  We demonstrate the operation of low-noise nano superconducting quantum
interference devices (SQUIDs) based on the high critical field and high
critical temperature superconductor YBa$_2$Cu$_3$O$_7$ (YBCO) as
ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The
nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have
been patterned by focused ion beam milling. This allows to precisely define the
lateral dimensions of the SQUIDs so as to achieve large magnetic coupling
between the nanoloop and individual MNPs. By means of focused electron beam
induced deposition, cobalt MNPs with typical size of several tens of nm have
been grown directly on the surface of the sensors with nanometric spatial
resolution. Remarkably, the nanoSQUIDs are operative over extremely broad
ranges of applied magnetic field (-1 T $< \mu_0 H <$ 1 T) and temperature (0.3
K $< T<$ 80 K). All these features together have allowed us to perform
magnetization measurements under different ambient conditions and to detect the
magnetization reversal of individual Co MNPs with magnetic moments (1 - 30)
$\times 10^6\,\mu_{\rm B}$. Depending on the dimensions and shape of the
particles we have distinguished between two different magnetic states yielding
different reversal mechanisms. The magnetization reversal is thermally
activated over an energy barrier, which has been quantified for the (quasi)
single-domain particles. Our measurements serve to show not only the high
sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these
sensors are exceptional magnetometers for the investigation of the properties
of individual nanomagnets.
