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 in direct current (DC) superconducting quantum interference devices (SQUIDs) which consist either of washers patterned in single YBa₂Cu₃O_7-δ (YBCO) films or which are patterned in multilayer structures from YBCO/SrTiO₃/YBCO thin films. Simultaneously, we are able to measure the low-frequency noise of our devices under test, which allows correlation of the local distribution of vortices with low-frequency noise in the SQUIDs. The vortex imaging and noise measurements are performed with the SQUIDs mounted on a liquid nitrogen cooled cryostage of a scanning electron microscope (SEM) for investigation at variable temperature (77 K<T<T_c) and in controllable magnetic fields up to several hundred μT. Our imaging technique, which yields a spatial resolution of about 1 μm, is based on the electron-beam-induced local displacement Δr of vortices, which is detected as a flux change ΔΦ = Δr(∂Φ/∂r) in the SQUID loop. Hence, the signal amplitude provides direct information on the coupling strength ∂Φ/∂r. Since ∂Φ/∂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 competing 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 with the surface morphology of the films.