferroelectric in nature. The room temperature frequency response of the dielectric constant was observed to obey Curie–von Schweidler power law with an exponent of 0.02 in the range of 10 kHz–1 MHz.

Elastic properties of magnetostrictive thin films using bending and torsion resonances of a bimorph

The modification of the elastic properties of giant magnetostriction alloy films due to an applied magnetic field (the ΔE effect), has been studied. Two different types of films were deposited on Si substrates: (i) single layers of TbDyFeCo alloys typically 1000 nm thick and (ii) nanocomposite multilayer films of FeCo/TbFeCo each having a typical thickness of 6 nm. Both types of films were rendered magnetically anisotropic with a well defined in-plane easy axis. Rectangular samples were cut out of these bimorphs and firmly glued at one end to a heavy base to form a simple cantilever structure. The variations of film elastic moduli were deduced from the shifts of the cantilever resonance frequencies as a function of bias field for two basic configurations: (i) field applied along the easy axis and (ii) field applied along the hard axis. In contrast with previous work, both flexural and torsion resonance modes were excited and studied. As a result the field induced variations of both planar traction modulus and the shear modulus were obtained and new interesting features were discovered. In particular strongly negative values of the shear modulus were observed (at least in the nanocomposite films) in the vicinity of the divergence in the transverse magnetic susceptibility at saturation field along the hard axis. A simple but complete theoretical analysis shows that the uniaxial anisotropy model together with the assumption of isotropic magnetoelastic coupling gives a good semiquantitative understanding of all the experimental results.

DIFFUSION UNDER CROSSED LOCAL AND GLOBAL BIASES IN DISORDERED SYSTEMS

Diffusion on 2D site percolation clusters at p = 0.7, 0.8, and 0.9 above pc on the square lattice in the presence of two crossed bias fields, a local bias B and a global bias E, has been investigated. The global bias E is applied in a fixed global direction whereas the local bias B imposes a rotational constraint on the motion of the diffusing particle. The rms displacement Rt ~ tk in the presence of both biases is studied. Depending on the strength of E and B, the behavior of the random walker changes from diffusion to drift to no-drift or trapping. There is always diffusion for finite B with no global bias. A crossover from drift to no-drift at a critical global bias Ec is observed in the presence of local bias B for all disordered lattices. At the crossover, value of the rms exponent changes from k = 1 to k &lt; 1, the drift velocity vt changes from constant in time t to decreasing power law nature, and the "relaxation" time τ has a maximum rate of change with respect to the global bias E. The value of critical bias Ec depends on the disorder p as well as on the strength of local bias B. Phase diagrams for diffusion, drift, and no-drift are obtained as a function of bias fields E and B for these systems.

Field-induced phase transitions in an antiferroelectric liquid crystal using the pyroelectric effect

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical polar phases. Measurements of pyroelectric response of these phases as a function of temperature and bias field have elucidated the ability of this method for investigating the nature of antiferroelectric phases and phase transitions under the bias field. The pyroelectric signal as a function of the bias field at fixed temperatures and as a function of temperature for fixed bias fields was measured for different phases of the investigated AFLC material. A theoretical model describing the pyroelectric response in different phases of AFLC is given, and the experimental results are interpreted. The threshold fields for field induced phase transitions are determined. The type of field induced phase transition from the AF phase in particular is found to be dependent on the temperature within its range. The properties of an unusual ferrielectric phase existing between ferrielectric chiral smectic-C (SmC*) and antiferroelectric AF phases are studied in a great detail. The results confirm that this phase is one of the incommensurate phases, predicted by the axial next-nearest neighbor Ising model and Landau model for this temperature region.

Dynamic magneto-mechanical properties of epoxy-bonded Terfenol-D composites

The dependence of the dynamic magneto-mechanical properties of epoxy-bonded Terfenol on the composition parameters of particle size ( P s) and volume fraction of Terfenol ( V f) is investigated as functions of bias field ( H) and high frequency ( f). Samples were prepared with powder in five size ranges between 106 and 710 μm using three volume fractions. The dynamic strain coefficient ( d) and magneto-mechanical coupling coefficient ( k) were found to be independent of P s and V f while the dynamic relative permeability ( μ r) was proportional to V f. d showed less variation with H up to 100 kA m −1, than for bulk Terfenol with d also exhibiting a flat response with frequency, up to 20 kHz. The frequency dependence of μ r, up to 200 kHz, was also flat apart from the occurrence, at ∼15 kHz, of a fundamental shape resonance which was also experienced by d. Classical eddy current theory was used to model the behaviour of μ r with f and it was found that experiment and theory were qualitatively similar.

Evolution of dielectric permittivity under applied field in PLZT 8.3/70/30 ceramics

The jump of dielectric permittivity as function of bias field applied after various duration of storage at the measurement temperature is investigated. The influence of storage time on ϵ(E=) depends on the value of bias field and frequency of measuring field. It is found that ϵ(E=) depends on the path of how particular field value is reached.