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Student-constructed magnetometers and commonly available software illustrate the use of autocorrelation functions to extract low-amplitude signals from background noise. The magnetometers are later used to measure forces at sampling rates over 50 Hz, providing data for an introduction to numerical integration. [DOI: 10.1119/1.2919741]

We present an extension of the nonlinear two-step estimation algorithm originally developed for the calibration of solid-state strapdown magnetometers. We expand the algorithm to include nonorthogonality within a sensor set for both two- and three-axis sensors. Nonorthogonality can result from manufacturing issues, installation geometry, and in the case of magnetometers, from soft iron bias errors. Simulation studies for both two- and three-axis sensors show convergence of the improved algorithm to the true values, even in the presence of realistic measurement noise. Finally the algorithm is experimentally validated on a low-cost solid-state three-axis magnetometer set, which shows definite improvement postcalibration. We note that the algorithm is general and can be applied to any two- or three-axis sensor set (such as accelerometers) with an error model consisting of scale, offset, and nonorthogonality errors.

A magnetic-field-sensing system that consists of a miniature zero-power magnetometer, an integrated micromachined corner-cube reflector (CCR), a commercially available diode laser, and a photodetector array, has been designed, fabricated, assembled, and tested. The zero-power-magnetometer design is based on a ferromagnetic MEMS magnetometer, which consists of a permanent magnet that is torsionally suspended to allow rotation about a single axis. A single mirror of the CCR is attached to the magnetometer torsion beam. When the magnet rotates in response to a change in magnetic field, the torsion beam will twist and cause the mirror to become misaligned. The non-ideality of the CCR can be interrogated with a laser and a photosensor array can be used to measure the reflected signal. This method of magnetic sensing completely eliminates sensor-node power consumed at the remote location. The sensor node developed for this paper occupies a volume of only 1.5 [mm.sup.3] and can detect a magnetic field between 820 A/m to 6 kA/m with an uncertainty of 90 A/m at a l-m optical-interrogation range. The primary application of this technology is in wireless sensing systems that must operate continuously without providing or maintaining the sensor-node energy (e.g., replacing batteries or scavenging energy) or in extremely harsh environments where power supplies and integrated circuits (IC) are not an option. [1616] Index Terms--Ferromagnetic materials, magnets, magnetometers, microelectromechanical devices, microsensors, multisensor systems, optical device fabrication, optical diffraction.

High-temperature Superconducting Quantum Interference Filters (SQIF's), which are multi-loop arrays of Josephson junctions possessing unconventional grating structures, were designed with voltage swings of more than 3 mV and a rather large dynamical range. This large dynamical range allows high-Tc SQIF's to be operated in commercial miniature cryocoolers at 50 K-80 K without any significant problems.

An alternative magnetometer device for low frequency applications, based on the combination of a superconducting flux-to-field transformer and a Giant-Magnetoresistance (GMR) element is presented. The performance of this device depends on the local enhancement of the applied field by the transformer and on the sensitivity of the sensing element.

Fabrication and characterization of nine Superconducting Quantum Interference Device (SQUID) magnetometers are done, which are integrated on a single chip. This magnetometer will be useful for multi-channel measurement of magnetic fields requiring highly spatial resolution.

Magnetometers with multiple-SQUID arrays are designed and fabricated to achieve higher signal to noise ratio compared to that for a single SQUID. To avoid forming super conducting loops between junctions of adjoining SQUIDs, a Au wiring layer interrupts a current bias line.

A new figure of merit based on spatial frequency features of the gradiometer is proposed to overcome the issue of combination of the associated field noise spectrum with description of the geometry. It consists of obtaining its spatial frequency response and extracting from it relevant parameters such as magnitude and bandwidth.

A dc superconducting quantum interference device magnetometer is used to develop a biomagnetic measurement system on mice. The results obtained indicate the ability of the system in evaluating the mice magentocardiogram (MCG) signals.

The fabrication of a three axis LTS SQUID magnetometer for us as a magnetic anomaly detector on an aircraft, is described.

The spatial resolution and sensitivity of a magnetometer for imaging magnetic fields of biological tissues and samples have been examined. Results indicate that the optimum sensitivity and spatial resolution can be attained when the magnetometer has a diameter of 600 micrometers. A magnetic field sensitivity of 120 fTHz raised to the -1/2 power is achieved with more than five loops. A field sensitivity of 320 fTHz raised to the -1/2 power requiring four loops is realized when the sensor size decreases to 300 micrometers.

A susceptometer based on planar DC superconducting quantum interference devices has been designed and constructed. The device is used in examining microcrystallites as a function of temperature from 0.3 K to 4 K in magnetic fields greater than 280G. Large, in-plane static fields can be applied through an external coil while perpendicular fields up to 2G can be applied by using a series-wound on-chip field coil. Paramagnetic, antifferomagnetic and superconducting transitions have been resolved on samples with sizes ranging from 20 to 50 micrometers.

A high-temperature superconducting fluxgate magnetic sensor with a YBCO core was built and used in inspecting damage in an aluminum plate. The magnetic sensor can work in unshielded environments and detected the presence of a slit that is 0.2 mm wide and 30 mm long on the aluminum plate. The sensor can detect the slit even if it is covered with another aluminum plate. It can sense the flaw when it is oriented perpendicular to the electric current. There was an increase in the detected magnetic field as the width became bigger.

Low-noise Y-BA-Cu-O flip-chip Superconducting Quantum Interference Devise (SQUID) magnetometers were developed using direct current SQUIDs with large flux-to-voltage transfer functions and superconducting flux transformers. The fabricated flip-chip magnetometers are characterized by a large voltage swing of about 60 microvolts. They are also observed to exhibit increased field sensitivity because of the use of the multiturn flux transformer.

An improved design for a radio frequency Superconducting Quantum Interference Devise (rf SQUID) magnetometer with a multi-turn flux transformer integrated with a superconducting labyrinth resonator is presented. The structure of the magnetometer was patterned from 200 nm-thick epitaxial YBa2Cu3O7 films deposited on 10x10 mm2 LAALO3 substrates. The proposed rf SQUID magnetometer was observed to have a field sensitivity of 16 fT/Hz and white field noise of 11.5 fT/Hz.

High-temperature superconducting direct current-Superconducting Quantum Interference Devise (HTS dc-SQUID) sensors and flip-chip magnetometers with quasiplanar Josephson junctions were developed using a high-oxygen pressure dc-sputtering technique. The fabricated dc-SQUID sensors and magnetometers were observed to operate in high magnetic fields without suppression of the critical current.

A high temperature superconducting direct current superconducting quantum interference device gradiometer was fabricated on the basis of a single YBa2Cu3O(sub 7-delta) film for a hand-held nondestructive evaluation system. Its performance was investigated by detecting holes in a carbon fiber reinforced plastic plate. Results demonstrate that the performance of the gradiometer was sufficient to detect holes in a carbon fiber reinforced plastic plate.

Research was conducted to fabricate high temperature superconducting direct current superconducting quantum interference device gradiometers on 24 degrees and 30 degrees SrTiO3 bicrystal substrates with slots and flux dams in the SQUID washers. A comparison of the measured spatial response of the devices with modelled results was carried out. Results show that single layer devices have characteristics that deviate only slightly from idealized first order gradiometers.

A vector magnetometer module that allows the recording of orthogonal vector components was designed as a cube with one liquid helium cooled superconducting quantum interference devices (SQUIDS) mounted at each of its six faces. The signals of the six SQUIDS can be combined to yield zero due to Maxwell's third equation. The module can be employed as a basic module for multichannel vector magnetometer systems.

A critical control system assessment of basic flux-locked loop systems is shown. A new superconducting quantum interference device (SQUID) linearization technique is also presented, where no magnetic flux feedback is required to cancel the applied flux. It was observed that a dual SQUID configuration can provide a true phase modulation system that is easily demodulated with a phase-locked loop.

The torquemeters designed as piezoresistive silicon cantilevers were developed to detect torque and work either in flexion or torsion mode. The design of the cantilevers utilized a standard two-leg flexion offering simplicity, sturdiness and increased sensitivity. The alternative version of the cantilever contained an extra central leg appended to support the metallic paths leading to the calibration loop. The microfabrication of the piezoresistive sensors involved alignment, ion implantation and annealing processes. The applications of the cantilevers are useful in magnetometry.

The development of a reproducible process for making high-quality SQUIDs and magnetometers has been stemmed by materials related problems. The use of GdBa2Cu3O7 and YBa2Cu3O7 thin films is proposed for the fabrication of bicrystal and step-edge magnetometers. Magnetometers with junction parameter variations that are as low as 57% from chip-to-chip and 25% for on-chip and with low noise performance have been fabricated.

Double relaxation oscillation superconducting quantum interference devices (SQUIDs) are used in the fabrication of a highly sensitive first-order gradiometer. The gradiometer exhibits an ideal white flux noise and white magnetic field noise level. Its noise level is not influenced by the relaxation oscillations in adjacent channels, indicating that double relaxation oscillation SQUIDs can be used in the design of multichannel magnetometers.

A vibrating reed magnetometer with high sensitivity is constructed. Using optical signal pickup, modified excitation coils, and an acoustical reference excitation system, the high sensitivity of former constructions is combined with the detection of the magnetization component perpendicular to the bias field H. The calibration procedure providing increased reliability is simplified and allows hysteresis measurements with the magnetic easy axis of the sample oriented at an arbitrary angle with respect to H. Measurement results are given in order to demonstrate the performance of the apparatus.

A direct current superconducting quantum interference device (SQUID) magnetometer with additional positive feedback designed to increase the SQUID's gain and avoid the complications of the traditional flux modulation technique is fabricated. The magnetometer is capable of very sensitive nuclear magnetic resonance measurements. Its bandwidth of about 300 KHz can be extended to 1 MHz.

An optically driven inductance modulation circuit is designed as part of a 1/f noise reduction scheme for superconducting quantum interference device magnetometers. Simulations show the design to be capable of a modulation depth of more than 17:1. Moreover, the design shows an increase in the mutual inductance between the washers, thus reducing the probability of flux jumping. The design is implemented in a pull-push chopping network.

Direct current superconducting quantum interference device (SQUIDs) magnetometers with directly-coupled square pick-up loops are developed from single-layer YBa2Cu3O7-pi thin films placed on step-edge junctions or bicrystal grain boundary junctions on SrTiO3 substrates. The films are developed through off-axis radio frequency sputtering or laser ablation. The current-voltage characteristics and voltage modulation of the SQUIDs in response to external fields are presented.

A low-noise direct superconducting quantum interference device (SQUID) magnetometer is developed using a single layer of YBa2Cu3O7-x on a bicrystal substrate. The magnetometer consists of a single-turn pickup loop directly coupled to a low-inductance SQUID. The magnetometer's white flux noise is measured with the use of conventional flux-locked loop electronics with bias current reversal. A white magnetic field is observed after the field-to-flux conversion efficiency is measured.

An integrated superconducting quantum interference device (SQUID) magnetometer based on Y1Ba2Cu3O7 step-edge Josephson junctions is developed. A coupling coefficent k of 0.9 allows tight coupling between the input coil and the SQUID washer. A nearly frequency independent flux noise is observed while bias reversal completely suppresses 1/f-noise contributions at lower temperatures. The magnetometer's field resolution is estimated with the use of field sensitivity and flux noise measurements.

Three types of high-transition-temperature magnetometers are developed with the use of a YBa2Cu3O7-x-SrTiO3-YBa2Cu3O7-x multilayer process. These are multiturn flux transformers combined with single layer direct current superconducting quantum interference devices (SQUIDs) in flip-chip arrangements, multiturn flux transformers integrated with dc SQUIDs and multiloop magnetometers.

A semi-portable direct current-superconducting quantum interference device magnetometer system is developed for studying diseases of the human heart and brain. The system is equipped with a liquid helium continuous cryostat with a cool down time of 20 minutes. The pickup coil is made of Nb-Ti-Cu wire. The magnetometer has a very low white noise level and allows reliable magnetocardiogram measurements.

Direct current-superconducting quantum interference devices and flux transformers are fabricated via KrF-excimer deposition and argon-ion etching and used in the construction of sensitive magnetometers. The measuring instruments are then studied for their superconducting properties and electromagnetic noise behavior. Results show that the magnetometers are capable of giving accurate readouts at varying magnetic field intensities.

Computer simulations ae employed to optimize the circuit parameters of a direct current superconducting quantum interference magnetometer. Among the parameters which are included in the optimization to minimize the magnetometer's field resolution are the critical currents of the Josephson junctions, flux quantum and inductance. A design methodology is then developed using the optimized parameters.

An integrated digital superconducting quantum interference device (SQUID) magnetometer is fabricated by using a single chip as a platform for a SQUID-based pre-amplifier, a high sensitivity comparator gate and feedback circuitries. The magnetometer has a very wide dynamic range and is capable of operating in unshielded environments. It slew rate can be significantly improved by adding Josephson junction-based counters for a faster clock rate.

The PTB 83-Superconducting Quantum Interference Device system is an ultra-low noise magnetometer designed for conducting magnetocardiography and magnetography of the peripheral nerve system. The 37-channel equipment uses additional positive feedback to realize electronic first- or second-order gradiometer configurations. It is capable of detecting extremely weak magnetic fields emitted by the human body.

A growing number of sensor technologies are using superconducting quantum interference devices magnetic anomaly detection. These systems include the superconducting gradiometer/magnetometer sensor, thin film gradiometer and autonomous mine reconnaissance systems. They used in a wide range of applications such as geomagnetic measurements, mine detection and nondestructive materials testing.

Analytical parameters can be found for the multiloop dc superconducting quantum interference (SQUID) device magnetometer developed from low or high transition temperature substances. T(sub c) is discussed and good agreement between the calculated and experimentally observed behavior of low-T(sub c) devices is found. A high-T(sub c) magnetometer design with 16 parallel lops and a 7 mm pickup coil produces reasonably small SQUID inductance of about 145 pH.

A fluxgate-like magnetometer using planar technology has been developed. The performance is comparable to conventional fluxgates. Due to minor inductive and capacitive effects of the planar coils, higher exciting frequencies can be used.

Magnetic feedback fields, applied in both hard and easy axis directions of a Permalloy sensor biased with a sinusoidal hard axis field and offset, pulsed easy axis field, allow the magnitude and direction of both orthogonal components of an external field to be determined. The two outputs from the feedback circuit, corresponding to these two components, are independent of sensitivity variations due to bias or temperature changes. The feedback circuit and sensor is a first order system with a corner frequency to sinusoidal inputs proportional to the hard axis bias frequency.