Abstract
We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity Sμ1/2 upon operation in intermediate magnetic fields B (tens of mT), applied perpendicular to the substrate plane. Our, so far, best SQUID can be continuously operated in fields up to B≈±50 mT with rms flux noise SΦ,w1/2≤250 nΦ0/Hz1/2 in the white noise regime and spin sensitivity Sμ1/2≤29 μB/Hz1/2. Furthermore, we demonstrate operation in B = 0.5 T with high sensitivity in flux SΦ,w1/2≈680 nΦ0/Hz1/2 and in electron spin Sμ1/2≈79 μB/Hz1/2. We discuss strategies to further improve the nanoSQUID performance.
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