Frequency Of Ac Field Research Articles

Harmonic AC magnetic susceptibility analysis of FeSe crystals with composite morphology

The AC magnetic response of multi-domain FeSe crystals has been explored by means of fundamental and third harmonic AC magnetic susceptibility (ACMS) analysis. Our previous studies have revealed a complex morphology which especially modifies the mixed state properties. The effects were expressed by an additional ‘pseudo’ peak feature of the magnetic hysteresis and in the present study we analyze its vortex dynamics nature and irreversibility behavior in the context of ACMS. We find that the effect is detectable mainly with the sensitive third harmonic component and especially at high AC field frequencies. From the analysis, the temperature dependence of the ‘pseudo’ peak effect at different DC fields ranges was determined, confirming the previously established superconductor–normal metal–superconductor type links between the domains in the crystal. The irreversibility line, the most important parameter for high power applications, shows a typical glass/liquid transition in the vortex matter. In addition we have observed surface barrier effects and vortex avalanche activity, the behavior of which appears to be influenced by the superconducting and magnetic nature coexistence and morphology. The presented results show the effective application of the harmonic AC magnetic susceptibility technique for a versatile analysis of complex nonlinear phenomena in materials with a sophisticated AC magnetic response.

Design and Development of Bismuth Ferrite Based Environmental Friendly Multiferroic for Devices

Abstract Recently, researches are mainly focussed on the development of eco-friendly materials for multifunctional devices. In the present communication, synthesis and detailed characterization of Bi(Ni0.30Ti0.30Fe0.40)O3 multiferroic material has been reported. This compound has been synthesized via a cost-effective (solid-state reaction or mixed oxide) technique in air atmosphere. Based on the analysis of X-ray diffraction data (collected on powder sample) crystal system, unit cell dimension and crystallite (particle) size of material were determined. The calculated particle size of the material is in the range of 28 to 52 nm. Analysis of the field emission scanning electron micrograph (FE-SEM) depicts uniform distribution of grains of varying dimension. Furthermore, the correlation between dielectric and impedance parameters is established using experimental data collected by a phase sensitive multimeter at various temperature and frequency. The impedance spectroscopic studies are performed in a broad range of temperature (300–780 K) and ac field frequencies (103 –106 Hz). As a result, the addition of Ni (nickel) and Ti (titanium) at the Fe (iron) site of BiFeO3 enhances electrical as well as multiferroic properties of the material.

Study of VLF wave with relativistic effect in Saturn magnetosphere in the presence of parallel A.C. electric field

Cassini radio and plasma wave surveys aim to study radio emissions, plasma waves, thermal plasma and dust near Saturn. Using the characteristic solution and dynamics method, the influence of electron beam on the loss cone and bi-Maxwellian distribution of whistler mode waves in the parallel alternating electric field and magnetic field is studied. The dispersion relation and the growth rate of Saturn's magnetic layer were deduced and calculated in detail. Parameter analysis is performed by changing the parameters of the plasma like number density, AC frequency, temperature anisotropy, etc. The influence of AC frequency on Doppler shift and the comparative study of growth rate of oblique and parallel propagating waves are analyzed using generalized distribution function. We found temperature anisotropy AT=1.25 can explain the linear spatiotemporal growth rate of whistler mode waves. It provides the majority of the observed frequency integral power. It can be seen that the effective parameters for the generation of Whistler mode waves are not only temperature anisotropy, but also the relativistic factors discussed in the results and discussion section, and the AC field frequency and width of the loss cone distribution function.

Investigation of Electrochemical, Optical and Thermal Effects during Flash Sintering of 8YSZ.

This paper reports the electrochemical, optical and thermal effects occurring during flash sintering of 8 mol % yttria-stabilized zirconia (8YSZ). In-situ observations of polycrystalline and single crystal specimens revealed electrochemical blackening/darkening during an incubation period prior to flash sintering. The phenomenon is induced by cathodic partial reduction under DC fields. When using a low frequency AC field (0.1–10 Hz) the blackening is reversible, following the imposed polarity switching. Thermal imaging combined with sample colour changes and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. The partial reduction at the cathode causes a modification of the electrical properties in the sample and the blackened regions, which are close to the cathode, are more conductive than the remainder of the sample. The asymmetrical nature of the electrochemical reactions follows the field polarity and causes an asymmetry in the temperature between the anode and cathode, with the positive electrode tending to overheat. It is also observed that the phenomena are influenced by the quality of the electrical contacts and by the atmosphere used.

Demagnetization Study of Pulse-Field Magnetized Bulk Superconductors

GdBa 2 Cu 3 O 7-δ bulk superconductors are a route to higher magnetic fields in rotating machines. Here, we examine the resistance of pulse-field magnetized bulks to the demagnetization fields they may experience in such a system. The bulks were magnetized at 77 K, after which several thousand cycles of AC field were applied. Subsequently, the decay of the trapped field was characterized. We found that the decay per cycle decreases with frequency and is, normalized to the initial trapped field, largest at the edge of the bulk. At 77 K, the reduction in trapped field proved significant (25% in the center for 150 mT (peak) AC field at 6 Hz), however, reducing below 1% when lowering the temperature to 60 K. We explain this observation as being due to increased flux pinning strength at low temperatures. When applying an AC field, we found a temperature rise that increased with applied field amplitude and frequency. However, when applying an AC field of amplitude 45 mT with a frequency of 48 Hz, we found an increase of the bulk temperature of only 100 mK. Therefore, we conclude that the temperature rise within the analyzed AC field frequency and amplitude range does not contribute significantly to the decay of trapped field.

High speed CMOS acquisition system based on FPGA embedded image processing for electro-optical measurements

Electro-optical measurements, i.e., optical waveguides and plasmonic based electrochemical impedance spectroscopy (P-EIS), are based on the sensitive dependence of refractive index of electro-optical sensors on surface charge density, modulated by an AC electrical field applied to the sensor surface. Recently, P-EIS has emerged as a new analytical tool that can resolve local impedance with high, optical spatial resolution, without using microelectrodes. This study describes a high speed image acquisition and processing system for electro-optical measurements, based on a high speed complementary metal-oxide semiconductor (CMOS) sensor and a field-programmable gate array (FPGA) board. The FPGA is used to configure CMOS parameters, as well as to receive and locally process the acquired images by performing Fourier analysis for each pixel, deriving the real and imaginary parts of the Fourier coefficients for the AC field frequencies. An AC field generator, for single or multi-sine signals, is synchronized with the high speed acquisition system for phase measurements. The system was successfully used for real-time angle-resolved electro-plasmonic measurements from 30 Hz up to 10 kHz, providing results consistent to ones obtained by a conventional electrical impedance approach. The system was able to detect amplitude variations with a relative variation of ±1%, even for rather low sampling rates per period (i.e., 8 samples per period). The PC (personal computer) acquisition and control software allows synchronized acquisition for multiple FPGA boards, making it also suitable for simultaneous angle-resolved P-EIS imaging.

Suppression of Flux Creep in HTS Coil by Applying Low AC Magnetic Field

The suppression of flux creep in the central magnetic field of an high-temperature superconducting (HTS) coil is experimentally validated by applying a low ac magnetic field using additional coils. The HTS coil is fabricated using a Gd-based coated conductor, and a pair of the additional coils wound using copper wires, which are connected in series in the opposite direction, are located coaxially inside and outside the HTS coil. These coils are immersed in liquid nitrogen, and the temporal evolutions of the central fields in the axial direction by applying ac fields for short periods of time using the additional coils after the ramping processes of the HTS coil are observed using a Hall probe. The influences of the amplitude and frequency of ac field on the flux creep in the HTS coil are investigated experimentally.

Field-controllable injection of virtual magnetic domain wall in discrete magnetic nanodot chains

Periodic injection behaviors of virtual magnetic domain wall (VDW) have been systematically investigated in asymmetrically shaped nanodot chains by means of micromagnetic simulations. Systematic investigation on a controllable VDW injection has been carried out. We demonstrate that precise control of VDW injection is achievable by using different nanodot shapes as well as by changing alternating magnetic field (AC field) profiles. The VDW position can be tuned by adjusting AC field frequency and amplitude. Field-controllable periodic VDW injection phenomenon is found to be sustainable over wide ranges of phase diagram spanned by AC field frequency and amplitude.

The New Concept of Nano-Device Spectroscopy Based on Rabi–Bloch Oscillations for THz-Frequency Range

We considered one-dimensional quantum chains of two-level Fermi particles coupled via the tunneling driven both by ac and dc fields in the regimes of strong and ultrastrong coupling. The frequency of ac field is matched with the frequency of the quantum transition. Based on the fundamental principles of electrodynamics and quantum theory, we developed a general model of quantum dynamics for such interactions. We showed that the joint action of ac and dc fields leads to the strong mutual influence of Rabi- and Bloch oscillations, one to another. We focused on the regime of ultrastrong coupling, for which Bloch- and Rabi-frequencies are significant values of the frequency of interband transition. The Hamiltonian was solved numerically, with account of anti-resonant terms. It manifests by the appearance of a great number of narrow high-amplitude resonant lines in the spectra of tunneling current and dipole moment. We proposed the new concept of terahertz (THz) spectroscopy, which is promising for different applications in future nanoelectronics and nano-photonics.

A test of giant electrorheological valve in DC and square wave AC fields with different frequencies

The high price and less content of current Braille displays keep most of visually impaired persons from using them. The compact arrangement of Braille dots regulated by Braille standard presents a rigid limitation to develop a full-page Braille display. Electrorheological valve gives a possibility to meet these severe demands, and its pressure drop is a key parameter to design a full-page Braille display. A giant electrorheological valve in DC field and square waveform AC field with different frequencies was investigated. It was shown that the pressure drop of valve increases with the electric field strength, and decreases with increasing frequency of square waveform AC field. The pressure drop of the giant electrorheological valve is high enough for the purpose in application of a full-page Braille display. The results also show that only square waveform AC field with low frequency can provide higher pressure drops. A forcing flow of giant electrorheological fluids in channel of valve may be a solution of solving the aggregations of nanoparticles on the electrodes.

Effect of a dc electric field on the high-frequency conductivity of a graphene superlattice

Longitudinal and transverse high-frequency conductivities of a graphene superlattice placed in an additional dc electric field are calculated. It is shown that in a sufficiently strong transverse field, the dependence of the longitudinal high-frequency conductivity of the superlattice on the ac field frequency changes. This effect is explained by the nonadditivity of the electronic spectrum of the investigated structure.

Mitigation of Demagnetization of Bulk Superconductors by Time-Varying External Magnetic Fields

Large single-grain high-temperature superconducting bulks have significant potential to replace permanent magnets in various engineering applications. However, based on our previous research, the trapped field in a bulk superconductor can be attenuated or even erased when a bulk is subjected to a time-varying external magnetic field. Therefore, it is important to develop a method to protect bulks from demagnetization by improving the thermal conduction of the bulk and/or reducing ac losses. Improvement in the thermal conduction of bulks involves modification of the material fabrication process, which may have a detrimental effect on its superconducting properties. Employing shielding materials around a bulk helps to decrease the ac losses, but also provides a durable way to maintain the original material properties. In this paper, two shielding cases are proposed and evaluated numerically: ring-shaped shielding with a copper coil and surface shielding with a ferromagnetic material. Based on the numerical modeling results, the ring-shaped coil works well for externally applied ac fields of larger magnitude and higher frequency. However, the ferromagnetic material was preferable for surface shielding for relatively lower fields. Finally, an optimal shield design is presented.

Performance of ITER Correction Busbar Conductor Samples CBCN2 and CBCN3

The correction busbar (CB) conductors for the ITER project are being produced in China. The second China CB conductor sample (CBCN2, Phase II of conductor testing) and the third CB conductor sample (CBCN3, Phase III testing) were made of the WST NbTi strands, cabled by Changtong, jacketed at ASIPP, and tested in the SULTAN facility. The current sharing $(T_\mathrm{cs})$ test results show that the conductor sections of CBCN2 and CBCN3 have high $T_\mathrm{cs}$ performance. Using the electrical method, the $T_\mathrm{cs}$ of both CBCN2 and CBCN3 samples was 7.50 K at 10 kA/2.9 T (maximum operating field) before and after electromagnetic loading. AC loss measurement was performed before any electromagnetic loading of the sample, and the measurements were repeated after 1000 or 2000 cyclic loads. The AC loss measurements were performed at 2-T background field, without transport current. The ac loss of the CBCN2 and CBCN3 conductors slightly differs at ±0.2T ac field and various frequencies. According to the SULTAN test result, $T_\mathrm{cs}$ of CBCN2 and CBCN3 conductor samples meet the ITER acceptance criteria of 7.0 K at Bpeak field is 2.9 T and 10 kA.

Iterative dipole moment method for the interaction of multiple dielectrophoretic particles in an AC electrical field

Dielectrophoresis (DEP) for bio-particle manipulation has been drawing much attention in recent years. The equivalent dipole moment method (EDM) has widely been used to calculate DEP forces on single particle, but this method falls short to describe the interaction between neighboring particles. The Maxwell stress tensor method (MST) is theoretically rigorous for particle interaction, but its complicated numerical computation makes it difficult to implement in practice. In this paper, an iterative dipole moment method (IDM) is presented to investigate the interaction of multiple dielectrophoretic particles in a two-dimensional AC electric field. Without cumbersome numerical computation, the inter-particle forces, the particle trajectories and chain patterns calculated by the IDM method are found to be well consistent with those by the MST method for some published results and experimental observations. Furthermore, it is found that the final stable particle chain patterns strongly depend on the initial configuration of the particle distribution. An arbitrary small disturbance to the particle locations may lead to dramatically different motion trajectories and final particle chains. By tuning the frequency of AC field, the particles can be transformed between positive DEP particles and negative DEP particles, which results in different particle chains as well.

Electrowetting Controls the Deposit Patterns of Evaporated Salt Water Nanodroplets.

So-called "coffee-ring" stains are the deposits remaining after complete evaporation of droplets containing nonvolatile solutes. In this paper we use molecular dynamics to simulate the evaporation of salt water nanodroplets in the presence of an applied electric field. We demonstrate, for the first time, that electrowetted nanodroplets can produce various deposit patterns, which vary substantially from the original ringlike deposit that occurs when there is no electric field. If a direct current (dc) electric field with strength greater than 0.03 V/Å is imposed parallel to the surface, after the water evaporates the salt crystals form a deposit on the substrate in a ribbon pattern along the field direction. However, when an alternating current (ac) electric field is applied the salt deposit patterns can be either ringlike or clump, depending on the strength and frequency of the applied ac field. We find that an ac field of high strength and low frequency facilitates the regulation of the deposit patterns: the threshold electric field strength for the transition from ringlike to clump is approximately 0.006 V/Å. These findings have potential application in fabricating nanostructures and surface coatings with desired patterns.

Operational research on a high-Tc rectifier-type superconducting flux pump

High-Tc superconducting (HTS) flux pumps are capable of injecting flux into a superconducting circuit, which can achieve persistent current operation for HTS magnets. In this paper, we studied the operation of a rectifier-type HTS flux pump. The flux pump employs a transformer to generate high alternating current in its secondary winding, which is connected to an HTS load shorted by an HTS bridge. A high frequency ac field is intermittently applied perpendicular to the bridge, thus, generating flux flow. The dynamic resistance caused by the flux flow ‘rectifies’ the secondary current, resulting in a direct current in the load. We have found that the final load current can easily be controlled by changing the phase difference between the secondary current and the bridge field. The bridge field of frequency ranging from 10 to 40 Hz and magnitude ranging from 0 to 0.66 T was tested. Flux pumping was observed for field magnitudes of 50 mT or above. We have found that both higher field magnitude and higher field frequency result in a faster pumping speed and a higher final load current. This can be attributed to the influence of dynamic resistance. The dynamic resistance measured in the flux pump is comparable with the theoretical calculation. The experimental results fully support a first order circuit model. The flux pump is much more controllable than the traveling wave flux pumps based on permanent magnets, which makes it promising for practical use.

Dynamic conductivity of ac–dc-driven graphene superlattice

The dynamic conductivity of graphene superlattice in the presence of ac electric field and dc electric field with longitudinal and transversal components with respect to superlattice axis was calculated. In the case of strong transversal component of dc field conductivity of graphene superlattice was shown to be such as if the electrons had got the effective mass. In the case of weak transversal component of dc field conductivity was shown to change its sign if the frequency of ac field was an integer multiple of half of Bloch frequency.

Increase in AC-Field Frequency and Recording Performance in Microwave-Assisted Magnetic Recording

Increase in AC-Field Frequency and Recording Performance in Microwave-Assisted Magnetic Recording

Photoassisted transport in silicon dangling bond wires

We theoretically investigate charge transport through dangling bond (DB) nanostructures built on a passivated silicon (100) surface by selectively removing hydrogen atoms. We focus on dangling bond wires and on T-junctions. In the latter case, destructive quantum interference effects lead to a strong suppression of charge transport mediated by the DB electronic states. We demonstrate, however, that by applying a time periodic voltage, mimicking irradiation with monochromatic light, a dramatic enhancement of the current up to the μA range can be achieved. This result is however limited by the restriction on the AC field strength and frequency that bulk states should minimally contribute to charge transport; otherwise current leakage will set in. Despite this constraint, transconductance values of the order of 10−6 A/V can be achieved, illustrating the potential of the discussed systems to find applications in nanoscale electronics.

The Importance of Depth-Varying Fields for MAMR Switching-Field Reduction

Microwave assisted magnetic recording (MAMR) was proposed to increase the data storage areal density in hard disk drives [1]. With a localized AC field generated by a spin torque oscillator (STO), MAMR significantly reduces the switching field when the AC field frequency matches the ferromagnetic resonance frequency of the media elements [2]. In this study we present micromagnetic modeling of media switching-field reduction due to AC fields at GHz frequencies. We employ a 4-layer media model based on current exchange-coupled media with the top layer describing a large lateral exchange “cap” with three low lateral exchange “oxide” layers below. We examine hysteresis loops in two scenarios: depth invariant applied fields (external and AC), and depth invariant external field with depth varying AC field. We then do a DC-band write study with realistic writer field and macrospin-based STO field to gauge switching-field reduction in a scenario more similar to actual recording.