Biblio

Reliable calculation model of electromagnetic mechanism characteristics of DC contactor is of great significance to its structural optimization. In this paper, the excitation process of contactor magnet is summarized, and a new calculation model of hysteresis-finite element method is proposed. It can effectively calculate the remanence of the electromagnetic mechanism under different excitation conditions, and give the relationship curve between the remanence flux and the anti-remanence gap.

An improved current transformer model (CT) developed in DIgSILENT™ is presented in this paper. The hysteresis characteristics of magnetic material described by Jiles-Atheron theory are included. The results show that model can represent the saturation and remanence characteristics of CT core accurately. The model accuracy is verified by comparing the simulation results with PSCAD/EMTDC™.

Previous efforts to detect cross-instance cloud remanence have consisted of searching current instance unallocated space for fragments easily attributable to a prior user or instance, and results were necessarily dependent on the specific instances tested and the search terms employed by the investigator. In contrast, this work developed, tested, and applied a general method to detect potential cross-instance cloud remanence that does not depend on specific instances or search terms. This method collects unallocated space from multiple cloud virtual machine instances based on the same cloud provider template. Empty sectors and sectors which also appear in the allocated space of that instance are removed from the candidate remanence list, and the remaining sectors are compared to sectors from instances based on other templates from that same provider; a matching sector indicate potential cross-instance remanence. Matching sectors are further evaluated by considering contiguous sectors and mapping back to the source file from the other instance template, providing additional evidence that the recovered fragments may in fact be content from another instance. This work first found that unallocated space from multiple cloud instances based on the same template is not empty, random, nor identical - in itself an indicator of possible cross-instance remanence. This work also found sectors in unallocated space of multiple instances that matched contiguous portions of files from instances created from other templates, providing a focused area for determining whether cross-instance data remanence exists. This work contributes a general method to indicate potential cross-instance cloud data remanence which is not dependent on a specific provider or infrastructure, instance details, or the presence of specific user-attributable remnant fragments. A tool to implement the method was developed, validated, and then run on Amazon's AWS cloud service.

This paper presents an analytical method for predicting the electromagnetic performance in permanent magnet (PM) machine with the spoke-type rotor (STR) and a proposed hybrid rotor structure (HRS), respectively. The key of this method is to combine magnetic field analysis model (MFAM) with the magnetic equivalent circuit model. The influence of the irregular PM shape is considered by the segmentation calculation. To obtain the boundary condition in the MFAM, respectively, two equivalent methods on the rotor side are proposed. In the STR, the average flux density of the rotor core outer-surface is calculated to solve the Laplace's equation with considering for the rotor core outer-surface eccentric. In the HRS, based on the Thevenin's theorem, the equivalent parameters of PM remanence BreB and thickness hpme are obtained as a given condition, which can be utilized to compute the air-gap flux density by conventional classic magnetic field analysis model of surface-mounted PMs with air-gap region. Finally, the proposed analytical models are verified by the finite element analysis (FEA) with comparisons of the air-gap flux density, flux linkage, back-EMF and electromagnetic torque, respectively. Furthermore, the performance that the machine with the proposed hybrid structure rotor can improve the torque density as explained.

The process of release of a single domain wall from the closure domain structure at the microwire ends and the process of nucleation of the reversed domain in regions far from the microwire ends were studied using the technique that consists in determining the critical parameters of the rectangular magnetic field pulse (magnitude-Hpc and length-τc) needed for free domain wall production. Since these processes can be influenced by the magnitude of the magnetic field before or after the application of the field pulse (Hi, τ), we propose a modified experiment in which the so-called three-level pulse is used. The three-level pulse starts from the first level, then continues with the second measuring rectangular pulse (Hi, τ), which ends at the third field level. Based on the results obtained in experiments using three-level field pulses, it has been shown that reversed domains are not present in the remanent state in regions far from the microwire ends. Some modification of the theoretical model of a single domain wall trapped in a potential well will be needed for an adequate description of the depinning processes.

A Stoner-Wohlfarth-type model is used to demonstrate the effect of the buildup of magnetic charges near the grain boundaries of low anisotropy polycrystalline materials, revealed by measuring the magnetization during positive-field warming after negative-field cooling. The remnant magnetization after negative-field cooling has two different contributions. The temperature-dependent component is modeled as an assembly of particles with thermal relaxation. The temperature-independent component is modeled as an assembly of particles overcoming variable phenomenological energy barriers corresponding to the change in susceptibility when the anisotropy constant changes its sign. The model is applicable to soft-magnetic materials where the buildup of the magnetic charges near the grain boundaries creates demagnetizing fields opposing, and comparable in magnitude to, the anisotropy field. The results of the model are in qualitative agreement with published data revealing the magneto-thermal characteristics of polycrystalline gadolinium.

To decouple the multi-axis motion in the 6 degrees of freedom magnetically levitated actuators (MLAs), this paper introduces a numerical method to model the force and torque distribution. Taking advantage of the Gaussian quadrature, the concept of coil node is developed to simplify the Lorentz integral into the summation of the interaction between each magnetic node in the remanence region and each coil node in the coil region. Utilizing the coordinate transformation in the numerical method, the computation burden is independent of the position and the rotation angle of the moving part. Finally, the experimental results prove that the force and torque predicted by the numerical model are rigidly consistent with the measurement, and the force and torque in all directions are decoupled properly based on the numerical solution. Compared with the harmonic model, the numerical wrench model is more suitable for the MLAs undertaking both the translational and rotational displacements.

The understanding of measured jitter is improved in three ways. First, it is shown that the measured jitter is not only governed by written-in jitter and the reader resolution along the cross-track direction but by remanence noise in the vicinity of transitions and the down-track reader resolution as well. Second, a novel data analysis scheme is introduced that allows for an unambiguous separation of these two contributions. Third, based on data analyses involving the first two learnings and micro-magnetic simulations, we identify and explain the root causes for variations of jitter with write current (WC) (write field), WC overshoot amplitude (write-field rise time), and linear disk velocity measured for heat-assisted magnetic recording.

The goal of this paper is to simulate the interactions between magnetic objects in a physically correct way. The simulation scheme is based on magnetization dynamics, which describes the temporal change of magnetic moments. For magnetization dynamics, the Landau-Lifshitz-Gilbert equation is adopted, which is widely used in micromagnetics. Through effectively-designed novel models of magnets, it is extended into the macro scale so as to be combined with real-time rigid-body dynamics. The overall simulation is stable and enables us to implement mutual induction and remanence that have not been tackled by the state-of-the-art technique in magnet simulation. The proposed method can be applied to various fields including magnet experiments in the virtual world.

One method to increase bit density in magnetic memory devices is to use multi-state structures, such as a ferromagnetic nanoring with multiple domain walls (DWs), to encode information. However, there is a competition between decreasing the ring size in order to more densely pack bits and increasing it to make multiple DWs stable. This paper examines the effects of ring geometry, specifically inner and outer diameters (ODs), on the formation of 360° DWs. By sequentially increasing the strength of an applied circular magnetic field, we examine how DWs form under the applied field and whether they remain when the field is returned to zero. We examine the relationships between field strength, number of walls initially formed, and the stability of these walls at zero field for different ring geometries. We demonstrate that there is a lower limit of 200 nm to the ring diameter for the formation of any 360° DWs under an applied field, and that a high number of 360° DWs are stable at remanence only for narrow rings with large ODs.

Micromagnetic simulations of coercivity as a function of external magnetic field direction were performed for a hexagonal array of hemispherical Permalloy nanocaps. The analysis was based on hysteresis loops for arrangements of nanocaps of variable thickness (5 nm and 10 nm). The angular dependence of coercivity had a maximum at about 80° with respect to the arrangement plane. An increase in coercivity with nanocap thickness is related to the magnetization reversal mechanism, where the dipole energy of individual caps generates an effective intermediate axis, locking the magnetic moments. The coercivity has maximum values of 109 Oe for 5 nm and 156 Oe for 10 nm thickness. The remanence decreases monotonically with angle. This is associated with the influence of shape anisotropy, where the demagnetizing field in the plane of the array is much smaller than the demagnetizing field perpendicular to the plane.

In order to study the stress detection method on long-distance oil and gas pipeline, the distribution characteristics of the surface remanence signals in the stress concentration regions must be known. They were studied by using the magnetic domain model in the non-magnetic saturation state. The finite element method was used herein with the aim to analyse the static and mechanical characteristics of a ferromagnetic specimen. The variation law of remanence signal in stress concentration regions was simulated. The results show that a residue signal in the stress concentration region exists. In addition, a one-to-one correspondence in the non-magnetic saturation environment is evident. In the case of magnetic saturation, the remanence signal of the stress concentration region is covered and the signal cannot be recognised.

Transition noise and remanence noise are the two most important types of media noise in heat-assisted magnetic recording. We examine two methods (spatial splitting and principal components analysis) to distinguish them: both techniques show similar trends with respect to applied field and grain pitch (GP). It was also found that PW50can be affected by GP and reader design, but is almost independent of write field and bit length (larger than 50 nm). Interestingly, our simulation shows a linear relationship between jitter and PW50NSRrem, which agrees qualitatively with experimental results.

Various critical state models have been developed to understand the hysteresis loss mechanism of high-temperature superconducting (HTSC) films. The analytic relation between the hysteresis loss and the remanent field was obtained based on Bean's critical state model for thin films in the full-penetration case. Furthermore, numerical calculation of local hysteresis loops was carried out by Kim's critical state model. In this paper, we investigated local hysteresis losses for a GdBCO coated conductor by using low-temperature scanning Hall probe microscopy and reproduced the experimental results by applying the critical state model. Because of the demagnetizing effect in thin films, analysis of local hysteresis losses can be useful approach to understand of total hysteresis losses.

We demonstrate how a unique nanomodulation within a continuous ferromagnetic film can induce magnetic dipoles at predefined, submicrometer scale locations, which can tune the global magnetic properties of the film due to dipole-dipole interactions. Arrays of tunable magnetic dipoles are generated with in-plane and out-of-plane directions, which can be rotated in-plane within the three-dimensional (3-D) modulated structure of a continuous film. In-plane magnetic dipole rotation enables a methodology to control overall magnetic properties of a ferromagnetic thin film. Formation of magnetic dipoles and their tunability were studied in detail by magnetic force microscopy, high-resolution magnetic measurements, and micromagnetic simulation of a nanomodulated Ni45Fe55 alloy film. A pattern larger than a single magnetic domain would normally form a vortex in the remanent state. However, here the unique 3-D nanostructure prevents vortex formation due to the competition between in-plane and out-of-plane dipole-dipole interaction giving rise to a metastable state. Experimentally, at zero remanence, the magnetization goes through a transformation from a metastable to a stable state, where the dipole-dipole interaction depends on their geometrical arrangement. Thus, the magnetic properties of the continuous film can be varied by the proposed pattern geometry. A detail analytical study of the dipolar energy for the system agrees well with the experimental and simulated results.

Having significant role in the storing, delivering and conversion of the energy, the permanent magnets are key elements in the actual technology. In many applications, the gap between ferrites and rare earths (RE) based sintered permanent magnets is nowadays filled by RE bonded magnets, used in more applications, below their magnetic performances. Therewith, the recent trends in the RE market concerning their scarcity, impose EU to consider alternative magnets (without RE) to fill such gap. The paper presents the chemical synthesis of the exchange coupled SrFe12O19/CoFe2O4 nanocomposites, based on nanoferrites. The appropriate annealing leads to the increasing of the main magnetic characteristics, saturation magnetization MS and intrinsic coercivity Hc, in the range of 49 - 53 emu/g, respectively 126.5 - 306 kA/m. The value reached for the ratio between remanent magnetization and saturation magnetization is higher than 0.5, fact that proved that between the two magnetic phases occurred exchange interaction.

Summary form only given. All-optical switching (AOS) has been observed in ferromagnetic (FM) layers and synthetic ferrimagnetic heterostructures [1-4]. In this work, we use anomalous Hall effect (AHE) measurements to demonstrate controlled helicity-dependent switching in synthetic ferrimagnetic heterostructures. The two FM layers are engineered to have different Curie temperatures Tc1 (fixed) and Tc2 (variable). We show that irrespective of whether Tc2 is higher or lower than Tc1, the final magnetic configuration of the heterostructure is controlled by using the laser polarization to set the magnetic state of the FM layer with the highest Tc. All samples were grown on glass substrates at room temperature by DC magnetron sputtering. Two sets of samples were prepared. The first set are single FM layers with layer composition Ta (3 nm)/Pt (4 nm)/FM1(2)/Pt capping (4 nm), where FM1 = Co (0.6 nm) is a Co layer and FM2 = CoFeB (tCoFeB)/Pt(0.4 nm)/ CoFeB (tCoFeB) (0.2 ≤ tCoFeB ≤ 0.6 nm) is a composite CoFeB layer where both CoFeB layers are ferromagnetically coupled and act as a single layer. FM1 and FM2 were used to produce the second set of synthetic ferrimagnetic samples with layer structure Ta (3 nm)/Pt (4 nm)/FM1/Pt (0.4 nm)/Ru (0.9 nm)/Pt (0.4 nm)/FM2/Pt capping (4 nm). The Ru layer provides the antiferromagnetic RKKY interlayer exchange coupling between the adjacent FM1 and FM2 layers while the Pt layers on either side of the Ru layer can tune the strength of the coupling and stabilize their perpendicular anisotropy [5]. To study the AOS, we use a Ti: sapphire fs-laser with a wavelength of 800 nm and a pulse duration of 43 fs. A quarter-wave plate is used to create a circularly polarized [right(σ+) and left-handed (σ-)] beam. We first measured the magnetic properties of the FM1 and FM2 layers using vibrating sample magnetometry (VSM). All FM samples show full remanence in perpendicular hyst- resis loops at room temperature (not shown). The temperature-dependent magnetization scans (not shown) give a Curie temperature Tc1 of 524 K for FM1. For FM2, increasing tCoFeB increases its Curie temperatureTc2. At tCoFeB = 0.5 nm, Tc2 - Tc1. Hall crosses are patterned by optical lithography and ion milling. The width of the current carrying wire is - 5 um, giving a DC current density of - 6 x 109 A/m2 during the measurement. Figure 1(a) shows the resulting perpendicular Hall hysteresis loop of the synthetic ferrimagnetic sample with tCoFeB = 0.2 nm. At remanence, the stable magnetic configurations are the two antiparallel orientations of FM1 and FM2 [State I and II in Fig. 1(a)]. To study the AOS, we swept the laser beam with a power of 0.45 mW and a speed of 1 μm/sec across the Hall cross, and the corresponding Hall voltage was constantly monitored. In Fig. 1(b), we show the normalized Hall voltage, VHall, as a function of the laser beam position x for both beam polarizations σ+ and σ-. The initial magnetic configuration was State I. When the beam is at the center of the cross (position B), both beam polarizations give VHall - 0. As the beam leaves the cross (position C), the σbeam changes the magnetic configurations from State I to State II (FM1 magnetization pointing down), while the system reverts to State I using the σ+ beam. Changing the initial configuration from State I to State II results in the same final magnetic configurations, determined by the laser beam polarizations (not shown). Similar results (not shown) were obtained for samples with tCoFeB ≤ 0.4 nm. However, at tCoFeB = 0.6 nm, the σbeam results in the final magnetic configurations with FM2 magnetization pointing down (State I) and the σ+ beam results in the State II configuration, suggesting that the final state is determined by the beam polar

Alternatives to rare earth permanent magnets, such as alnico, will reduce supply instability, increase sustainability, and could decrease the cost of permanent magnets, especially for high-temperature applications, such as traction drive motors. Alnico magnets with moderate coercivity, high remanence, and relatively high-energy product are conventionally processed by directional solidification and (significant) final machining, contributing to increased costs and additional material waste. Additive manufacturing (AM) is developing as a cost effective method to build net-shape 3-D parts with minimal final machining and properties comparable to wrought parts. This paper describes initial studies of net-shape fabrication of alnico magnets by AM using a laser engineered net shaping (LENS) system. High-pressure gas atomized pre-alloyed powders of two different modified alnico “8” compositions, with high purity and sphericity, were built into cylinders using the LENS process, and followed by heat treatment. The magnetic properties showed improvement over their cast and sintered counterparts. The resulting alnico permanent magnets were characterized using scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, and hysteresisgraph measurements. These results display the potential for net-shape processing of alnico permanent magnets for use in next generation traction-drive motors and other applications requiring high temperatures and/or complex engineered part geometries.

The heat load of the original cryomodules for the continuous electron beam accelerator facility is 50% higher than the target value of 100 W at 2.07 K for refurbished cavities operating at an accelerating gradient of 12.5 MV/m. This issue is due to the quality factor of the cavities being 50% lower in the cryomodule than when tested in a vertical cryostat, even at low RF field. Previous studies were not conclusive about the origin of the additional losses. We present the results of a systematic study of the additional losses in a five-cell cavity from a decommissioned cryomodule after attaching components, which are part of the cryomodule, such as the cold tuner, the He tank, and the cold magnetic shield, prior to cryogenic testing in a vertical cryostat. Flux-gate magnetometers and temperature sensors are used as diagnostic elements. Different cool-down procedures and tests in different residual magnetic fields were investigated during the study. Three flux-gate magnetometers attached to one of the cavities installed in the refurbished cryomodule C50-12 confirmed the hypothesis of high residual magnetic field as a major cause for the increased RF losses.

In order to investigate the relationship and effect on the performance of magnetic modulator among applied DC current, excitation source, excitation loop current, sensitivity and induced voltage of detecting winding, this paper measured initial permeability, maximum permeability, saturation magnetic induction intensity, remanent magnetic induction intensity, coercivity, saturated magnetic field intensity, magnetization curve, permeability curve and hysteresis loop of main core 1J85 permalloy of magnetic modulator based on ballistic method. On this foundation, employ curve fitting tool of MATLAB; adopt multiple regression method to comprehensively compare and analyze the sum of squares due to error (SSE), coefficient of determination (R-square), degree-of-freedom adjusted coefficient of determination (Adjusted R-square), and root mean squared error (RMSE) of fitting results. Finally, establish B-H curve mathematical model based on the sum of arc-hyperbolic sine function and polynomial.

Ensemble waveform analysis is used to calculate signal to noise ratio (SNR) and other recording characteristics from micromagnetically modeled heat assisted magnetic recording waveforms and waveforms measured at both drive and spin-stand level. Using windowing functions provides the breakdown between transition and remanence SNRs. In addition, channel bit density (CBD) can be extracted from the ensemble waveforms using the di-bit extraction method. Trends in both transition SNR, remanence SNR, and CBD as a function of ambient temperature at constant track width showed good agreement between model and measurement. Both model and drive-level measurement show degradation in SNR at higher ambient temperatures, which may be due to changes in the down-track profile at the track edges compared with track center. CBD as a function of cross-track position is also calculated for both modeling and spin-stand measurements. The CBD widening at high cross-track offset, which is observed at both measurement and model, was directly related to the radius of curvature of the written transitions observed in the model and the thermal profiles used.

A significant advance in magnetic field management in a fully assembled superconducting radiofrequency cryomodule has been achieved and is reported here. Demagnetization of the entire cryomodule after assembly is a crucial step toward the goal of average magnetic flux density less than 0.5 μT at the location of the superconducting radio frequency cavities. An explanation of the physics of demagnetization and experimental results are presented.

The start-up value of an SRAM cell is unique, random, and unclonable as it is determined by the inherent process mismatch between transistors. These properties make SRAM an attractive circuit for generating encryption keys. The primary challenge for SRAM based key generation, however, is the poor stability when the circuit is subject to random noise, temperature and voltage changes, and device aging. Temporal majority voting (TMV) and bit masking were used in previous works to identify and store the location of unstable or marginally stable SRAM cells. However, TMV requires a long test time and significant hardware resources. In addition, the number of repetitive power-ups required to find the most stable cells is prohibitively high. To overcome the shortcomings of TMV, we propose a novel data remanence based technique to detect SRAM cells with the highest stability for reliable key generation. This approach requires only two remanence tests: writing `1' (or `0') to the entire array and momentarily shutting down the power until a few cells flip. We exploit the fact that the cells that are easily flipped are the most robust cells when written with the opposite data. The proposed method is more effective in finding the most stable cells in a large SRAM array than a TMV scheme with 1,000 power-up tests. Experimental studies show that the 256-bit key generated from a 512 kbit SRAM using the proposed data remanence method is 100% stable under different temperatures, power ramp up times, and device aging.

The Cloud Computing is a developing IT concept that faces some issues, which are slowing down its evolution and adoption by users across the world. The lack of security has been the main concern. Organizations and entities need to ensure, inter alia, the integrity and confidentiality of their outsourced sensible data within a cloud provider server. Solutions have been examined in order to strengthen security models (strong authentication, encryption and fragmentation before storing, access control policies...). More particularly, data remanence is undoubtedly a major threat. How could we be sure that data are, when is requested, truly and appropriately deleted from remote servers? In this paper, we aim to produce a survey about this interesting subject and to address the problem of residual data in a cloud-computing environment, which is characterized by the use of virtual machines instantiated in remote servers owned by a third party.

Several applications adopt electromagnetic sensors, that base their principle on the presence of magnets realized with specific magnetic materials that show a rather high remanence, but low coercivity. This work concerns the production, analysis and characterization of hybrid composite materials, with the use of metal powders, which aim to reach those specific properties. In order to obtain the best coercivity and remanence characteristics various "recipes" have been used with different percentages of soft and hard magnetic materials, bonded together by a plastic binder. The goal was to find out the interdependence between the magnetic powder composition and the characteristics of the final material. Soft magnetic material (special Fe powder) has been used to obtain a low coercivity value, while hard materials were primarily used for maintaining a good induction remanence; by increasing the soft proportion a higher magnetic permeability has been also obtained. All the selected materials have been characterized and then tested; in order to verify the validity of the proposed materials two practical tests have been performed. Special magnets have been realized for a comparison with original ones (AlNiCo and ferrite) for two experimental cases: the first is consisting in an encoder realized through a toothed wheel, the second regards the special system used for the electric guitars.