Local Bias Field Research Articles

Adaptive active contour model based on local bias field estimation and saliency

Some active contour models proposed based on intensity inhomogeneity are sensitive to initialization and cannot achieve ideal segmentation results for real images. An adaptive active contour model based on local bias field estimation and saliency is proposed in this paper. First of all, this model proposes an adaptive multi-local search algorithm, which avoids the initialization sensitivity by adaptively setting of the initial contour; Secondly, the local bias field is estimated by fusing the saliency map and fuzzy c-means clustering; Finally, the new bias field and the corrected energy fitting constant are used to define the new energy functional. The desired target object is obtained by minimizing the energy functional. The experimental results show that the segmentation accuracy of the model proposed in this paper is higher than that of the models participating in the comparison. The proposed model can not only avoid the interference of initialization and redundant information, but also segment images with intensity inhomogeneity effectively.

Enhanced unipolar electrical fatigue resistance and related mechanism in grain-oriented Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 piezoceramics

Piezoceramics with high and fatigue-resisted piezoelectric properties are strongly desired for actuator applications. In this work, textured Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 ceramics with Lotgering factor F001∼98% were fabricated by templated grain growth technique. Strong [001]c-grain orientation (f∼90% and r∼0.22) of the textured ceramics effectively produced about 230% enhanced piezoelectric coefficient d33* (i.e., Smax/Emax) and substantially improved unipolar electrical fatigue resistance. Unipolar polarization Pmax and d33* of the textured ceramics were nearly maintained up to 106 unipolar cycles, while 19% and 14% degradations were respectively observed from randomly oriented counterparts. Especially, normalized d33* of the textured ceramics shows better unipolar fatigue resistance than those of piezoceramics reported previously. Much lower bipolar strain asymmetry γs (∼4%) was observed from the textured samples fatigued after 106 unipolar cycles as compared to γs∼23% for randomly oriented counterparts. While charged defect accumulation model described the serious fatigue deteriorations in randomly oriented ceramics, the current work revealed that substantially enhanced unipolar fatigue resistance of the textured ceramics is mainly associated with the inherent fatigue anisotropy, weakened local bias fields owing to both enhanced domain mobility and lower defect density near grain boundaries/interfaces, and increased intrinsic contribution due to more tetragonal content. These superior characteristics suggest the great potential of textured ceramics for high-performance and robust actuator applications.

Advanced Hall Magnetometer with improved performance

A sharp jump in the magnetic response near the first critical twins magnetic field Hic1 was generated under the combined action of a constant biased and a local oscillating damped magnetic fields on the surface of the YBa2Cu3O7-x epitaxial film. The possibility of increasing due to this jump the sensitivity of a standard Hall magnetometer from ~2.5·10-3 Gs/Hz1/2 to ~8·10-7 Gs/Hz1/2 in the frequency range ~1 kHz has been demonstrated. The range of the measured magnetic fields of the magnetometer was extended to the region of weak fields ~(8·10-7-2.5·10-3) Oe, while maintaining the accuracy and linearity of the order of ~0.01%. To increase the sensitivity, linearity and noise sustainability the beginning of the operating point of the magnetometer was shifted to the twins values Hic1. The increased spatial resolution of the magnetometer, determined by the linear dimensions of the YBa2Cu3O7-x film twins, was provided by an increase in the total magnetic field directed perpendicular to the film surfaces and the Hall transducer and reached ~300 nm. Keywords: the twin high temperature superconductor, the density jump of the trapped magnetic flux, biased magnetic field, oscillating damped local magnetic bias field, two-stage Hall magnetometer.

Non-plasmonic improvement in photoconductive THz emitters using nano- and micro-structured electrodes.

We investigate here terahertz enhancement effects arising from micrometer and nanometer structured electrode features of photoconductive terahertz emitters. Nanostructured electrode based emitters utilizing the palsmonic effect are currently one of the hottest topics in the research field. We demonstrate here that even in the absence of any plasmonic resonance with the pump pulse, such structures can improve the antenna effect by enhancing the local d.c. electric field near the structure edges. Utilizing this effect in Hilbert-fractal and grating-like designs, enhancement of the THz field of up to a factor of ∼ 2 is observed. We conclude that the cause of this THz emission enhancement in our emitters is different from the earlier reported plasmonic-electrode effect in a similar grating-like structure. In our structure, the proximity of photoexcited carriers to the electrodes and local bias field enhancement close to the metallization cause the enhanced efficiency. Due to the nature of this effect, the THz emission efficiency is almost independent of the pump laser polarization. Compared to the plasmonic effect, these effects work under relaxed device fabrication and operating conditions.

Involvement of Unsaturated Switching in the Endurance Cycling of Si‐doped HfO2 Ferroelectric Thin Films

AbstractIn the past decade, extensive studies on HfO2‐based ferroelectric thin films have been carried out in order to realize their application in high‐density, low‐power, and high‐speed nonvolatile memories. However, understanding the field cycling endurance, which is extremely important for device performance, is still a serious issue. In this work, the frequency and field dependent subcycling behavior is carefully examined. The differences in split‐up effects induced at different subcycling conditions are analyzed by comparing the evolution of polarization‐electric field and transient current hysteresis loops monitored by two subsequent triangular voltage waveforms with different amplitudes. Using the first‐order reversal curves, the evolution of the switching and backswitching field distribution before and after numerous subcycles is characterized. An intuitive model based on the migration of oxygen vacancies from switchable regions to non‐switchable regions and subsequent movement toward top/bottom electrode interface driven by the transient depolarization field is proposed to explain the generation and enhancement of local bias fields.

Programmable Spin–Orbit‐Torque Logic Device with Integrated Bipolar Bias Field for Chirality Control

AbstractDriven by the need to address both the von Neumann bottleneck and scaling limits predicted by Moore's law, spintronic devices have been shown to be strong contenders for logic‐in‐memory applications. While several field‐free spin–orbit torque (SOT)‐driven logic devices have been proposed, their operation typically requires additional initialization or reset pulses, the exchange‐coupled canted spins reduce both anomalous Hall sign‐to‐noise ratio as well as thermal stability of the ferromagnetic layer, and device‐to‐device variation in exchange coupling strength is expected. A reconfigurable SOT‐driven logic device using a double Hall cross structure with an integrated bias field line for the generation of a local bias field is experimentally demonstrated. The on‐chip bipolar bias field can be toggled to flip the SOT‐induced switching chirality, and to assist with deterministic SOT magnetization switching, thereby enabling on‐the‐fly reconfigurability of the logic device to function as one of the several possible logic gates, e.g., AND, NOR, XNOR, XOR, NIMP, and converse NIMP. It is then shown through compact‐modeling and circuit simulation that the applications of such reconfigurable logic devices can be further expanded to build half‐adders.

Local difference‐based active contour model for medical image segmentation and bias correction

This study proposes a local bias field and difference estimation (LBDE) model for medical image segmentation and bias field correction. Firstly, the LBDE model uses a linear combination of a given set of smooth orthogonal basis functions, which is called Chebyshev polynomial, to estimate the bias field. Then, a clustering criterion function is defined by considering the difference between the measured image and approximated image in a small region. By applying this difference in the local region, the LBDE model can obtain accurate segmentation results and estimation of the bias field. Finally, the energy functional is incorporated into a level set formulation with a regularisation term, and it is minimised via the level set evolution process. The LBDE model first appears as a two-phase model and then extends to the multi-phase one. Extensive experiments on medical images demonstrate that the LBDE model achieves more precise segmentation results in terms of Jaccard similarity and dice similarity coefficient than the comparative models. Therefore the proposed model can increase the segmentation accuracy and robustness to noise.

AFM-tip written normal and anomalous domains in PMN-0.4PT crystals

At present, crystals of the solid solutions Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) are under intensive investigation due to their excellent piezoelectric characteristics. Studies in domain engineering are of importance for an insight into the contribution from the domain formation and resulting domain-wall density to piezoelectric coefficients. In this work, the fabrication of domains by dc AFM-tip voltages was studied in the tetragonal PMN-0.4PT crystals. Two types of domains were observed, either uniformly polarized along the poling field (the “normal” ones), or, following the commonly accepted term, “anomalous” ones containing a small area with the polarization directed oppositely to the poling field. In this work, for the first time, the correlation between the domain shape and the local piezoelectric hysteresis loops Hω−Utip was found, namely, the larger is the local bias voltage Ub, the higher is the tip voltage Utip at which the anomalous domains appear. This finding relates the observed scatter in the exposure conditions corresponding to the anomalous domains formation, to the nonuniform spatial distribution of the local bias fields. The domain diameter D vs Utip and pulse duration tp were investigated for Utip ≤ 50 V and tp from 10 ms to 10 s. The exposure characteristics are independent of the domain shape. For a given tp, D(Utip) is described by a unified linear function in the whole Utip range. The curves D(tp) follow a power law D∼tpk with the exponent k varying very weakly with Utip. The relaxation kinetics of anomalous domains depends on the writing conditions. The normal domains are found to decay significantly faster than the anomalous ones, with the decay kinetics depending on the domain spacing.

Active contour model based on local bias field estimation for image segmentation

The active contour model is a commonly used image segmentation method. When applied to complex images, such as images with grayscale inhomogeneity, most existing active contour models produce poor segmentation results. In order to solve this problem, we propose an active contour model based on local bias field estimation (LBFE), which makes the improved model better able to segment complex images. Firstly, we propose a new function to compute the value of bias field with fuzzy c-means clustering algorithm. This computation is completed before the iteration, which greatly improves the compute speed. Secondly, compute minimization with the energy function in the bias correction model (BC) proposed by Li et al. Thirdly, a new variational level set function is proposed to limit the segmentation range and greatly improve the robustness. Experiment results have proved that the proposed model not only segments images with intensity inhomogeneity effectively and shortens time spent, but also shows a better robustness to initialization and a higher segmentation accuracy than other classic models.

Automated 3-D lung tumor detection and classification by an active contour model and CNN classifier

The World Health Organization (WHO) recently reported that the lung tumor was the leading cause of death worldwide. In this study, a practical computer-aided diagnosis (CAD) system is developed to increase a patient's chance of survival. Segmentation is acritical analysis tool for dividing a lung image into several sub-regions. This work characterized an automated 3-D lung segmentation tool modeled by an active contour model for computed tomography (CT) images. The proposed segmentation model is used to integrate the local image bias field formulation with the active contour model (ACM). Here, a local energy term is specified by using the mean squared error to reconcile severely in homogeneous CT images and used to detect and segment tumor regions efficiently with intensity inhomogeneity. In addition, a Multiscale Gaussian distribution was applied to the CT images for smoothening the evolution process, and features were determined. For proposed model evaluation, were used the Lung Image Database Consortium (LIDC-IDRI) data set that consisted of 850 lung nodule-lesion images that were segmented and refined to generate accurate 3D lesions of lung tumor CT images. Tumor portions were extracted with 97% accuracy. Using continuous feature extraction of 3-D images leads to attributing the deformation and quantifies the centroid displacement. In this work, predict the centroid displacement and contour points by a curve evolution method which results in more accurate predictions of contour changes and than the extracted images were classified using an Enhanced Convolutional Neural Network (CNN) Classifier. The experimental result shows that the modified Computer Aided Diagnosis (CAD) system has a high ability to acquire good accuracy and assures automated diagnosis of a lung tumor.

Tunable ground state in heterostructured artificial spin ice with exchange bias

We describe an artificial spin ice (ASI) composed of exchange biased heterostructured nanomagnetic elements with unidirectional anisotropy and compare it with a conventional ASI constituted by ferromagnets with uniaxial anisotropy (Ising spins). The introduction of a local exchange bias field, aligned along one of the sublattices of the square ASI, lifts the spin-reversal symmetry of the vertices. By varying the lattice constant of the square array, we control the ratio of exchange bias (EB) to dipolar field (${H}_{\mathrm{EB}}/{H}_{\mathrm{dip}}$) and tune the ground state from an antiferromagnetic to a ferromagnetic configuration with an effective magnetic moment. The geometric frustration of dipolar interactions is moderated by a nonfrustrated local field, leading to a mesoscopic system with specific metastable states observed during the demagnetization process.

Adiabatic quantum simulations with driven superconducting qubits

We propose a quantum simulator based on driven superconducting qubits where the interactions are generated parametrically by a polychromatic magnetic flux modulation of a tunable bus element. Using a time-dependent Schrieffer-Wolff transformation, we analytically derive a multi-qubit Hamiltonian which features independently tunable $XX$ and $YY$-type interactions as well as local bias fields over a large parameter range. We demonstrate the adiabatic simulation of the ground state of a hydrogen molecule using two superconducting qubits and one tunable bus element. The time required to reach chemical accuracy lies in the few microsecond range and therefore could be implemented on currently available superconducting circuits. Further applications of this technique may also be found in the simulation of interacting spin systems.

Local and Global Active Contour Model for Image Segmentation With Intensity Inhomogeneity

The active contour model (ACM) is a popular approach for image segmentation. Many existing ACMs perform poorly in severe inhomogeneous images. To address this issue, a novel local and global ACM (LaG_ACM) is proposed in this paper. First, we define a global fitting image formulation that encodes the global property of an image and a global energy term using the relative entropy between the original image and the proposed global fitting image formulation. Then, a local image bias field formulation is defined to extract the local image information and to estimate the bias field. By integrating the proposed local image bias field formulation with the ACM, we specify a local energy term using the mean squared error to accommodate severe inhomogeneous images. More importantly, we define an adaptive weighting function using image entropy, which can automatically adjust the weight between the local and global energy terms according to the degree of intensity inhomogeneity. Finally, the experimental results on images with different degrees of intensity inhomogeneity validate the favorable performance of the LaG_ACM.

Dynamic in situ observation of voltage-driven repeatable magnetization reversal at room temperature

Purely voltage-driven, repeatable magnetization reversal provides a tantalizing potential for the development of spintronic devices with a minimum amount of power consumption. Substantial progress has been made in this subject especially on magnetic/ferroelectric heterostructures. Here, we report the in situ observation of such phenomenon in a NiFe thin film grown directly on a rhombohedral Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) ferroelectric crystal. Under a cyclic voltage applied perpendicular to the PMN-PT without a magnetic field, the local magnetization of NiFe can be repetitively reversed through an out-of-plane excursion and then back into the plane. Using phase field simulations we interpret magnetization reversal as a synergistic effect of the metastable ferroelastic switching in the PMN-PT and an electrically rotatable local exchange bias field arising from the heterogeneously distributed NiO clusters at the interface.

Magnetic characteristics of a high-layer-number NiFe/FeMn multilayer

We report the static and dynamic magnetic characteristics of a high-layer-number NiFe/FeMn multilayer test structure with potential applications in broadband absorber and filter devices. To allow fine control over the absorption linewidths and to understand the mechanisms governing the resonances in a tailored structure similar to that expected to be used in real world applications, the multilayer was intentionally designed to have layer thickness and interface roughness variations. Magnetometry measurements show that the sample has complex hysteresis loops with features consistent with single ferromagnetic film reversals. Characterisation by transmission electron microscopy allows us to correlate the magnetic properties with structural features, including the film widths and interface roughnesses. Analysis of resonance frequencies from broadband ferromagnetic resonance measurements as a function of field magnitude and orientation provide values of the local exchange bias, rotatable anisotropy, and uniaxial anisotropy fields for specific layers in the stack and explain the observed mode softening. The linewidths of the multilayer are adjustable around the bias field, approaching twice that seen at larger fields, allowing control over the bandwidth of devices formed from the structure.

Complex Internal Bias Fields in Ferroelectric Hafnium Oxide.

For the rather new hafnia- and zirconia-based ferroelectrics, a lot of questions are still unsettled. Among them is the electric field cycling behavior consisting of (1) wake-up, (2) fatigue, and (3) the recently discovered subcycling-induced split-up/merging effect of transient current peaks in a hysteresis measurement. In the present work, first-order reversal curves (FORCs) are applied to study the evolution of the switching and backswitching field distribution within the frame of the Preisach model for three different phenomena: (1) The pristine film contains two oppositely biased regions. These internal bias fields vanish during the wake-up cycling. (2) Fatigue as a decrease in the number of switchable domains is accompanied by a slight increase in the mean absolute value of the switching field. (3) The split-up effect is shown to also be related to local bias fields in a complex situation resulting from both the field cycling treatment and the measurement procedure. Moreover, the role of the wake-up phenomenon is discussed with respect to optimizing low-voltage operation conditions of ferroelectric memories toward reasonably high and stable remanent polarization and highest possible endurance.

Embedded ferromagnetic microwires for monitoring tensile stress in polymeric materials

Considerable efforts have been made to develop testing non-destructive methods for polymer composite materials. We would like to introduce researchers in the field of smart materials to a new method of monitoring internal stresses. The method can be classified as an embedded sensing technique, where the sensing element is a glass-coated ferromagnetic microwire with a specific magnetic anisotropy. With a diameter 10–100μm, the microwire impedance acts as the controlled parameter which is monitored for a weak alternating current (AC) in the MHz range. The microwire impedance becomes stress sensitive in the presence of a weak constant axial bias magnetic field. This external parameter allows the impedance stress sensitivity to be easily tuned. In addition, a local bias field may also allow the reconstruction of stress profile when it is scanned along the microwire. The experimental results are analysed using simple magnetostatic and impedance models.

Single-domain shape anisotropy in near-macroscopic Ni80Fe20 thin-film rectangles

Shape anisotropy provides a simple mechanism to adjust the local bias field in patterned structures. It is well known that for ellipsoidal particles <1 μm in size, a quasi-single domain state can be realized with uniform anisotropy field. For larger patterned ferromagnetic thin-film elements, domain formation is thought to limit the effectiveness of shape anisotropy. In our work, we show that very soft lithographically patterned Ni80Fe20 films with control of induced magnetic anisotropy can exhibit shape anisotropy fields in agreement with single-domain models, for both hysteresis loop measurements at low field and ferromagnetic resonance measurements at high field. We show the superiority of the fluxmetric form over the magnetometric form of anisotropy estimate for thin films with control dimensions from 10 μm to 150 μm and in-plane aspect ratios above 10.

Bias field reduction by localized Lloyd–Max quantization

Bias field reduction is a common problem in medical imaging. A bias field usually manifests itself as a smooth intensity variation across the image. The resulting image inhomogeneity is a severe problem for posterior image processing and analysis techniques such as registration or segmentation. In this article, we present a novel debiasing technique based on localized Lloyd–Max quantization (LMQ). The local bias is modeled as a multiplicative field and is assumed to be slowly varying. The method is based on the assumption that the global, undegraded histogram is characterized by a limited number of gray values. The goal is then to find the discrete intensity values such that spreading those values according to the local bias field reproduces the global histogram as good as possible. We show that our method is capable of efficiently reducing (even strong) bias fields in 3D volumes.

Geomagnetic observations on Tristan da Cunha, South Atlantic Ocean

Few geomagnetic ground observations exist of the Earth's strongest core field anomaly, the South Atlantic Anom- aly (SAA). The geomagnetic repeat station on the island Tristan da Cunha, located half-way between South Africa and South America at 37° 05' S, 12° 18' W, is therefore of crucial importance. We have conducted several sets of repeat station measurements during magnetically quiet conditions (Kp 2o or less) in 2004. The procedures are de- scribed and the results are compared to those from earlier campaigns and to the predictions of various global field models. Features of the local crustal bias field and the solar quiet daily variation are discussed. We also evaluate the benefit of continuous magnetic field recordings from Tristan da Cunha, and argue that such a data set is a very valuable addition to geomagnetic satellite data. Recently, funds were set up to establish and operate a magnetome- ter station on Tristan da Cunha during the Swarm magnetic satellite mission (2011-2014).