Nb nano superconducting quantum interference devices with high spin sensitivity for operation in magnetic fields up to 0.5\,T

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 $S_{\mu }^{1\mathrm{/}2}$ 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 $B\approx \pm 50\mathrm{m}\mathrm{T}$ with rms flux noise $S_{\mathrm{\Phi },\mathrm{w}}^{1\mathrm{/}2}\le 250\mathrm{n}{\mathrm{\Phi }}_0\mathrm{/}\mathrm{H}{\mathrm{z}}^{1\mathrm{/}2}$ in the white noise regime and spin sensitivity $S_{\mu }^{1\mathrm{/}2}\le 29{\mu }_{\mathrm{B}}\mathrm{/}\mathrm{H}{\mathrm{z}}^{1\mathrm{/}2}$. Furthermore, we demonstrate operation in $B=0.5\mathrm{T}$ with high sensitivity in flux $S_{\mathrm{\Phi },\mathrm{w}}^{1\mathrm{/}2}\approx 680\mathrm{n}{\mathrm{\Phi }}_0\mathrm{/}\mathrm{H}{\mathrm{z}}^{1\mathrm{/}2}$ and in electron spin $S_{\mu }^{1\mathrm{/}2}\approx 79{\mu }_{\mathrm{B}}\mathrm{/}\mathrm{H}{\mathrm{z}}^{1\mathrm{/}2}$. We discuss strategies to further improve the nanoSQUID performance.