Self-Assembly of Synthetic and Biological Polymeric Systems of ...

In particular, we use SQUID to measure the magnetic properties of magnetic nanoparticles and
hybrids. In this regard, magnetic nanoparticles show a wide variety of unusual magnetic
properties as compared to the respective bulk materials. In a paramagnetic material, the
thermal energy overcomes the coupling forces between neighbouring atoms above the Curie
temperature, causing random fluctuations in the magnetization directions that result in a null
overall magnetic moment. However, in superparamagnetic materials the fluctuations affect
the direction of individual entire crystallites. The magnetic moments of individual crystallites
compensate each other and the overall magnetic moment becomes null. When an external
magnetic field is applied, the behaviour is similar to paramagnetic materials except that,
instead of each individual atom being independently influenced by an external magnetic field,
the magnetic moment of entire crystallites aligns with the magnetic field (Figure 2.32) (58)(59).
For magnetic nanoparticles, superparamagnetism occurs in magnetic materials composed of
very small crystallites (the threshold size depends on the nature of the material, i.e. Fe-based
materials becomes superparamagnetic at sizes