Influence of AC magnetic field amplitude on the surface magnetoimpedance tensor in amorphous wire with helical magnetic anisotropy

We have performed experimental and theoretical studies on the influence of ac magnetic field amplitude on the magnetoimpedance tensor in an amorphous wire with helical magnetic anisotropy. For the experimental measurements, we used an amorphous wire of composition (Co/sub 0.94/Fe/sub 0.06/)/sub 72.5/Si/sub 12.5/B/sub 15/ with negative, nearly zero magnetostriction constant, excited either by an ac circular h/sub /spl phi// or by an axial h/sub z/ magnetic field created by an ac electric current. We changed the ac current amplitude from 7.5 to 40 mA and the current frequency f from 1.5 to 20 MHz. The values of the asymmetric giant magnetoimpedance ratio associated with the sweeping direction of the dc field H/sub ex/ and the corresponding sensitivity were 211% and 0.64 V/Oe, respectively, for an ac current of 37.5 mA at 3 MHz. For the theoretical study based on the magnetization rotation, we obtained the second-order harmonic of the ac magnetization m/spl I.oarr//sup (2)/ induced by the relatively high ac magnetic field by solving the Landau-Lifshitz-Gilbert (LLG) equation. We also considered a second-order surface impedance tensor /spl sigmav//spl circ//sup (2)/, which allowed us to analyze quantitatively the influence of the ac magnetic field amplitude on the impedance tensor of the wire. We obtained the domain model of the wire with helical magnetic anisotropy having multidomains and the magnetization vector /spl plusmn/M/sub 0/ directed in the easy direction, and the corresponding static magnetic configurations, by solving the static LLG equation. For the given magnetic configurations, we calculated the second-order impedance tensor /spl sigmav//spl circ//sup (2)/. The results can well explain the irregular field characteristics of the voltage responses at low dc field value, when the wire was excited at high frequency and at large ac magnetic field.     

Direct Measurement of AC Current by Measuring the Voltage Drop on the Coaxial Current Shunt

An accurate direct measurement of ac current that avoids complex and time-consuming ac–dc current transfer measurements by directly measuring the ac voltage drop of coaxial current shunts is described. Evaluation of custom-developed coaxial current shunts for long-term drift, self-heating effects, and loading using a commercial ac voltmeter is presented. By correcting only the shunts' long-term drift, direct ac current measurement uncertainties below 55 $muhbox{A/A}$ are attained in the frequency range of 20 Hz–30 kHz and for currents ranging from 100 mA up to 10 A.      

The effect of alternating current on the plasma frequency of the tunnel Josephson junctions

The effect of alternating current on the plasma frequency of the tunnel Josephson junctions simultaneously carrying both direct (dc) and alternating (ac) currents has been studied. The presence of the ac component leads to a decrease in the plasma frequency. An analytical expression describing the plasma frequency as a function of the ac current amplitude is proposed. The obtained results agree with the recent experimental data on the properties of tunnel Josephson junctions.     

An additional loss source in cabled multifilamentary superconductors

It is shown that irregularities in the twist rate of a multifilamentary superconducting strand, which may occur in the cabling or braiding process of a high current conductor, give rise to additional matrix currents and enhanced ac losses. Electrodynamic equations for matrix and filament currents in a strand with a sudden change in twist rate are derived and damped wave solutions are given for an external harmonic field as well as for a ramp field. Inder unfavourable conditions the associated loss enhancement may become comparable to the regular matrix losses. This is also verified experimentally by magnetization measurements. Some recommendations for cable design are given.     

ac losses of multifilament superconductors reduced below the level of filament hysteresis losses due to field screening by filament coupling currents

In measurements on multifilament superconductors, it was found that overall ac losses fall below the level of filament hysteresis losses at sufficiently high frequencies, due to field screening by filament coupling currents. At low frequencies, overall loss should also fall below the hysteresis loss level, provided f?>_m a critical frequency defined in the next text). This may be arranged by the choice of large values of the twist pitch, or small values of the mean transverse resistivity p_e.     

Nonlinear magnetization reversal in copper-permalloy composite wires induced by a high-frequency current

The phenomenon of magnetization reversal in copper-permalloy composite wires induced by a high-frequency ac current field was studied. The frequency spectrum of a voltage measured in the probing coil wound on the wire contains only even harmonics in a broad range of amplitudes of the ac excitation current and the bias magnetic field strength. The amplitude of the even harmonics measured is highly sensitive with respect to the bias field. The results can be used to develop weak magnetic field sensors.     

Transport ac loss of a superconducting cylinder with field dependent critical current density

The transport ac loss per cycle per unit length of a hard superconducting cylinder is calculated from the critical state model assuming a Kim-type and an exponential field dependent critical current density. Without such dependence, the results are consistent with Norris’ equations for an ellipse bar, in which the critical current density is assumed not to depend on the flux density. Based on Norris’ equations, the expressions of the loss are derived for a finite length cylinder. It is shown that the field dependence decreases and increases the loss at low and high ac currents, respectively, and the effects of the parameter p on the loss are related to the magnetization process. Compared to Norris’ prediction, the results for the Kim and exponential model show the same trend with respect to the external transport current.     

Deterministic Magnetization Switching Using Lateral Spin–Orbit Torque

Current-induced magnetization switching by spin–orbit torque (SOT) holds considerable promise for next generation ultralow-power memory and logic applications. In most cases, generation of spin–orbit torques has relied on an external injection of out-of-plane spin currents into the magnetic layer, while an external magnetic field along the electric current direction is generally required for realizing deterministic switching by SOT. Here, deterministic current-induced SOT full magnetization switching by lateral spin–orbit torque in zero external magnetic field is reported. The Pt/Co/Pt magnetic structure is locally annealed by a laser track along the in-plane current direction, resulting in a lateral Pt gradient within the ferromagnetic layer, as confirmed by microstructure and chemical composition analysis. In zero magnetic field, the direction of the deterministic current-induced magnetization switching depends on the location of the laser track, but shows no dependence on the net polarization of external out-of-plane spin currents. From the behavior under external magnetic fields, two independent mechanisms giving rise to SOT are identified, i.e., the lateral Pt–Co asymmetry as well as out-of-plane injected spin currents, where the polarization and the magnitude of the SOT in the former case depends on the relative location and the laser power of the annealing track.     

Alternating field losses in superconducting wires carrying dc transport currents. Part 2: multifilamentary composite conductors

Investigations of transient field losses in multifilamentary composite conductors carrying dc transport currents, I, are presented. As shown previously, the total loss is best characterized by a normalized transport current i = l/l_c and a dimensionless field-change rate β = τB_e/B_p, where τ is the relaxation time of the coupling current and B_p is the full-penetration field of a solid conductor equivalent to the multifilamentary composite. For β ? 1, the composite conductor behaves like a solid conductor and a saturation effect occurs in both the magnetization and the ac losses. The characteristic feature of the composite appears for β ? 1, and the total losses for transport currents below i < 1 ? β are almost independent of i. Beyond the limit given, losses rise sharply. Experimental results over a wide range of the field change rate and the transport-current level agree sufficiently with the calculations.     

Digital signal processing for step-scan phase and electrochemical potential double-modulation Fourier transform infrared spectrometry

Step-scan double-modulation (phase and electrochemical potential) Fourier transform infrared (FT-IR) spectrometry provides both spectroscopic and dynamic information about faradaic reactions. Recently introduced digital signal processing (DSP) can be used, instead of two lock-in amplifiers, for the optical signal demodulation at two modulation frequencies. In order to establish the merits of double-modulation FT-IR spectrometry with DSP, spectro-electrochemical experiments are performed in the attenuated total reflection configuration and with the commonly used ferri/ferrocyanide redox couple. Because of a large potential drop associated with the uncompensated resistance, a satisfactory signal-to-noise ratio for the alternating current (ac) optical measurements is obtained only with the employment of positive feedback compensation. In this arrangement, the amplitude of electrochemical modulation is sufficiently large to convert a significant fraction of the reduced form to the oxidized form and back to the reduced form. Large amplitude ac voltammetry demonstrates that the phase of faradaic admittance at the formal potential is approximately 45 degrees at 2.00 Hz. In addition, these experiments allow for calculation of the interfacial ac potential. This variable is needed for the normalization of the in-phase and the quadrature spectra in order to overcome the problem associated with the iR(u) drop. Because of the integral relationship between the faradaic current and the electromodulation reflectance coefficient, the phases of electromodulation reflectance coefficient with respect to the interfacial ac potential are expected to be -45 degrees and 135 degrees for the reduced and oxidized forms, respectively. However, dynamic information from double-modulation FT-IR spectrometry is available only if demodulation at the electrochemical potential modulation frequency is performed with respect to a defined phase. Because of an undefined demodulation phase implemented in the current version of DSP software, step-scan double-modulation FT-IR spectrometry with DSP is suitable only to provide spectroscopic information. In order to overcome this limitation, the demodulation of the ac optical signal at the electrochemical modulation frequency must be synchronized in phase with the ac potential modulation applied to the electrochemical cell.     

Anisotropy of the critical current density in superconducting niobium-titanium wires

The critical current density in wires of an Nb-64 at % Ti alloy was determined in axial and circular directions by various methods. Owing to the cell structure due to cold deformation an anisotropy in the critical current density (j_{c, axially} <j_c, circularly) is present in the wires There is no major difference between magnetization currents and transport currents. Therefore it should be possible in principle to obtain the high critical current densities determined from circular magnetization currents by suitable defect structure with axial transport currents too.     

Effective magnetization and forces due to eddy currents

A simple method for evaluating the effective magnetization due to eddy currents excited by AC magnetic fields in finite conductive objects is presented. The values of magnetization enable the estimation of the forces exerted on the object by the effect of eddy currents. The method relies on assuming a similarity between eddy current magnetization and the magnetization due to diamagnetic effects, which is easier to evaluate. Its validity is checked by comparing results from the eddy current forces with data obtained from conventional but much more complicated methods or from experimental data. The approach may be useful for evaluating the eddy current losses in finite conductive objects excited by AC magnetic fields or the influence of small conductive objects on AC excitation coils. The method combines techniques related to the conventional evaluation of eddy currents in certain 1-D geometries with techniques that approximate behavior of the AC magnetic field in finite objects in a way similar to that followed when the magnetic forces acting on a diamagnetic object are calculated     

Computer-aided design of air-gapped magnetic core inductors with minimum DC winding resistance

The method of computer-aided design of air-gapped inductors operating with de bias current is described. An algorithm for the minimum de resistance of the winding for a desired value of the inductance is given. Simultaneously, this permits us to determine the optimal length of the air gap, to choose a suitable operating point on the normal magnetization curve and to calculate the minimum number of turns of the winding under given inductor operation conditions, i.e., the ac voltage value across the inductor and its frequency, and the value of the de bias current superimposed in the same winding. The method presented here uses the normal magnetization curve of the core material and the family of relative incremental permeability curves of this material.     

A High-Resolution Electrodynamic AC-to-DC Power Transfer Instrument

The design of an electrodynamic instrument for measurement of ac power at line frequency is described. The deflection of its moving coil is kept at zero by passing alternating and direct currents simultaneously through both coils. An automatic control circuit generates the balancing direct current related to the ac power to be measured. The high resolution of the instrument, 2 × 10-7 of rated power, makes a detailed investigation of its performance in the error range below 1 × 10-5 possible.     

Development of critical current measurement standards

This paper deals with the development of criteria for critical current measurements. The two tasks investigated were: (1) the determination of critical current of short samples as a function of transition criterion, and (2) analysis of current transfer from sample holder to sample. Critical currents were measured using the equivalent resistivity criterion with sensitivities ranging from 10^-7Ω-cm to 10^-12Ω-cm and using the electric field criterion with sensitivities of 1 mV/cm to 100 nV/cm. Current transfer measurements were performed on monolithic conductors with critical currents greater than 1,000 amps. The results of the program are that no single measurement standard and no single test holder are suitable for all types of critical current measurements, and that sample holders must be designed with sufficiently large copper current contacts in order to minimize current transfer effects.     

Three-phase phase-lock loop for distorted utilities

A novel three-phase phase-locked loop (PLL) structure suitable for phase and angular frequency derivation from distorted ac utility voltages is presented. The proposed PLL has a simple structure; a conventional three-phase PLL followed by a proportional-integral (Pl)-controlled moving average filter together with a phase-locking algorithm. The objective of the proposed technique is to capture the fundamental phase angle and angular frequency of three-phase clean, distorted, balanced or unbalanced ac utilities. The PLL gives fast, accurate angular frequency and phase locking and is robust to utility distortion such as line notching, random noise, voltage imbalance, phase loss, phase imbalance, harmonics, dc offsets and frequency variation. The analysis presented substantiates the immunity of the proposed PLL to unbalanced and distorted utility conditions. The PLL technique is simulated and digital signal processor (DSP)-implemented for a three-phase system to verify the analytical results. The simulated and experimental results, for numerous utility conditions, demonstrate its phase-tracking ability, whereas the conventional technique fails to lock accurately in highly distorted, three-phase grid-connected operation.     

Adjustable Complex-Ratio Transformer

A four-terminal network in which the complex output-to-input ac current or voltage ratio may be adjusted is described. The design of an experimental Adjustable Complex-Ratio Transformer for use at a frequency of 60 Hz, in which the output-to-input current ratio (Io/Ii = ?+j?) can be adjusted with two sets of 5 decade dials to 0.1 percent accuracy, is presented. The application of the instrument for the direct reading of the difference of two alternating currents is discussed.     

Real-time impedance measurements during electrochemical experiments and their application to aniline oxidation

Development of an in situ technique for measuring electrochemical impedance spectra in real time during an electrochemical experiment is described. The technique is based on staircase voltammetry with relatively large step heights, in which a series of increasing potential steps are applied to an electrochemical system, and the resulting currents are sampled. The first derivatives of the currents thus obtained are then converted to ac current signals in frequency domain, and impedances are computed from them. To demonstrate the technique as a tool for studying the electrode/electrolyte interface during the electrochemical reaction, we chose an electrochemical oxidation reaction of aniline, whose reaction products have been known to continuously change the electrode surface due to the polymer film growth on its surface, and report a number of observations that would not have been obtained without such in situ experiments. A suggestion is also made on the use of staircase voltammetry for mechanistic studies on complex electrochemical reactions by simply varying the sampling time.     

Spin transfer effect in magnetic tunnel junction with a nano-current-channel Layer in free layer

The current-induced magnetization switching (spin transfer effect) in a low resistance-area (RA) product magnetic tunnel junction (MTJ) device with critical current density of 1.4/spl times/10/sup 7/ A/cm/sup 2/ was demonstrated. The RA product of the MTJ is 4.2 /spl Omega//spl mu/m/sup 2/ and the magnetoresistance (MR) ratio induced by current is up to 16%. An MTJ structure with a novel nano-current-channel (NCC) layer inserted into the free layer for the current-induced magnetization switching by lower current density was proposed and prototyped. By using the current confined effect, the local current density in the integrated free layer was sufficiently high to switch the magnetization locally. Such local magnetization reversal helped to reverse the magnetic moments around together with the polarized current and spread out the switching of the entire free layer through the superparamagnetic nano-channels. The critical current density was reduced to 4.2/spl times/10/sup 6/ A/cm/sup 2/, which is only one quarter of that for a pure MTJ structure.     

ac Response and losses of differently pinned type-II superconductors

Measurements of the penetration of flux and losses in ac fields (20–2000 cps) at 4.2 K together with quasistatic magnetization curves on three samples of Ta ₉₂ Nb ₈ having different degrees of pinning are presented. The results are explained in terms of surface shielding currents taking into account both the pinning strength and the flux-flow resistivity exhibited by the bulk. A quantitative approach is used in which the concept of flux-flow resistivity proves applicable when the sample is subjected to an ac magnetic field.