Spintronics detection technology can add significant functionality to bioassay detection systems and improve their precision. Photonic detection is presently the dominant technology in bioassay systems. Optical detectors on mechanical scanners are used to read the vast majority of commercially produced DNA chips. Introduction of these optical systems into non-laboratory applications is greatly impeded by the cost, size, and complexity of optical systems. Magnetics can help by providing detectors and actuators that are inherently rugged, small, and mass-manufacturable. Magnetic nanoparticles are used in many biochemical assays as labels for concentration, manipulation and, more recently, detection. Typically one attaches the magnetic particles to the biochemical species of interest (target) using a chemically specific binding interaction. Once bound, the labels enable the manipulation of the target species through the application of magnetic forces. Spintronic sensors, specifically Giant Magnetoresistive (GMR) and Spin Dependent Tunneling (SDT), sensors have been developed to detect and quantify labels in two main formats: flowing in a microfluidic channel, and immobilized labels on a chip surface.
A 3-axis magnetometer has been constructed using 3 Spin Dependent Tunneling (SDT) magnetic field sensors as transducers. This magnetometer has been designed for use in Unattended Ground Sensor (UGS) applications. As such, there has been an emphasis on low cost, size, and power. The present version is smaller than previous versions, and is ready for prototype sampling. This paper describes the basic properties of the SDT 3-axis magnetometer, including size, power, and noise floor.
Spin Dependent Tunneling (SDT) magnetic field sensors are under development as high performance magnetometers for Unattended Ground Sensing applications. These sensors have been successfully incorporated into a 3-axis magnetometer circuit for prototype level demonstrations with noise floors on the order of 1 nT / root Hz. at 1 Hz. Future versions of these magnetometers will require lower power and lower noise floor than the existing prototype. This paper examines near term developments in the SDT magnetometer power requirements, considering the transducer + amplifier combination as the basic unit. An emphasis is placed on discussing the trade-offs between low power and high sensitivity. The impact of both SDT transducer impedance and amplifier power and noise are considered. Projections show that a 1 pT / root Hz at 1 Hz noise floor is achievable in a transducer + amplifier unit consuming 1 to 10 mW. SDT sensors are being made in an effort to reach these goals. Special emphasis in this paper is on efforts to make an intrinsically low power transducer. Further discussion will explain some fundamentals of SDT sensor operation and how they impact ultimate expected magnetometer performance.
Micro Unattended Ground Sensor Networks will likely employ magnetic sensors, primarily for discrimination of objects as opposed to initial detection. These magnetic sensors, then, must fit within very small cost, size, and power budgets to be compatible with the envisioned sensor suites. Also, a high degree of sensitivity is required to minimize the number of sensor cells required to survey a given area in the field. Solid state magnetoresistive sensors, with their low cost, small size, and ease of integration, are excellent candidates for these applications assuming that their power and sensitivity performance are acceptable. SDT devices have been fabricated into prototype magnetic field sensors suitable for use in micro unattended ground sensor networks. They are housed in tiny SOIC 8-pin packages and mounted on a circuit board with required voltage regulation, signal amplification and conditioning, and sensor control and communications functions. The best sensitivity results to date are 289 pT/rt. Hz at 1 Hz, and and 7 pT/rt. Hz at f > 10 kHz. Expected near term improvements in performance would bring these levels to approximately 10 pT/rt Hz at 1 Hz and approximately 1 pT/rt. Hz at > 1 kHz.
A handhold mine detection system is under development using a 2D array of spin dependent tunneling (SDT) magnetorestrictive sensors, which will measure the x, y and z scalar components of the electromagnetic (EM) field. SDT sensors directly measure the EM field component along an axis of the sensor over a wide frequency and intensity range, which make them ideal EM sensors. The sensors are small and are relatively inexpensive due to the massive investment in this technology by the computer industry for their use in disc storage devices. A system was designed with primary emphasis on the unique capabilities of the sensor elements and sensor array design for landmine detection and discrimination. Much of the early work concentrated on theoretical models verified with measured laboratory time domain EM response of metallic components of typical low metal landmines. The modeling results have provided the information needed to define performance requirements for the SDT sensor and a design of an array of SDT sensor to measure the x, y and z spatial components expected from the landmines. A parallel effort to develop the supporting theory for optimal interpretation of the multi-axis sensor array, has resulted in significant progress in developing an improved methodology for distinguishing the signature of landmine targets from metallic clutter. We have adopted an integrated approach to the sensor design in which the data requirements for effective discrimination have driven the design while meeting the practical and engineering requirements as well.
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