Weak-link investigation of a YBaCuO thin film DC SQUID by SEM

Weak links of an unintentionally realized SQUID in a YBaCuO thin film microbridge have been visualized by SEM. We measured a perfectly periodic magnetic field response with a maximum voltage-to-flux transfer function of 70 mu V/Phio at 60 K, where 2LIc/Phio approximately=9. Using the visualization technique, weak-link diameters were observed to be 2 and 3 mu m, each weak link consisting of four to ten different grain boundaries. However, in spite of multigrain composition, the weak links showed ideal Josephson junction behaviour. The local critical current density of the weak links and the microbridge bulk have been measured to be equal to an order of magnitude, which resulted in the conclusion that YBaCuO thin films with Jc approximately=103-105 A cm-2 at 77 K are Josephson media. A method for controlled fabrication of thin film SQUIDS using visualization of weak links by SEM has been proposed. (author)