  Nanometer-sized magnetic particles have generated continuous interest as the
study of their properties has proved to be scientifically and technologically
very challenging. In this article we reviewed the most important theories and
experimental results concerning the magnetization reversal of single-domain
particles,clusters and molecular clusters. Sect.1 reviews briefly the commonly
used measuring techniques. Among them, electrical transport measurements, Hall
probes and micro-SQUID techniques seem to be the most convenient techniques for
low temperature measurements. Sect.2 discusses the mechanisms of magnetization
reversal in single domain particles at zero Kelvin. The influence of
temperature on the magnetization reversal is reported in Sect.3. Finally,
Sect.4 shows that for very small systems or very low temperature, magnetization
can reverse via quantum tunneling. The boundary between classical and quantum
physics has become a very attractive field of research. This section discusses
detailed measurements which demonstrated that molecular magnets offer an unique
opportunity to explore the quantum dynamics of a large but finite spin. We then
discussed tunneling in nanoparticles and showed how one might give a definite
proof of their quantum character at low temperature.
