Dieter Koelle, II. Physikalisches Institut, Lehrstuhl für Angewandte Physik, Universitaet zu Koeln, Zuelpicherstr 77, D-50937 Koeln, Tel: +49-221-470-3583, Fax: +49-221-470-5178, email:  koelle@colorix.ph2.uni-koeln.de

Superconducting QUantum Interference Devices (SQUIDs) are the most sensitive detectors for magnetic flux and many of their applications (e.g. in biomagnetism, nondestructive evaluation or geophysics) are based on their ability to detect tiny magnetic fields if equipped with a suitable input circuit. The advent of high transition temperature (T_c) superconductivity above 77K has opened the perspective for the development of smaller and less expensive liquid nitrogen cooled SQUID based systems compared to their low T_c counterparts. This has stimulated strong efforts towards the development of sensitive high-T_c SQUIDs.
An introduction to the basic principles of SQUIDs will be given, which is followed by a review of the progress towards sensitive thin film high-T_c SQUIDs and magnetometers: Theoretical predictions of the signal and thermal noise will be compared to experimental results. Furthermore low frequency 1/f noise is a major issue in this field and will be discussed. The improvement of film quality reduces the 1/f noise from thermally activated motion of vortices and the use of proper readout schemes eliminates 1/f-noise contributions form critical current fluctuations in the Josephson junction(s), intersecting the SQUID loop. This has led to low levels of 1/f noise in SQUIDs based on YBa₂Cu₃O₇ thin films. Various approaches are shown to enhance the magnetic field sensitivity of the SQUIDs, like the use of inductively coupled flux transformers with multiturn input coils, multiloop SQUIDs and directly coupled single layer designs. Impressive progress has led to magnetometers with field resolutions as low as 10-20 fT Hz^-1/2 in the white noise limit at 77K, with somewhat larger values at 1Hz. The implications of these improved field resolutions for some applications will be discussed.