Vortex imaging by low-temperature scanning electron microscopy and correlation with low-frequency noise in YBCO DC SQUIDs

We present a technique for direct imaging of magnetic flux quanta trapped i n direct current (DC) superconducting quantum interference devices (SQUIDs) which consist either of washers patterned in single YBa2Cu3O7-delta (YBCO) films or which are patterned in multilayer structures from YBCO/SrTiO3/YBC O thin films. Simultaneously, we are able to measure the low-frequency nois e of our devices under test, which allows correlation of the local distribu tion of vortices with low-frequency noise in the SQUIDs. The vortex imaging and noise measurements are performed with the SQUIDs mounted on a liquid n itrogen cooled cryostage of a scanning electron microscope (SEM) for invest igation at variable temperature (77 K < T< T-c) and in controllable magneti c fields up to several hundred mu T Our imagine technique, which yields a s patial resolution of about 1 mu m, is based on the electron-beam-induce loc al displacement Delta r of vortices, which is detected as a flux change Del ta Phi = Delta r(partial derivative Phi/partial derivative r) in the SQUID loop. Hence, the signal amplitude provides direct information on the coupli ng strength partial derivative Phi/partial derivative r. since partial deri vative Phi/partial derivative r determines the amount of flux noise which a fluctuating vortex induces in the SQUID, we obtain valuable information on possible low-frequency noise sources in the SQUIDs, We investigated washer SQUIDs with regular arrays of micron-sized holes (antidots) to image the c ompering formation of multiquanta trapped in antidots vs. the formation of interstitial vortices. In most cases, the interstitial vortices are pinned reproducibly at the same locations. These pinning sites do not correlate wi th the surface morphology of the films. (C) 2000 Elsevier Science B.V. All rights reserved.