High-speed Magnetic Field Research Articles

Mapping AC susceptibility with quantum diamond microscope

We present a technique for determining the micro-scale AC susceptibility of magnetic materials. We use the magnetic field sensing properties of nitrogen-vacancy (NV-) centers in diamond to gather quantitative data about the magnetic state of the magnetic material under investigation. A quantum diamond microscope with an integrated lock-in camera is used to perform pixel-by-pixel, lock-in detection of NV- photo-luminescence for high-speed magnetic field imaging. In addition, a secondary sensor is employed to isolate the effect of the excitation field from fields arising from magnetic structures on NV- centers. We demonstrate our experimental technique by measuring the AC susceptibility of soft permalloy micro-magnets at excitation frequencies of up to 20Hz with a spatial resolution of 1.2 µm and a field of view of 100 µm. Our work paves the way for microscopic measurement of AC susceptibilities of magnetic materials relevant to physical, biological, and material sciences.

Application of magnetic field to reduce the forced response of steel bridges to high speed train

This paper uses a train-track-bridge interaction system to assess the dynamic performance of railway bridges exposed to a high-speed train and magnetic field. A 24° of freedom 3D train model and thin steel bridge beam are considered. In the interaction of train and bridge, a new six-parameter track system consisting of rail, sleeper, and ballast is modeled. The governing equations of the bridge, track and train motions are derived based on the Lagrange method. The Lorentz force induced by the directed magnetic field in the axial direction is obtained by Maxwell's equation. Using state-space forms, the second-order equations of motion are transformed into first-order differential equations, which are then solved using the Runge-Kutta method. Studies using parametric data are done to show how the suggested approach may be used to investigate the dynamic interaction of the entire system. The magnetic field intensities and moving train speed on the interaction of the railway bridge system were investigated and analyzed for the first time in the literature. Depending on the speed of the vehicle, when the dimensionless magnetic field is Hxm=30, it can be seen that the train body's vertical displacement falls by around 50%. The obtained results are helpful for the design of railway bridges and the safe and comfortable ride of high-speed trains over flexible structures.

Simulation of Special Distribution of energy of plasma in a High-speed Changing Magnetic Field

Tokamak and inertial confinement fusion(ICF) are two kinds of devices to achieve fusion. Magnetic and gravitational fields are the drivers used in tokamak and ICF, respectively. In other fusion devices like MIF, both of them are used. In this work, different from these fusion devices mentioned above, a new promising method to confine plasma is proposed. Spacial distribution of energy of plasma in a high-speed changing magnetic field is simulated. This method is expected to achieve fusion.

Switching of magnetoresistive light-emitting diode by external magnetic field

A magnetic switch consisting of an integrated magnetoresistive element and a light-emitting diode has been developed and investigated. Switching the state of a magnetoresistive part allows one to control the electron vs hole current ratio in a light-emitting diode. In the high-resistance state, the hole current is high enough to trigger the electroluminescence. Switching to a low-resistance state decreases the hole current drastically, thus completely switching off the light-emitting diode. The effect was found to be strongly dependent on the operating current of the integrated circuit. We believe that the observed effect has multiple applications as a high-speed magnetic field sensor with an opto-coupler, a magnetic field visualizer, or an optical device for reading magnetoresistive random access memory.

The acceleration of laser plasma in a strong non-stationary magnetic field

Based on experimental and computer simulation the acceleration of deuterons from laser plasma in a strong non-stationary magnetic field was studied. The possibility of reaching an energy of ∼100 keV, corresponding to the effective course of the nuclear reactions D (d, n) 3He and T (d, n) 4He, was demonstrated. YAG: Nd3+ laser (W ≤ 0.85 J, τ ≈ 10 ns) was used in the experiment with focusing laser radiation on a deuterated polyethylene target. The high voltage pulse generator with a conical spiral coil was used to generate a high-speed magnetic field (2·107 T/s). A mathematical model of the process is proposed. According to this model, the acceleration of a laser plasma is analyzed by means of a computer. The algorithm is based on a numerical solution of the system of Newton-Lorentz equations.

Study of the effect of an external magnetic field on the photoelectric properties of a copper phthalocyanine film

In the article the results of a study of the influence of a magnetic field on the change in current-voltage char- acteristics and charge carrier mobility in a solid film of copper phthalocyanine (CuPc) are presented. A solid CuPc film on a substrate with a conductive ITO surface (Indium Tin Oxide) was obtained by thermal evapo- ration in a vacuum. The absorption spectrum of CuPc was measured; as a result, copper phthalocyanine mol- ecules on the substrate surface were found to be in the metastable α-phase. Measurements of current-voltage characteristics (IVC) were carried out using a potentiostat-galvanostat P20X in the linear sweep mode. To study the effect of a magnetic field on the photoelectric characteristics, a sample was placed between the poles of an electromagnet. A xenon lamp with radiation intensity of 50 mW/cm2 was used as a simulator of sunlight. As a result of experiments, it was found that the external magnetic field affects the value of the short-circuit current, at the same time, the charge carrier mobility in a magnetic field does not change. The studied effect of a change in the current-voltage characteristics in a solid CuPc film upon application of an external magnetic field can be used as a high-speed magnetic field sensor.

A multi-channel high-speed magnetic field detection system based on FPGA for transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS), a popular technology, acts on the brain by using a pulse magnetic field to cause a series of physiological and biochemical reactions. In order to detect the magnetic field generated by the TMS coil with high-speed and multi-channel performance, a novel magnetic field detection system based on a field programmable gate array (FPGA) is designed and implemented. The detection system includes an induction coil array, a data acquisition (DAQ) card, and upper computer monitor software. The DAQ card contains analog signal processing circuits, a multiplexer, an analog-to-digital converter, and a FPGA with a high-speed, parallel, and switching idea. The system can sample at a rate of 500 ksps, with 14-bit resolution and 12 channels. The three dimensional (3D) magnetic field can be monitored on the screen with a waveform display and 3D magnetic field vector display. The DAQ card has a good signal noise and distortion and cross talk of 88.35 dB and -79.69 dB, respectively. Compared with the NI DAQ card, the proposed system has a relative error smaller than 1.81% and a mean square error smaller than 2.89 × 10-6, which verifies that the proposed detection system has a good performance. The multi-channel high-speed magnetic field detection system provides an important platform for the study of TMS in medical, engineering, and other fields.

Dynamic response of tunneling magnetoresistance sensors to nanosecond current step

We investigate the dynamic response of state-of-the-art CoFeB/MgO/CoFeB sensors to nanosecond current step. We use sensors with different field sensitivity (FS) that is induced by voltage controlled magnetic anisotropy (VCMA). In the sensors rise and response times reach down to 15ns and 11ns, respectively and are sensitivity independent. The settling time of the sensors changes with field sensitivity and reaches 335ns for FS=24V/T and 137ns for FS=10V/T. The sensor 3dB bandwidth estimated from the rise time reaches 15MHz. This superior dynamic performance of the TMR sensors opens possibilities of using them for high-speed magnetic field and current sensing and controlling their dynamic response by the sensitivity.

Effect of MgO thickness and bias voltage polarity on frequency response of tunneling magnetoresistance sensors with perpendicular anisotropy

We investigate the frequency response of CoFeB/MgO/CoFeB based tunneling magnetoresistance sensors with the perpendicular anisotropy. The 3 dB cut-off frequency (f3dB) strongly and non-monotonically depends on resistance-area product (RA), which is controlled by MgO thickness. Both high and low RA sensors achieved wide frequency bandwidth; however, we observed a maximum f3dB of 45 MHz at around 20 kΩ μm2. The bandwidth of the sensors increases with bias voltage of both polarity and is higher under positive polarity. This opens possibilities of using the sensors for high-speed magnetic field sensing and for improving their bandwidth by tuning RA and the bias.

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.

High Q factor over 3000 due to out-of-plane precession in nano-contact spin-torque oscillator based on magnetic tunnel junctions

We investigated microwave oscillation in a sombrero-shaped nano-contact spin-torque oscillator under an external magnetic field with an out-of-plane component. By increasing the out-of-plane field, we observed an abrupt change from in-plane precession to out-of-plane precession around the effective demagnetization field of the free layer. The out-of-plane precession under optimized conditions yielded not only a fundamental oscillation peak with a very large Q factor (3200) but also higher-order harmonic peaks with frequencies up to at least 45 GHz. A high Q factor and high-frequency oscillations are of great importance for device applications such as telecommunications, radar sensors, and high-speed magnetic field sensors.

Fast magneto-optic switch based on nanosecond pulses

The paper studies an all fiber high-speed magneto-optic switch which includes an optical route, a nanosecond pulse generator, and a magnetic field module in order to reduce the switching time of the optical switch in the all optical network. A compact nanosecond pulse generator can be designed based on the special character of the avalanche transistor. The output current pulse of the nanosecond pulse generator is less than 5 ns, while the pulse amplitude is more than 100 V and the pulse width is about 10 to 20 ns, which is able to drive a high-speed magnetic field. A solenoid is used as the magnetic field module, and a bismuth-substituted rare-earth iron garnet single crystal is chosen as the Faraday rotator. By changing the direction of current in the solenoid quickly, the magnetization of the magneto-optic material is reversed, and the optical beam can be rapidly switched. The experimental results indicate that the switching time of the device is about 100 to 400 ns, which can partially meet the demand of the rapid development of the all optical network.

Magnetic pulse generation for high-speed magneto-optic switching

In this article, the magnetic pulse characteristics needed to achieve high-speed magneto-optic (MO) switching are investigated. A fiber-based, MO, low-voltage optical switch capable of 200 ns switching is presented, along with the special circuit characteristics for magnetic field generation for high-speed switching. The switch consists of the optical system, the MO material (bismuth substituted iron garnet [(Bi1.1Tb1.9)(Fe4.25Ga0.75)O12]), and a high-speed magnetic field driving circuit. A Faraday rotator is placed within the interferometric loop of a fiber-optic Sagnac interferometer, and interference at the output ports is controlled by the applied field. The fast switching speed is accomplished via the special design of the magnetic pulse generation circuitry. The applied magnetic field overshoots the field necessary to achieve the desired Faraday rotation and then settles to a steady state field. If the duration of the overshoot is less than the time it takes the material to saturate, a fast optical switching time can be achieved without saturating the material. The effects of the overshoot amplitude and duration and steady-state amplitude on optical rise time (determined by domain wall velocity) are studied and experimental results are presented.

Visualization of current sheet evolution in a pulsed plasma accelerator

High-speed photography and magnetic field probes were used to visualize and study the formation and propagation of current sheets in a pulsed plasma accelerator. Magnetic field measurements complement photographic records, as the latter indicate the location of the plasma but not necessarily current, whereas magnetic field data provide an unambiguous picture of the current location, but do not directly yield the location of the plasma. The observed current sheet evolution was found to be rich in features and has produced fundamental insight into the phenomena of current sheet canting and trailing wake formation.

Zero-field magnetic resonance of the photo-excited triplet state of pentacene at room temperature

The pulsed EPR free induction decay (FID) signals of the photo-excited pentacene triplet state are reported for three mixed crystals at room temperature: pentacene-h14 in p-terphenyl, pentacene-h14 in benzoic acid, and pentacene-d14 in p-terphenyl. The recorded FID signals have relatively long decay times of about four microseconds, presumably due to the reduced hyperfine interactions in the zero magnetic field. The time domain FID signals transform to spectral components typically narrower than 500 kHz, allowing us to determine the pentacene triplet zero field splitting parameters to better accuracy than previously reported. Further, a new experimental technique using the high speed magnetic field jumping capability enables us to examine the anisotropic hyperfine and quadrupole interactions.

Increased electron mobility of InAsSb channel heterostructures grown on GaAs substrates by molecular beam epitaxy

We have designed and grown high-electron-mobility heterostructures that use InAsySb1−y group V alloys as a channel material and that can be used in high-speed transistors and magnetic field sensors. The group V alloys were formed by modulating As2 and Sb2 beams during growth. The composition was controlled by changing the group V shutter cycle. The electron mobility in the InAsySb1−y channel, which is only 20–30 nm thick and is sandwiched between Al0.15In0.85Sb high-resistivity barrier layers, was increased to 28 000 cm2 V−1 s−1 at room temperature by reducing the lattice mismatch between the channel layer and the barrier layer. This mobility is an order of magnitude greater than that of the strained Al0.15In0.85Sb/InSb/Al0.15In0.85Sb heterostructure grown as a reference. The electron mobility in the InAsySb1−y channel sandwiched between Al0.5Ga0.5Sb barrier layers was also increased from 19 500 cm2 V−1 s−1 (y=1.0) to 24 500 cm2 V−1 s−1 (y=0.86) at room temperature by reducing the lattice mismatch between the channel layer and the barrier layer. These increases in mobility indicate that the lattice mismatch must be reduced in order to achieve a high electron mobility of such heterostructures grown mismatched on GaAs substrates.

Comparison of ferrite materials for pulse applications

Materials are the limiting factor in many pulse power projects. The magnetic materials available from several manufacturers were experimentally compared for their usefulness in high-speed magnetic field applications. This particular application is a high-speed kicker magnet for injection of 150 GeV antiprotons into the Tevatron at Fermilab.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Extended Capacity and High Speed Magnetic Field Modulation Recording for 3.5" Magnetooptical Disk

The authors attempted recording by magnetic field modulation overwriting with a flying magnetic head, using a (1,7) RLL code, mark-edge recording, write precompensation, equalization, and the ZCAV format. A flying magnetic head for high-speed magnetooptic recording was developed. It was confirmed that the magnetic head has a magnetic efficiency of 1.38 Oe/mA at a spacing of 18 ?m and a modulation frequency of 8.16 MHz. Under these conditions, a linear recording density of 0.55 ?m/bit and an overwrite power margin in excess of ±25% were achieved. A 3.5 magnetooptic disk drive with a storage capacity greater than 300 MB and a high data transfer rate of 2.7 MB/s at the outermost track was realized.

Large-Capacity and High-Speed Magnetic Field Modulation Recording for 3.5′′ Magneto-Optical Disk Drive

Recording by magnetic field modulation overwriting with a flying magnetic head and adopting a 1-7 RLL code, mark-edge recording, and ZCAV format, was attempted. A 3.5′′ magneto-optical disk drive with a large storage capacity of greater than 300 MBytes and a high data transfer rate of 2.7 MBytes/s on the outermost track was realized.

High speed magnetic field measurement of a large spectrometer magnet

Magnetic field mapping was carried out using a search coil method for a large gap spectrometer magnet. The magnetic field volume of 3 × 3.5 × 1 m 3 was scanned by 18 sets of coils, each of which had three-dimensional coils. The performance of the high speed mapping is described.