PROGRESS IN RF-SQUIDS

At present, dc-SQUIDS made of conventional superconductors are recognised as the most sensitive devices for measuring magnetic flux. The growing interest in high-Tc superconducting electronics, however, has also increased the interest in rf-SQUIDS again. Here, rf-SQUIDS offer several advantages like the requirement for only one single Josephson junction and a low l/f noise. With relatively high bias frequencies (> 100 MHz) it is possible to obtain values of the white flux noise comparable to dc-SQUIDS. Starting from bulk material devices with relatively high flux noise levels, at preseat, the best high-Tc rf-SQUIDS offer a field sensitivily of less than 200 fT/root Hz (@ 1Hz). This is already sufficient for a number of biomedical applications. This paper reviews recent developments towards practical rf-SQUIDs made of high-Tc as well as conventional superconductors.