Development of high field SQUID magnetometer for magnetization studies up to 7 T and temperatures in the range from 4.2 to 300 K

We present the design, fabrication, integration, testing, and calibration of a high field superconducting quantum interference device (SQUID) magnetometer. The system is based on dc SQUID sensor with flux locked loop readout electronics. The design is modular and all the subsystems have been fabricated in the form of separate modules in order to simplify the assembly and for ease of maintenance. A novel feature of the system is that the current induced in the pickup loop is distributed as inputs to two different SQUID sensors with different strengths of coupling in order to improve the dynamic range of the system. The SQUID magnetometer has been calibrated with yttrium iron garnet (YIG) sphere as a standard reference material. The calibration factor was determined by fitting the measured flux profile of the YIG sphere to that expected for a point dipole. Gd(2)O(3) was also used as another reference material for the calibration and the effective magnetic moment of the Gd(3+) could be evaluated from the temperature dependent magnetization measurements. The sensitivity of the system has been estimated to be about 10(-7) emu at low magnetic fields and about 10(-5) emu at high magnetic fields ～7 T.     

Note: Low temperature superconductor superconducting quantum interference device system with wide pickup coil for detecting small metallic particles

A one-channel low temperature superconductor superconducting quantum interference device system comprising a second-order axial gradiometer with a sensing area of 10 mm × 190 mm was developed. The gradiometer was mounted in a liquid-helium dewar (450-mm diameter; 975-mm length), with a gap of 12 mm between the pickup coil and the dewar-tail surface. The magnetic field sensitivity was measured to be 16 fT∕Hz(1∕2) in the white noise regime above 2 Hz. The system was used to measure stainless steel particles of different sizes passing through the sensing area. A 100-μm diameter SUS304 particle was readily detected passing at different positions underneath the large pickup coil by measuring its 1.3-pT magnetic field. Thus, the system was shown to be applicable to quality control of lamination sheet products such as lithium ion batteries.     

77K VHF SQUID低噪声磁强计的研制

本文阐述了为提高７７Ｋ超导量子干涉器（ｓｕｐｅｒｃｏｎｄｕｃｔｉｎｇｑｕａｎｔｕｍｉｎｔｅｒｆｅｒｅｎｃｅｄｅｖｉｃｅ,简称ＳＱＵＩＤ）磁强计的磁场灵敏度,选择ＳＱＵＩＤ载波电流频率在甚高频（简称ＶＨＦ）频段的优点,并介绍了在ＳＱＵＩＤ磁强计电路中采用频率合成及锁相技术来实现电路低噪声工作原理,使磁测有比较高的信噪比,该磁强计在地球物理勘查的弱磁测量上将得到广泛的应用。     

Characteristics of the dual-bridge giant magnetoresistance magnetometer

The responsivity and the field noise characteristics of the giant magnetoresistance (GMR) magnetometer driven by an alternating magnetic field are investigated. The harmonic spectrum of the sensor voltage shows that only the odd harmonics of excitation frequency are sensitive to the external dc field. To cancel out the even harmonics, the dual-bridge GMR magnetometer with the balanced output is proposed. With the dual-bridge configuration, the field-to-voltage transfer coefficient is doubled and the field noise is reduced by a factor of 1.4. The minimum field noise is 7 nT∕√Hz at 1 Hz with the ac excitation power of 5.5 mW. The proposed sensor is suitable for the electronic compass application.     

Hardware comb filter enhances dynamic range and noise performance of sensors in noisy environments

We present the results of combining a hardware implementation of an analog comb filter with an ultralow noise electromagnetic sensor. The comb filter is designed to attenuate mains related interference, at either 50 or 60 Hz, and related harmonics. The sensor chosen for this work is an induction magnetometer, but the method is applicable to any low noise high dynamic range sensor. The resultant system, in this case, uses only a single coil, not a gradiometric configuration, thus providing a magnetometer capable of sensing field as opposed to field gradient. This combination of filter and sensor allows additional gain to be added and the full sensitivity of the system to be achieved, previously only realized in an electromagnetically screened room. At the same time, the high dynamic range, low noise performance, and original bandwidth of the sensor are maintained. The technique is illustrated by using the system in an urban environment to observe Schumann resonance phenomena. This approach to acquiring small signals in a noisy environment is compared with conventional analog filter and digital signal processing techniques.     

星载感应式空心磁芯磁传感器的研究

优化设计了频率带宽为1 Hz~10 k Hz的空心磁芯感应式磁传感器。首先提出空心结构的磁芯,分析空心磁芯的退磁因数和有效磁导率,并仿真、实测空心磁芯感应线圈的磁通,得出一定壁厚的空心磁芯感应线圈的磁通可以等于相同长径比棒状磁芯感应线圈的磁通。然后分析空心磁芯感应式磁传感器的灵敏度与噪声,通过数学算法优化噪声公式达到要求的噪声指标和较低的重量。为验证理论分析,设计空心磁芯感应式磁传感器,在屏蔽室对空心磁芯感应式磁传感器的性能进行测试,频率高于400 Hz时灵敏度为0.73 V/n T。100 Hz时噪声为0.06 p T/Hz1/2,总重量为80 g。实测的空心磁芯感应式磁传感器的灵敏度与噪声和理论分析一致。空心磁芯感应式磁传感器与THEMIS相比具有噪声低、重量轻的优点,能够满足空间电磁探测的实用条件。     

Accurate periodicity measurement of superconducting quantum interference device magnetic flux response

It is theoretically explained that a response of a superconducting quantum interference device (SQUID) is periodically dependent on total magnetic flux coupling to the SQUID ring (Φ) and its period is a flux quantum (Φ(o)=h/2e, where h and e, respectively, express Planck's constant and elementary charge). For example, the voltage of an electromagnetically oscillated rf-SQUID or a current biased dc-SQUID is thought to be periodically dependent on Φ with a period of Φ(o). In this paper, we propose an accurate method to check the periodicity of a SQUID response by using a set of sensing coils covered with a superconducting sheath. As a demonstration, we measured periodicity of a commercially available thin-film type rf-SQUID response in magnetic flux ranging up to approximately 4300Φ(o). Its flux dependence was periodic below about 3400Φ(o).     

Gradiometric micro-SQUID susceptometer for scanning measurements of mesoscopic samples

We have fabricated and characterized micro-SQUID susceptometers for use in low-temperature scanning probe microscopy systems. The design features the following: a 4.6 mum diameter pickup loop; an integrated field coil to apply a local field to the sample; an additional counterwound pickup-loop/field-coil pair to cancel the background signal from the applied field in the absence of the sample; modulation coils to allow setting the SQUID at its optimum bias point (independent of the applied field), and shielding and symmetry that minimizes coupling of magnetic fields into the leads and body of the SQUID. We use a SQUID series array preamplifier to obtain a system bandwidth of 1 MHz. The flux noise at 125 mK is approximately 0.25 mu Phi 0/ sqrt Hz above 10 kHz, with a value of 2.5 mu Phi 0/ sqrt Hz at 10 Hz. The nominal sensitivity to electron spins located at the center of the pickup loop is approximately 200 muB/ sqrt Hz above 10 kHz, in the white-noise frequency region.     

High-performance UHF SQUID magnetometer

We describe a lumped-component stabilized point contact SQUID magnetometer, biased at 430 MHz and utilizing a cooled (4.2 K) GaAs FET preamplifier. The system incorporateds a full-bandwidth (5-10 MHz) flux-locked loop and has an overall flux sensitivity of 1x10(-5) Phi0/(Hz)(1/2), corresponding to an energy sensitivity approximately 4.5x10(-31) J/Hz.     

A high-sensitive static vector magnetometer based on two vibrating coils

A static vector magnetometer based on two-dimensional (2D) vibrating coils actuated by a piezoelectric cantilever is presented. Two individual sensing coils are orthogonally fastened at the tip of cantilever and piezoelectric sheets are used to excite the cantilever bending. Due to off-axis coupler on the tip, the cantilever generates bending and twisting vibrations simultaneously on their corresponding resonant frequencies, realizing the 2D rotating vibrations of the coils. According to Faraday-Lenz Law, output voltages are induced from the coils. They are amplified by a pre-amplifier circuit, decoupled by a phase-sensitive detector, and finally used to calculate the vector of magnetic field at the coil location. The coil head of a prototype magnetometer possesses a dc sensitivity of around 10 μV/Gs with a good linearity in the measuring range from 0 to 16 μT. The corresponding noise level is about 13.1 nT in the bandwidth from 0.01 Hz to 1 Hz.     

Cryogenic ultra-low-noise SiGe transistor amplifier

An ultra-low-noise one-stage SiGe heterojunction bipolar transistor amplifier was designed for cryogenic temperatures and a frequency range of 10 kHz-100 MHz. A noise temperature T(N) ≈ 1.4 K was measured at an ambient temperature of 4.2 K at frequencies between 100 kHz and 100 MHz for a source resistance of ~50 Ω. The voltage gain of the amplifier was 25 dB at a power consumption of 720 μW. The input voltage noise spectral density of the amplifier is about 35 pV/√Hz. The low noise resistance and power consumption makes the amplifier suitable for readout of resistively shunted DC SQUID magnetometers and amplifiers.     

GMR array uniform eddy current probe for defect detection in conductive specimens

The usage of eddy current probes (ECP) with a single magnetic field sensor represents a common solution for defect detection in conductive specimens but it is a time consuming procedure that requires huge amount of scanning steps when large surface specimens are to be inspected. In order to speed-up the nondestructive testing procedure, eddy current probes including a single excitation coil and an array of sensing coils present a good solution. The solution investigated in this paper replaces the sensing coils for giant magneto-resistors (GMRs), due to their high sensitivity and frequency broadband response. Thus, the ECP excitation coil can be driven at lower frequencies than the traditional ones allowing defects to be detected in thicker structures.In this work an optimized uniform eddy current probe architecture including two planar excitation coils, a rectangular magnetic field biasing coil and a GMR magnetometer sensor array is presented. An ac current is applied to the planar spiral rectangular coil of the probe, while a set of GMR magnetometer sensors detects the induced magnetic field in the specimens under test. The rectangular coil provides the DC uniform magnetic field, assuring appropriate biasing of the GMR magnetometers of the probe, setting-up the functioning point on the linear region and at the same branch of the GMR static characteristics. The differences on the images obtained for the same specimen for each GMR are reduced if all sensors are biased on the same working point. Elements of the automated measurement system used to inspect the plate under test using the proposed eddy current probe, including a validation procedure based on a 2D template matching algorithm and the corresponding experimental results are included in the paper.     

插接式智能组合电器中电子式电流传感器的结构特性分析

电力系统中电子式电流传感器替代传统电磁式电流互感器是发展方向。首先简要介绍一种应用在插接式智能组合电器中的电子式电流传感器的基本测量原理 ,然后对传感器的结构特性进行分析 ,主要包括温度、线圈的多层绕制与不均匀绕制、电流源的放置方式以及磁场的不均匀分布等因素对测量结果的影响 ,最后对多种因素共同作用下的影响进行综合分析 ,分析结果对改善电流传感器的性能有指导作用。     

High temperature superconductor micro-superconducting-quantum-interference-device magnetometer for magnetization measurement of a microscale magnet

We have developed a high temperature superconductor (HTS) micrometer-sized dc superconducting quantum interference device (SQUID) magnetometer for high field and high temperature operation. It was fabricated from YBa2Cu3O7-delta of 92 nm in thickness with photolithography techniques to have a hole of 4x9 microm2 and 2 microm wide grain boundary Josephson junctions. Combined with a three dimensional magnetic field coil system, the modulation patterns of critical current Ic were observed for three different field directions. They were successfully used to measure the magnetic properties of a molecular ferrimagnetic microcrystal (23x17x13 microm3), [Mn2(H2O)2(CH3COO)][W(CN)8]2H2O. The magnetization curve was obtained in magnetic field up to 0.12 T between 30 and 70 K. This is the first to measure the anisotropy of hysteresis curve in the field above 0.1 T with an accuracy of 10(-12) J T(-1) (10(-9) emu) with a HTS micro-SQUID magnetometer.     

Fabrication and characterization of a new MEMS fluxgate sensor with nanocrystalline magnetic core

This paper presents a new MEMS fluxgate sensor with a Fe-based nanocrystalline ribbon magnetic core and 3D micro-solenoid coils. The excitation coils were placed vertically to the sensing coil on the chip plane. Second harmonic operation principle was adopted in this fluxgate sensor. The total size of the fluxgate sensor was 6.25 mm × 4.85 mm × 120 μm. A simple testing system was established to characterize the fabricated devices. A band pass filter was used to pick up the second harmonic signals in the sensing coils. When excitation rms current of 120 mA and the operational frequency of 200 kHz were selected for the testing of the fabricated devices, the sensitivity of the developed fluxgate sensor was 1005 V/T in the linear range of −500 μT to +500 μT. Due to the combination of the 3D structure coils with the nanocrystalline core, relatively low sensor noise was achieved. The noise power density was 544 pT/Hz^0.5@1 Hz and the noise rms level was 9.68 nT in the frequency range of 25 mHz-10 Hz.     

A high frequency sensor for optical beam deflection atomic force microscopy

We demonstrate a novel electronic readout for quadrant photodiode based optical beam deflection setups. In our readout, the signals used to calculate the deflections remain as currents, instead of undergoing an immediate conversion to voltages. Bipolar current mirrors are used to perform all mathematical operations at the transistor level, including the signal normalizing division. This method has numerous advantages, leading to significantly simpler designs that avoid large voltage swings and parasitic capacitances. The bandwidth of our readout is solely limited by the capacitance of the quadrant photodiode junctions, making the effective bandwidth a function of the intensity of photocurrents and thus the applied power of the beam deflection laser. Using commercially available components and laser intensities of 1-4 mW we achieved a 3 dB bandwidth of 20 MHz with deflection sensitivities of up to 0.5-1 V/nm and deflection noise levels below 4.5 fm/Hz. Atomic resolution imaging of muscovite mica using FM-AFM in water demonstrates the sensitivity of this novel readout.     

基于马赫-曾德干涉仪的光纤电流互感器设计

电流互感器作为一次侧电流的常用计量设备之一,它在电力系统的交流电测量、继电保护、电力设备检修控制等相关领域均具有十分重要的地位。针对传统电流互感器存在的线性度低、抗干扰能力差及精度低等问题,尤其在工频小电流精确测量这一难题方面更是捉襟见肘。鉴于此,本文提出一种基于马赫-曾德干涉仪的光纤电流互感器。该互感器利用电流经过电阻所产生的焦耳热改变光纤中的光信号特征,从而引起马赫-曾德干涉仪的输出信号变化,然后通过光电探测器完成对干涉仪的光信号捕捉,并将其转换为电信号输出,完成工频小电流检测。结构上通过采用PCB型罗氏线圈作为感应取电装置,并利用MATLAB完成其动态响应分析。另外针对信号采集、传输、处理过程中所出现的噪声信号,设计了基于FPGA技术的信号处理系统,以提高信噪比。最后通过实验对工频小电流进行测量,线性度为0.996 1,检测精度可达到0.14%。实验结果表明提出的光纤电流互感器相较于传统光学电流互感器相比,具有更高的线性度和测量精度,为基于热效应的电流互感器发展提供了新的思路,也为工频小电流的测量提供了一种新的方法。     

Development of superconducting contacts for the CRESST II 66-channel superconducting quantum interference device readout system

The CRESST experiment is designed to search for weakly interacting massive particle dark matter with cryogenic detectors. CRESST II will use up to 33 CaWO(4) crystals with a total mass of approximately 10 kg. These many detectors require a readout system based on 66-channel superconducting quantum interference devices (SQUIDs). In this article we report on the development of a modular superconducting connector for the 66-channel SQUID readout circuit. We show that the technique developed reliably produces superconducting contacts.     

A Radio-Frequency High-Temperature Superconducting Magnetometer

A radio-frequency high-temperature superconducting magnetometer is described. It is based on the generation of combination frequencies in a nonlinear ceramic-superconductor sensor with a resonance circuit at its input. Measurements at the combination frequency allows for an improvement of the noise and frequency characteristics as compared to a magnetometer based on the generation of the second harmonic in the superconductor. The instrument has a level of intrinsic noise of (2–4) × 10^–8 Oe/Hz^1/2 within a frequency range of 17 kHz.     

光纤电流互感器中Rogowski线圈的设计与分析

Rogowski线圈作为传感头是光纤电流互感器的关键部分,文章分析了Rogowski线圈的测量原理和等效电路,得出了测量关系,并根据分析设计了一个Rogowski线圈。实验表明该Rogowski线圈具有良好的线性度;频率特性分析显示该Rogowski线圈有非常充裕的带宽,进而为设计高精度的光纤电流互感器奠定了基础。