SQUIDs (Superconducting Quantum Interference Devices) are magnetic field sensors with unsurpassed sensitivity. They are amazingly versatile, being able to measure all physical quantities which can be converted to magnetic flux. They are routinely fabricated in thin film technology from two classes of superconducting materials: high-temperature superconductors (HTS) which are usually cooled to 77 K, and low-temperature superconductors (LTS), which have to be cooled to 4.2 K.
SQUIDs have many applications, two of which shall be discussed in this talk. In SQUID microscopy, a SQUID scans a sample, which preferrably is at room temperature, and measures the two-dimensional magnetic field distribution at its surface. In order to achieve a relatively high spatial resolution, the stand-off distance between the sample and the SQUID is made as small as possible. For SQUIDs made from high-temperature superconductors this spacing can be as low as 0.01 mm, whereas for low-temperature superconductors, spacings of the order of 0.05 mm have been realized.
SQUIDs show also promising results in the field of nondestructive testing of various materials. For example, ferromagnetic impurities in stainless steel formed by aging processes in the material can be detected with high probability, and cracks in conducting materials, for example aircraft parts, can be located using eddy current methods. Especially for the case of thick, highly conductive, or ferromagnetic materials, as well as sintered materials, it can be shown that a SQUID based NDE system exhibits a much higher sensitivity compared to conventional eddy current NDE and ultrasonic testing.

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