  The magnetic sensing at nanoscale level is a promising and interesting
research topic of nanoscience. Indeed, magnetic imaging is a powerful tool for
probing biological, chemical and physical systems. The study of small spin
cluster, like magnetic molecules and nanoparticles, single electron, cold atom
clouds, is one of the most stimulating challenges of applied and basic research
of the next years. In particular, the magnetic nanoparticle investigation plays
a fundamental role for the modern material science and its relative
technological applications like ferrofluids, magnetic refrigeration and
biomedical applications, including drug delivery, hyper-thermia cancer
treatment and magnetic resonance imaging contrast-agent. In this framework,
several efforts have been devoted to the development of a high sensitivity
magnetic nanosensor pushing sensing capability to the individual spin level.
Among the different magnetic sensors, Superconducting QUantum Interference
Devices (SQUIDs) exhibit an ultra high sensitivity and are widely employed in
numerous applications. In the recent years, it has been proved that the
magnetic response of nano-objects can be effectively measured by using a SQUID
with a very small sensitive area (nanoSQUID). In fact, the sensor noise,
expressed in terms of the elementary magnetic moment (spin or Bohr magneton),
is linearly dependent on the SQUID loop side length. For this reason, SQUIDs
have been progressively miniaturized in order to improve the sensitivity up to
few spin per unit of bandwidth. With respect to other techniques, nanoSQUIDs
offer the advantage of direct measurement of magnetization changes in small
spin systems. In this review, we focus on nanoSQUIDs and its applications. In
particular, we will discuss the motivations, the theoretical aspects, the
fabrication techniques, the different nanoSQUIDs and the relative nanoscale
applications.
