1/f noise characteristics of SEJ Y-Ba-Cu-O rf-SQUIDs on LaAlO3 substrate and the step structure, film, and temperature dependence

Step edge junction (SEJ) rf-SQUID magnetometers and gradiometers were fabricated using PLD Y-Ba-Cu-O films on LaAlO₃(100) and SrTiO₃(100) substrates. Effects of different step structure and the film properties on the yield, optimal operating temperature, and the 1/f noise of the SQUIDs were investigated. The step structure was controlled using various IBE processes. The devices on LaAlO₃ showed higher sensitivity to the step structure compared to those on SrTiO₃. This was due to re-deposition of substrate material at the steps prepared using the conventional IBE process resulting in a very low yield of unstable SQUIDs. High yield of low 1/f noise stable SQUIDs was obtained on LaAlO₃ substrates with sharp steps prepared using an optimized IBE process. A typical 1/f noise corner frequency of about 10 Hz at 77 K with two major temperature dependencies was obtained. The temperature dependencies of the 1/f noise could be correlated to the junction and the film of washer area of the SQUIDs. The white noise of our devices showed a dependency mainly on the amplitude of the flux to voltage transfer function signal. The operating temperature range of the SQUIDs could be controlled by the step structure and narrowed when the optimal operating temperature range was increased. All the measured junctions of our devices on the modified steps showed RSJ type behavior with a moderate decrease of the R_N versus temperature