Dc Bias Current Research Articles

Effects of lateral dimensions of the magnetic thin films on the characteristics of thin-film type orthogonal fluxgate sensors

Orthogonal fluxgate sensors with Co–Nb–Zr magnetic thin films as the core material are fabricated using conventional photolithography and their performances characterized. The sensor structure consists of a central Cu thin film and surrounding Co–Nb–Zr magnetic layers. The magnetic layers are excited by an AC current applied along the Cu film, and the output voltage is detected with an external pickup coil surrounding the sensor. A DC bias current, sufficient to saturate the magnetic layers, is applied along the Cu film, thus allowing for the fundamental mode operation. The lateral dimensions of the magnetic thin films are the main parameters investigated, and are varied widely; the width is in the range 0.5–1.1mm, and the length is in the range 1.6–9.6mm. The device characteristics measured under the optimum conditions show that the output voltage is linear and the sensitivity is constant over a wide range of external magnetic field (−2400A/m–+2400A/m). The normalized output voltage with respect to the magnetic area is high for a high aspect ratio and a small width of the magnetic thin films. The highest values of peak-to-peak output voltage and normalized output voltage achieved in this study are 14,120mV and 2783V/mm2, respectively.

Modeling of AC Loss in Coils Made of Thin Tapes Under DC Bias Current

Many applications, such as magnets and SMES, are usually charged and discharged under a bias DC current, which may increase the AC loss. For their design, it is necessary to understand and predict the AC loss. This article analises the AC loss in magnet-like coils under DC bias contribution super-imposed to the AC current. The analysis is based on a numerical model that takes the interaction between magnetization currents in all turns into account. The studied example is a stack of 32 pancake coils with 200 turns each made of thin tape, such as $Re$BCO coated conductor. We present the current density, the instantaneous power loss, and loss per cycle. We have found that the loss increases with the DC bias current. The instantaneous power loss is the largest in the initial rise of the the AC current. In following cycles, the power loss is higher in the current increase than in the decrease. The loss per cycle is the largest at the end pancakes. In conclusion, the highest cooling power should be supplied to the top and bottom pancakes and during current rise, specially the initial one. The presented model has a high potential to predict the AC loss in magnet-size coils, useful for their design.

Nonlinear magnetotransport theory and Hall induced resistance oscillations in graphene

The quantum oscillations of nonlinear magnetoresistance in graphene that occur in response to a dc current bias are investigated. We present a theoretical model for the nonlinear magnetotransport of graphene carriers. The model is based on the exact solution of the effective Dirac equation in crossed electric and magnetic fields, while the effects of randomly distributed impurities are perturbatively added. To compute the nonlinear current effects, we develop a covariant formulation of the migration center theory. The current is calculated for short- and large-range scatterers. The analysis of the differential resistivity in the large magnetic field region, shows that the extrema of the Shubnikov de Hass oscillations invert when the dc currents exceed a threshold value. These results are in good agreement with experimental observations. In the small magnetic field regime, corresponding to large filling factors, the existence of Hall induced resistance oscillations are predicted for ultra clean graphene samples. These oscillations originate from Landau–Zener tunneling between Landau levels, that are tilted by the strong electric Hall field.

Magnetization reversal process and peculiarities of giant magneto‐impedance effect in amorphous ferromagnetic microwire with helical anisotropy

The micromagnetic simulation of the magnetization reversal process in a Co‐rich microwire with a weak helical anisotropy under the influence of applied magnetic field and dc bias current has been carried out. It is shown that due to the influence of the magneto–elastic interaction in a wire with helical anisotropy, the change of the sign of the longitudinal magnetization component under the influence of the axial magnetic field leads to a subsequent jump of the circular magnetization component at some critical value of the applied magnetic field. This leads to a non‐trivial behavior of the off‐diagonal giant magneto‐impedance (GMI) component of the wire as a function of applied magnetic field. The effect is confirmed experimentally for two Co‐rich microwires, having significantly different amplitudes of the residual quenching stress.

Fabrication and characteristics of high speed InGaAs/GaAs quantum-wells superluminescent diode emitting at 1053 nm

A high speed 1053 nm superluminescent diode (SLD) with a ridge-waveguide structure has been fabricated for the first time to the best of our knowledge. InGaAs/GaAs quantum well epitaxial structure, the etched depth of the insulation channel and the area of p-side electrode were optimized to enhance the modulation bandwidth of the SLD. Bend-waveguide unpumped absorbing region structure and facet coating methods have been adopted to suppress the lasing oscillation. As a result, a −3 dB cutoff frequency of 1.7 GHz is obtained at a dc bias current of 100 mA and 25 °C heat-sink temperature, corresponding to 2.5 mW output power from single-mode fiber with spectral modulation of less than 0.15 dB and spectral width of 24 nm. The SLD module shows a good reliability.

Sensitivity Enhancement of Lorentz Force MEMS Resonant Magnetometers via Internal Thermal-Piezoresistive Amplification

This letter presents sensitivity enhancement of MEMS resonant magnetometers using the thermal-piezoresistive internal amplification effect in silicon microstructures. Preliminary results show up to ~15 X improvement in sensitivity per bias current for a resonator operated as a Lorentz force magnetometer. Magnetometer sensitivity figure-of-merit, defined as sensitivity (mV/T) over sensor dc bias current, has increased from 0.29 Ω/T(mV/Tesla/mA) to 4.22 Ω/T via internal thermal-piezoresistive amplification that also led to resonator effective quality factor (Q) increasing from its intrinsic value of 1140 to 16900 (in air). Previous work on the thermal-piezoresistive amplification effect suggests that amplification factors up to 3-4 orders of magnitude can be achieved using optimally designed structures, which can lead to ultra-high sensitivities for the presented sensors. It should be noted that the main focus of this letter is not to demonstrate a highly sensitive magnetometer, but rather to demonstrate the ability to improve magnetometer sensitivity as the resonator internal Q-amplification kicks-in. Although the resonant structure in this letter has not been optimized to operate as a magnetometer, sensitivities as high as 262 mV/T in air (minimum detectable field in the μT range) have been achieved.

High-speed magnetic field detection using a spin valve with a perpendicularly magnetized free layer and an in-plane magnetized reference layer

The magnetic field response of a spin valve having a perpendicularly magnetized free layer (FL) and an in-plane-magnetized reference layer (RL) under a DC bias current was studied theoretically. Above a certain bias current value, the magnetization of the FL points in the equilibrium direction in the film plane. If the sign of the applied field is changed, the magnetization moves to another equilibrium direction that is symmetric with respect to the axis parallel to the magnetization of the RL. We found that this field response is rapid because the spin-transfer torque enhances the damping.

50 MHz-10 GHz low-power resistive feedback current-reuse mixer with inductive peaking for cognitive radio receiver.

A low-power wideband mixer is designed and implemented in 0.13 µm standard CMOS technology based on resistive feedback current-reuse (RFCR) configuration for the application of cognitive radio receiver. The proposed RFCR architecture incorporates an inductive peaking technique to compensate for gain roll-off at high frequency while enhancing the bandwidth. A complementary current-reuse technique is used between transconductance and IF stages to boost the conversion gain without additional power consumption by reusing the DC bias current of the LO stage. This downconversion double-balanced mixer exhibits a high and flat conversion gain (CG) of 14.9 ± 1.4 dB and a noise figure (NF) better than 12.8 dB. The maximum input 1-dB compression point (P1dB) and maximum input third-order intercept point (IIP3) are −13.6 dBm and −4.5 dBm, respectively, over the desired frequency ranging from 50 MHz to 10 GHz. The proposed circuit operates down to a supply headroom of 1 V with a low-power consumption of 3.5 mW.

Soft ferrite cores characterization for integrated micro-inductors

Ferrite-based micro-inductors are proposed for hybrid integration on silicon for low-power medium frequency DC-DC converters. Due to their small coercive field and their high resistivity, soft ferrites are good candidates for a magnetic core working at moderate frequencies in the range of 5–10 MHz. We have studied several soft ferrites including commercial ferrite film and U70 and U200 homemade ferrites. The inductors are fabricated at wafer level using micromachining and assembling techniques. The proposed process is based on a sintered ferrite core placed in between thick electroplated copper windings. The low profile ferrite cores of 1.2 × 2.6 × 0.2 mm3 are produced by two methods from green tape-casted films and ferrite powder. This paper presents the magnetic characterization of the sintered ferrite films cut and printed in rectangular shape and sintered at different temperatures. The comparison is made in order to find out the best material for the core that can reach the required inductance (470 nH at 6 MHz) under 0.6A current DC bias and that generate the smallest losses. An inductance density of 285 nH/ mm2 up to 6 MHz was obtained for ESL 40011 cores that is much higher than the previously reported devices. The small size of our devices is also a prominent point.

Hysteresis regime in the operation of a dual-free-layer spin-torque nano-oscillator with out-of-plane counter-precessing magnetic moments

We studied the operation of a dual-free-layer (DFL) spin-torque nano-oscillator (STNO) and demonstrated that in a practically interesting regime when the magnetizations of the two free layers (FLs) precess in opposite directions along large-angle out-of-plane trajectories, thus doubling the generation frequency, the operation of the DFL STNO is strongly hysteretic as a function of a bias dc current. The stable magnetization dynamics starts at a rather large magnitude of the bias dc current density Jdc>Jthhigh when the bias current is increased, but the regime of stable counter-precession of the FLs persists till rather low magnitudes of the bias dc current density Jthlow<Jdc<Jthhigh when the bias current is decreased. This hysteresis is caused by the dipolar coupling between the FLs, and is especially pronounced for small distances between the FLs and the small magnetic damping in them. The discovered hysteretic behavior of the DFL STNO implies the possibility of application of a strong initial pulse of the bias current (greater than the upper threshold Jthhigh of the stable dynamics) and subsequent reduction of the bias current to a working point (Jthlow<Jdc<Jthhigh) corresponding to the required output frequency f(Jdc). The obtained results are important for the practical development of DFL STNOs with optimized operation characteristics.

Error Analysis for Metering Current Transformer with DC Bias

DC bias widely exists in power system. In order to analyze its influence on metering current transformer (MCT), a new digital simulation model based on Preisach theory is constructed firstly. Using this digital model, the ratio error and angle error of a running MCT are calculated under different levels of DC bias. Simulation results show that the current DC bias in power system has only little influence on the error of MCT.

Active Frequency Tuning for Magnetically Actuated and Piezoresistively Sensed MEMS Resonators

This letter, for the first time, reports an active frequency tuning for a magnetically actuated and piezoresistively sensed MicroElectroMechanical Systems resonator. Magnetic transduction is used to drive the device into the resonance. Piezoresistive transduction is used as resonance sensing technique and to tune the device's frequency by the Joule heating generated in the Wheatstone bridge without fabricating additional electrothermal heaters. The experiments are performed in air at room temperature and atmospheric pressure. The measurements shows that, by increasing the DC bias current of Wheatstone bridge from 0.5 to 5 mA, a maximum tuning range of -3403 ppm is achieved with a device resonating at 25.77 kHz.

NiZn 페라이트코어를 이용하여 제작한 직교형 플럭스게이트 센서의 출력에 미치는 바이어스전류의 영향

Orthogonal fluxgate sensor was fabricated with cylinder-shaped NiZn ferrite core, Cu wire through the core and pickup coil wound on the core, and the bias current effect on the output sensitivity of it was investigated. The output ( sensitivity) of the sensor was largely dependent on the operation frequency, and the tendency of sensor output was similar to that of the impedance of pickup coil. The maximum output was obtained by adding the DC bias current of which value was over 50% of the excitation current. The output was saturated when the DC bias current was larger than 50% of the excitation current.

Giant magneto-impedance effect in amorphous ferromagnetic wire with a weak helical anisotropy: Theory and experiment

An adequate description of the results of experimental measurement of both diagonal and off-diagonal Giant magneto-impedance (GMI) components has been obtained for Co-rich amorphous microwire at moderate frequencies assuming the existence of a small off-diagonal tensor component of the residual quenching stress. The latter is the origin of a weak helical anisotropy of amorphous microwire. The micromagnetic simulation of the magnetization reversal process in the microwire under the influence of the applied magnetic field and dc bias current has been carried out. It is shown that due to the influence of the magneto–elastic interaction in a wire with a weak helical anisotropy, the behavior of the longitudinal and circular magnetization components is significantly correlated. Namely, the change of the sign of the longitudinal magnetization component under the influence of the axial magnetic field leads to a subsequent jump of the circular magnetization component at some critical value of the applied magnetic field. As a result of the jump of the circular magnetization, the off-diagonal GMI component also changes sign during the wire magnetization reversal. This effect is confirmed experimentally for a Co-rich wire with a small negative magnetostriction. It is also shown that the jump of the circular magnetization can be eliminated by a circular magnetic field of a weak dc bias current flowing along the wire. This effect allows one to design sensitive magnetic field sensor based on the measurement of the off-diagonal GMI component.

Estimate of the limitations in data transmission imposed on digitally modulated laser diodes by the bit pattern

In this paper, the cut-off bit-pattern phenomenon of a digitally-modulated semiconductor laser diode (LD) is calculated. The cut-off bit-pattern \(\left( {Q_{\textit{off}}^{\textit{cut}\text{- }\textit{off}} }\, \right) \) is defined as the number of “0” bits (i.e., OFF pulses) preceding the considered “1” bit (i.e., ON pulse) above which an LD cannot respond normally to an injected data pattern and will consequently shutdown or become inoperable. The latter case occurs when the turn-on time delay of an LD exceeds the bit-time interval of the injected current pulse. An analytical calculation for an LD’s cut-off bit-pattern is derived in terms of laser cavity parameters, bit rate, modulation and dc-bias currents. This calculation can be used to predetermine, based on typical values for an LD, accurate dc-bias and modulation current values that should be utilized to ensure an LD is operational for high-speed optical communications applications.

Current-Biased Kinetic Inductance Detector Using $ \hbox{MgB}_{2}$ Nanowires for Detecting Neutrons

We propose a current-biased kinetic inductance detector (CB-KID). This detector senses a change in kinetic inductance under a dc bias current, in contrast to a current-biased transition edge detector, reported previously, which probes the transient change in the bias current at the transition edge. Our CB-KID consists of a 200-nm-thick thin-film meander line with 3- wire. It is operated at 4 K. A scanning laser spot can be achieved by an XYZ piezo-driven stage and an optical fiber with an aspheric focused lens. We succeeded in observing a signal image in the contour at 4 K. The magnitude of the signal should be proportional to the number of quasi-particles excited by the laser. Because our CB-KID has a typical signal of 3-ns pulse width and can operate at 4 K, it will be possible to assemble a large-scale array of neutron detectors with single flux quantum readout circuits.

Synchronized dynamics of Josephson vortices in artificial stacks of SNS Josephson junctions under both dc and ac bias currents

Nonlinear dynamics of Josephson vortices (fluxons) in artificial stacks of superconducting-normal-superconducting Josephson junctions under simultaneously applied time-periodic ac and constant biasing dc currents is studied using the time dependent Ginzburg-Landau formalism with a Lawrence-Doniach extension. At zero external magnetic field and dc biasing current the resistive state of the system is characterized by periodic nucleation and annihilation of fluxon-antifluxon pairs, relative positions of which are determined by the state of neighboring junctions. Due to the mutual repulsive interaction, fluxons in different junctions move out of phase. Their collective motion can be synchronized by adding a small ac component to the biasing dc current. Coherent motion of fluxons is observed for a broad frequency range of the applied drive. In the coherent state the maximal output voltage, which is proportional to the number of junctions in the stack, is observed near the characteristic frequency of the system determined by the crossing of the fluxons across the sample. However, in this frequency range the dynamically synchronized state has an alternative---a less ordered state with smaller amplitude of the output voltage. Collective behavior of the junctions is strongly affected by the sloped sidewalls of the stack. Synchronization is observed only for weakly trapezoidal cross sections, whereas irregular motion of fluxons is observed for larger slopes of the sample edge.

Radio-frequency amplification property of the MgO-based magnetic tunnel junction using field-induced ferromagnetic resonance

The radio-frequency (RF) voltage amplification property of a tunnel magnetoresistance device driven by an RF external-magnetic-field-induced ferromagnetic resonance was studied. The proposed device consists of a magnetic tunnel junction and an electrically isolated coplanar waveguide. The input RF voltage applied to the waveguide can excite the resonant dynamics in the free layer magnetization, leading to the generation of an output RF voltage under a DC bias current. The dependences of the RF voltage gain on the static external magnetic field strength and angle were systematically investigated. The design principles for the enhancement of the gain factor are also discussed.

Design and demonstration of an on-chip AC power source for adiabatic quantum-flux-parametron logic

Adiabatic quantum-flux-parametron (AQFP) logic is a promising technology for use in future high-end computers because its bit energy can be potentially reduced to the order of thermal energy. AQFP gates are driven by an AC bias current, which changes their potential energy adiabatically. In the present paper, we propose an on-chip AC power source, which is composed of a Josephson relaxation oscillator and a superconducting resonator, to drive the AQFP gates. We designed and implemented the AC power source using a Nb Josephson integrated circuit process. We confirmed that the AC power source can generate a 4.4 GHz sinusoidal current from a DC bias current and that the output amplitude of the AC power source can be controlled by varying the DC bias current.

Electrical Spin Injection and Detection in Silicon Nanowires through Oxide Tunnel Barriers

We demonstrate all-electrical spin injection, transport, and detection in heavily n-type-doped Si nanowires using ferromagnetic Co/Al(2)O(3) tunnel barrier contacts. Analysis of both local and nonlocal spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency (up to ~30%) and long spin diffusion lengths (up to ~6 μm) are achieved. These values exceed those reported for spin transport devices based on comparably doped bulk Si. The spin valve signals are found to be strongly bias and temperature dependent and can invert sign with changes in the dc bias current. The influence of the nanowire morphology on field-dependent switching of the contacts is also discussed. Owing to their nanoscale geometry, ~5 orders of magnitude less current is required to achieve nonlocal spin valve voltages comparable to those attained in planar microscale spin transport devices, suggesting lower power consumption and the potential for applications of Si nanowires in nanospintronics.