Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops containing Josephson junctions. They are sensitive enough to measure fields as low as 5 aT (5×10-18T) within a few days of averaged measurements. Their noise levels are as low as 3 fT·Hz-½. For comparison, a typical refrigerator magnet produces 0.01 tesla (10-2 T), and some processes in animals produce very small magnetic fields between 10-9 T and 10-6 T. Recently invented SERF atomic magnetometers are potentially more sensitive and do not require cryogenic refrigeration but must be operated near zero field.
The traditional superconducting materials for SQUIDs are pure niobium or a lead alloy with 10% gold or indium, as pure lead is unstable when its temperature is repeatedly changed. To maintain superconductivity, the entire device needs to operate within a few degrees of absolute zero, cooled with liquid helium.
"High temperature" SQUID sensors are more recent; they are made of high temperature superconductors, particularly YBCO, and are cooled by liquid nitrogen which is cheaper and more easily handled than liquid helium. They are less sensitive than conventional "low temperature" SQUIDs but good enough for many applications.
Probably the most common use of SQUIDs is in magnetic property measurement systems. These are turn-key systems, made by several manufacturers, that measure the magnetic properties of a material sample. This is typically done over a temperature range from that of liquid helium (~4K), to a couple of hundred degrees above room temperature.
For example, SQUIDs are being used as detectors to perform Magnetic Resonance Imaging. While high field MRI uses precession fields of one to several tesla, SQUID-detected MRI uses measurement fields that lie in the microtesla regime. Since the NMR signal drops off as the square of the magnetic field, a SQUID is used as the detector because of its extreme sensitivity. The SQUID coupled to a second-order gradiometer and input circuit, along with the application of gradients are the fundamental entities which allows a research group to retrieve noninvasive images. SQUID-detected MRI has advantages over high field MRI systems such as the low cost required to build such a system and its compactness. The principle has been proven by imaging human extremities, and its future application may involve tumor screening.
Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. The use of SQUIDs in oil prospecting, mineral exploration, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops; they are also used as precision movement sensors in a variety of scientific applications, such as the detection of gravitational waves. Four SQUIDs were employed on Gravity Probe B in order to test the limits of the theory of general relativity.
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