  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 $S_\mu^{1/2}$
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 $B\approx\pm50\,\rm{mT}$ with rms flux noise
$S_{\Phi,\rm w}^{1/2}\leq250\,\rm{n\Phi_0/Hz^{1/2}}$ in the white noise regime
and spin sensitivity $S_{\mu}^{1/2}\leq29\,\rm{\mu_B/Hz^{1/2}}$. Furthermore,
we demonstrate operation in $B=0.5\,\rm{T}$ with high sensitivity in flux
$S_{\Phi,\rm w}^{1/2}\approx680\,\rm{n\Phi_0/Hz^{1/2}}$ and in electron spin
$S_{\mu}^{1/2}\approx79\,\rm{\mu_B/Hz^{1/2}}$. We discuss strategies to further
improve the nanoSQUID performance.
